Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journal homepage: www.elsevier .com/locate /saa

Quantum chemical computations, vibrational spectroscopic analysis andantimicrobial studies of 2,3-Pyrazinedicarboxylic acid

http://dx.doi.org/10.1016/j.saa.2014.11.0341386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Department of Physics and Research centre, Women’sChristian College, Nagercoil 629 001, Tamil Nadu, India. Tel. : +91 9994769009.

E-mail address: benaezhil@yahoo.com (V.B. Jothy).

T. Joselin Beaula a, A. Packiavathi a, D. Manimaran b, I. Hubert Joe b, V.K. Rastogi c, V. Bena Jothy a,⇑a Department of Physics and Research Centre, Women’s Christian College, Nagercoil 629 001, Tamil Nadu, Indiab Department of Physics and Research Centre, Mar Ivanios College, Nalancira 695015, Kerala, Indiac Indian Spectroscopy Society, KC 68/1, Old Kavinagar, Ghaziabad 201 002, India

h i g h l i g h t s

� Red shift of OAH stretchingfrequencies in FT IR is a clear evidencefor the occurrence of OAH� � �Ointeraction.� Strong intermolecular hydrogen bond

formation in PDCA enhancesantimicrobial activity.� Antimicrobial activity results of the

viability assay have proved PDCA tohold excellent antimicrobial nature.� Molecular docking study confirms the

interaction between PDCA and 1GSK.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 September 2014Received in revised form 6 November 2014Accepted 20 November 2014Available online 28 November 2014

Keywords:Antimicrobial2,3-Pyrazinedicarboxylic acidFTIRFT-RamanDFTNBO analysis

a b s t r a c t

Density Functional Theory (DFT) calculations at B3PW91 level with 6-311G (d) basis sets were carried outfor 2,3-Pyrazinedicarboxylic acid (PDCA) to analyze in detail the equilibrium geometries and vibrationalspectra. Calculations reveal that the optimized geometry closely resembles the experimental XRD data.Vibrational spectra were analyzed on the basis of potential energy distribution (PED) of each vibrationalmode, which provides quantitative as well as qualitative interpretation of IR and Raman spectra. Informa-tion about size, shape, charge density distribution and site of chemical reactivity of the molecule wereobtained by mapping electron density isosurface with the electrostatic potential surface (ESP). Basedon optimized ground state geometries, NBO analysis was performed to study donor–acceptor (bond–anti-bond) interactions. TD–DFT analysis was also performed to calculate energies, oscillator strength of elec-tronic singlet–singlet transitions and the absorption wavelengths. The 13C and 1H nuclear magneticresonance (NMR) chemical shifts of the molecule in the ground state were calculated by gauge indepen-dent atomic orbital (GIAO) method and compared with the experimental values. PDCA was screened forits antimicrobial activity and found to exhibit antifungal and antibacterial effects. Molecular docking wasalso performed for the different receptors.

� 2014 Elsevier B.V. All rights reserved.

Introduction

Pyrazine and its derivatives form an important class of com-pounds present in several natural flavours and complex organicmolecules [1,2]. It is a building block of phenazine and pteridines

724 T.J. Beaula et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735

and occurs in many compounds with pharmaceutical applications.Pyrazine family is known as a very effective antimycobacterialagent, with a well established role in tuberculosis treatment [3–5]. Pyrazinamide is bactericidal to semidormant mycobacteriaand reduces total treatment time. 2,3-Pyrazinedicarboxylic acid(PDCA) is an antibacterial compound [6] and its anions have beenof great interest in coordination and supra molecular chemistry asthey generally tend to react with metal salts to yield insolublepolynuclear materials. Among them, substituted pyrazines havebecome increasingly important biologically active compounds.

The antimicrobial activity together with the extensive analysisof the quantum chemical studies, structural characteristics, vibra-tional spectral investigations with the aid of normal coordinateanalysis (NCA), molecular orbital interactions and electronicproperties are discussed in detail.

Experimental details

FTIR absorbance spectrum of the purchased PDCA compoundfrom Sigma Aldrich was recorded using KBr pellet method at roomtemperature in the region between 4000 and 400 cm�1 usingPerkin Elmer Spectrometer with resolution of 1.0 cm�1. FT Ramanspectrum was recorded with BRUKER RFS 27: Stand alone FT-Raman Spectrometer. The laser source used was Nd: YAG1064 nm in the spectral range of 4000–2 cm�1 at room tempera-ture with spectral resolution of 2.0 cm�1. The UV–Vis absorptionspectrum of the compound was recorded in water solution usinga Cary 5E UV–VIS–NIR spectrophotometer in the spectral regionof 200–800 nm. 1H NMR and 1C NMR spectra were analyzed withBruker Bio Spin GmbH NMR spectrometer using D2O as solvent.The Correlation Spectroscopy (COSY) spectrum of PDCA wasrecorded using Bruker AVANCE III 500 MHz (AV 500) multi nucleisolution NMR Spectrometer. Thermal analysis was carried out withNetzsch STA 409 PC/PG where the sample was scanned in the tem-perature range 30–1000 �C at the rate off 10 �C/min. in inert nitro-gen atmosphere.

The antimicrobial activity of PDCA was screened by agar welldiffusion method by Perez et al. [7] and it was tested against fourhuman pathogenic microorganisms namely two bacterial strainssuch as, Escherichia coli and Staphylococcus aureus and two fungalstrains such as, Aspergillusochraceus and Candida albicans. Both,the antibacterial and antifungal activities of the title compoundwere observed and the diameter of the inhibition zone was mea-sured around the well.

Computational details

Density Functional Theoretical (DFT) quantum chemical com-putations were carried out at the B3PW91 level with 6-311G (d)basis set using Gaussian’09 program package [8] to get a clearknowledge of optimized parameters, vibrational wavenumbers,IR intensities, harmonic vibrational frequencies, depolarizationratios, Raman activity and energies of PDCA. Further, the interac-tion energies of dimer were corrected for the Basis Set Superposi-tion Error (BSSE), which were approximated using counterpoisemethod [9,10]. The amount of p-electron delocalization in the sixmembered heterocycles were also calculated from the geometrybased Harmonic Oscillator Model of Aromaticity (HOMA) indexof aromaticity to get information about the nature of aromatization[11–14]. NBO calculations [15] were performed using NBO 3.1 pro-gram to understand inter and intra-molecular delocalization orhyperconjugation. The characterization of normal modes usingpotential energy distribution (PED) was performed with MOLVIB

– 7.0 program written by Sundius [16,17]. To improve theagreement between the predicted and observed frequencies, thecomputed harmonic frequencies were scaled for comparison andthe scaling of the force field was performed according to theSQMFF procedure [18]. The Cartesian representation of the forceconstants were transferred to a non-redundant set of symmetrycoordinates, chosen in accordance with the recommendations ofPulay et al. [19]. The descriptions of the predicted frequencies dur-ing the scaling process were followed by the potential energy dis-tribution (PED) matrix.

The electronic properties such as HOMO and LUMO energieswere determined by time-dependent DFT (TD–DFT) approach, bytaking solvent effect into account [20–23]. The important quanti-ties such as electronegativity (v), hardness (g), softness (f), andelectrophilicity index (p) were deduced from ionization potentialand electron affinity values [24–26]. 1H and 13C NMR isotropicshielding were calculated by GIAO method [27,28] using optimizedparameters obtained from B3PW91with 6-311G (d) method. Iso-tropic shielding values were used to calculate isotropic chemicalshifts with respect to tetramethylsilane (TMS). AutoDock4 (version4.2) with the Lamarckian genetic algorithm was used to performdocking studies.

Results and discussion

Optimized geometry

Optimized structures of the monomer and dimer PDCA mole-cules obtained by DFT computations using Gaussian ‘09 are shownin Fig. 1(A) and (B) respectively. Global minimum energy of mono-mer and dimer calculated by DFT structure optimization methodare �641.3482 and �1282.7468 Hartrees respectively. The mole-cules in dimer are bound together through hydrogen bonds. TheBSSE corrected intermolecular energy and the BSSE energy are�1282.7084 and 0.00407 Hartrees. This interaction arises largelythrough bonded O9AH10� � �O17 and O29AH30� � �O14 contacts whichresult in the increased stabilization of the molecule. This is wellreflected as distortion in the molecular geometry with respect tothe isolated molecule.

Structural analysisThe selected bond lengths and bond angles tabulated in Table 1

and the dihedral angles presented in Table S1 are compared withthe XRD data [29]. Pyrazine ring is found to be nearly planar within1� twist and appears little distorted with the angles C18AC21AN22

as 114.03� (theoretical value is 116.30�) and C18AC21AC26 as123.72� (theoretical value is 122.27�) which are slightly out of per-fect trigonal angle for both molecules due to the substitutions ofthe carboxylic acid in the place of the hydrogen atoms. The pyra-zine ring is co-planar with the carboxylic acid and the computeddihedral angles N4AC5AC13AO14, N4AC5AC13AO15, C6AC5AC13-

AO14 and C6AC5AC13AO15 got by DFT calculations are �180�.Since PDCA is a heterocyclic molecule, resonance effects are

observed in the ring and the CAC bond lengths in the six mem-bered ring are similar to the benzene molecule. Generally, CACbond length in benzene ring is about 1.396 Å [30] but in PDCA,the optimized bond lengths C2AC3 and C5AC6 got by DFT computa-tions are 1.394 and 1.397 Å showing good agreement with theexperimental values 1.392 and 1.400 Å respectively.

Hydrogen bondingThe optimized molecular structure of PDCA dimer (Fig. 1(B))

reveals the presence of intermolecular hydrogen bonding

Fig. 1. The optimized structure of the (A) monomer and (B) dimer at DFT/B3PW91/6-311G (d) level.

T.J. Beaula et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735 725

interaction. The intermolecular contacts O8� � �H30 and O17� � �H11

occur with O� � �H distances of 2.233 and 2.906 Å respectively,which are significantly shorter than the van der Walls separationbetween O and H atoms [31], indicating the possibility of the inter-molecular OAH� � �O and CAH� � �O hydrogen bonding in PDCA. Theintramolecular distance of N4� � �H16 (3.223 Å) is also observed.

Bond order

Normally bond order provides indication about the stability of abond. According to molecular orbital theory, bond order is theamount of charge located on a bond and is of great importancein the calculation of molecular parameters like dipole moment.Selected bond orders calculated at the B3PW91/6-311G (d) levelby natural atomic orbital theory and molecular orbital theory basisare tabulated along with the theoretical and experimental bondlengths to understand the bond length-bond order relation asgiven in Table S2. As the C7@O8 and C13@O14 bond lengths increase,there is decrease in bond order from monomer to dimer. The pos-sible reason for this is the presence of strong conjugation pathbetween monomer and dimer. The bond order analysis of mono-mer and dimer reveals that the CAO bond orders are nearly equal

1.0 with increasing bond order from monomer to dimer and C@Obond orders are 1.7 with decreasing bond order which are the pos-sible consequences of p-delocalization around the central carbonatom of carbocyclic acid.

Aromaticity

Aromaticity is a chemical property which describes the way inwhich a conjugated ring of unsaturated bonds, lone pairs or emptyorbitals can exhibit stabilization stronger than that expected by thestabilization of conjugation alone. Aromaticity can also be consid-ered as a manifestation of cyclic delocalization and resonance [32–35]. Aromaticity index HOMA is used for the estimation of delocal-ization and to get information about the nature of aromatization.

The calculated values of HOMA, GEO and EN of the pyrazine ringare tabulated in Table S3. The HOMA index values of the pyrazinering obtained experimentally and theoretically are 0.8837 and0.9878 respectively illustrating that this pyrazine ring belongs toaromatic compounds and also there is a slight decrease in aroma-ticity when compared to that of the ideal benzene molecule(HOMA = 1) due to the presence of two nitrogen atoms replacedby the methine of benzene ring. The increase of bond alternation

Table 1Optimized bond lengths (Å) and bond angles (�) of PDCA by B3PW91with 6-311G (d) basis sets along with the XRD data.

Bond length Dimer (Å) Monomer (Å) Experimental (Å) Bond angle Experimental (�) Monomer (�) Dimer (�)

N1AC2 1.327 1.328 1.320 C2AN1AC6 117.83 116.76 116.58N1AC6 1.331 1.332 1.344 N1AC2AC3 121.68 121.90 121.92C2AC3 1.392 1.393 1.379 N1AC2AH11 116.48 116.72 116.77C2AH11 1.086 1.087 0.969 C3AC2AH11 121.77 121.36 121.29C3AN4 1.324 1.326 1.327 C2AC3AN4 121.11 121.56 121.66C3AH12 1.086 1.087 1.924 C2AC3AH12 118.99 121.43 121.31N4AC5 1.330 1.331 1.328 N4AC3AH12 119.89 117.00 117.02N4AO20 2.216 5.148 C3AN4AC5 117.85 116.95 116.83C5AC6 1.400 1.401 1.391 C3AN4AO20 70.99 117.34C5AC13 1.493 1.495 1.502 C5AN4AO20 86.64 105.93C6AC7 1.507 1.508 1.502 N4AC5AC6 120.82 121.497 121.49C7AO8 1.200 1.198 1.199 N4AC5AC13 116.92 118.650 118.67C7AO9 1.337 1.336 1.306 C6AC5AC13 122.06 119.846 119.82O9AH10 0.968 0.968 0.814 N1AC6AC5 120.40 121.308 121.46C13AO14 1.207 1.206 1.206 N1AC6AC7 115.01 114.595 114.35C13AO15 1.334 1.334 1.294 C5AC6AC7 124.49 123.991 124.16O15AH16 0.967 0.968 0.890 C6AC7AC8 122.61 123.118 123.95O17AC18 1.200 1.185 C6AC7AO9 111.78 111.628 111.51C18AO19 1.336 1.315 O8AC7AO9 125.51 125.106 124.36C18AC21 1.507 1.505 C7AO8AH30 66.85 130.905O19AO20 0.968 0.856 C7AO9AH10 112.27 107.301 107.23C21AN22 1.331 1.343 C5AC13AO14 122.52 123.024 123.16C21AC26 1.400 1.386 C5AC13AO15 112.25 113.321 113.45N22AC23 1.329 1.315 O14AC13AO15 125.21 123.653 123.38C23AC24 1.392 1.382 C13AO15AH16 112.54 106.582 106.41C23AH31 1.087 0.959 O17AC18AC21 122.29 123.81C24AN25 1.326 1.327 O19AC18AC21 112.43 111.52C24AH32 1.087 0.995 C18AO19AO20 105.88 107.25N25AC26 1.330 1.344 C18AC21AN22 114.03 114.39C26AC27 1.494 1.501 C18AC21AC26 123.72 124.06C27AO28 1.206 1.201 N22AC21AC26 122.17 121.50C27AO29 1.331 1.298 C21AN22AC23 116.16 116.63O29AH30 0.967 0.936 N22AC23AC24 122.59 121.85

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factor (GEO) and bond elongation factor (EN) causes dearomatiza-tion by 0.1163 and 0.0122 in the theoretical and experimental val-ues of pyrazine ring.

Natural bond orbital analysis

NBO analysis clearly manifests the evidence for the formation oftwo strong H-bonded interactions between oxygen lone electronpair and r⁄(OAH) antibonding orbitals. The stabilization energyE(2) associated with hyperconjugative interactions n1(O8) ? r⁄(-O29AH30), is obtained as 13.79 kJ/mol (Table 2) which quantifythe extend of intermolecular hydrogen bonding. The difference inE(2) energies are reasonably due to the fact that the accumulationof electron density in the OAH bond is not only drawn from then(O) of hydrogen-acceptor but also from the entire molecule.Rehybridization plays a dominant role in weakening and elonga-tion in OAH bond. It is observed from Table 3 that the s-characterof O9AH10 and O15AH16 hybrid orbitals decreases (1.44%) fromsp3.63 to sp3.96 and (1.39%) from sp3.65 to sp3.97 respectively leadingto the weakening and elongation of O9AH10 and O15AH16 bonds.

The ED in the carbonyl C@O antibonding orbitals p⁄(C7@O8) andp⁄(C13@O14) are increased significantly (0.0113e and 0.0077e)upon dimerization which yields to weakening of the bond and itselongation and this is evident from the E(2) energy (47.20 and49.30 kcal mol�1) of the hyperconjugative intramolecular interac-tion n2(O9) ? p⁄(C7AO8) and n2(O15) ? p⁄(C13AO14). In addition,the s character of spn hybrid orbital for the C7@O8 bond increasesfrom sp1.95 to sp1.90 upon dimerization, which substantiates thebond weakening.

The strengthening and contraction of OAH bonds due to rehy-bridization, is revealed by the low value of electron density0.0112e and 0.0094e in the r⁄(O9AH10) and r⁄(O15AH16) orbitalsas tabulated in Table 2. The formation of intermolecular improper

OAH� � �O hydrogen-bonding interaction in PDCA is due to the pres-ence of electron donor carboxylic group, which is very important inthe enhancement of the biological activity of this compound[36,37].

The r and p electron bonds of N1AC2, C3AN4 and C5AC6 bondsshow a typical double bond characteristics whereas the only relectron bond of C2AC3, N4AC5 and C6AN1 show the single bondcharacteristics which are substantiated by the NBO computations.

The bending angles of different bonds are expressed as theangle of deviation from the line joining the two nuclei centers.The r(O17AC18) bond (10.6�) and the r(O27AC28) bond (10.1�)are more bent away from the line of centers as a result of lyingin the strong charge transfer path towards the substituted electrondonating oxygen atom (Table S3).

Vibrational spectral analysis

Detailed vibrational assignments of PDCA were carried out withthe aid of Normal Coordinate Analysis. Non-redundant set of inter-nal coordinates have been defined (Table S4) and used as data filefor MOLVIB program while selective scaling has been incorporatedaccording to the SQM scheme using a set of 20 transferable scalefactors (given in the Table S6) with the RMS frequency error10 cm�1. Detailed spectral assignments with PED contributionsare tabulated in Table 4. The observed FTIR and FT Raman spec-trum along with the simulated spectra is shown in Figs. 2 and 3for visual comparison. The vibrational assignments of the differentfunctional groups are discussed below.

Pyrazine ring vibrationsThe hetero aromatic structure confirms the presence of CAH

stretching vibration in the region 3100–3000 cm�1, which is thecharacteristic region for the ready identification of CAH stretching

Table 2Second order perturbation theory analysis of Fock matrix in NBO basis.

Donor (i) ED (i) (e) Acceptor (j) ED (j) (e) E(2)a (kJ mol�1) E(j)–E(i)b (a.u) F(i,j)c (a.u)

r(N1AC2) 1.9844 r⁄(C1AC6) 0.0218 3.46 1.23 0.059p(N1AC2) 1.6885 p⁄(C5AC6) 0.3302 26.89 0.33 0.081r(C2AC3) 1.9919 r⁄(N1AC2) 0.0153 1.26 1.27 0.036r(C3AN4) 1.9854 r⁄(C4AC5) 0.2750 1.28 1.38 0.038p(C3AN4) 1.6945 p⁄(C5AC6) 0.3302 23.71 0.33 0.079r(N4AC5) 1.9829 r⁄(C5AC6) 0.0491 3.40 1.39 0.062r(C5AC6) 1.9797 r⁄(N1AC6) 0.0218 2.25 1.26 0.048p(C5AC6) 1.5787 p⁄(N1AC2) 0.0153 17.19 0.27 0.064r(N1AC6) 1.9795 p⁄(C5AC6) 0.3302 26.89 0.33 0.085r(C6AC7) 1.9728 r⁄(O9AH10) 0.0112 2.29 1.04 0.044r(C5AC13) 0.0094 r⁄(O15AH16) 0.0094 1.87 1.05 0.040p(C7@O8) 1.9920 r⁄(C5AC6) 0.0491 1.26 0.93 0.031r(C7@O8) 1.9948 r⁄(C6AC7) 0.0832 2.14 1.48 0.051r(C5AC6) 1.9783 r⁄(C7@O8) (Monomer) 0.0266 1.10 1.37 0.035

1.9797 r⁄(C7@O8) (Dimer) 0.0505 2.66 0.90 0.049n2(O9) 1.8187 p⁄(C7@O8) (Monomer) 0.2012 39.84 0.37 0.109

1.8070 p⁄(C7@O8) (Dimer) 0.2125 47.20 0.36 0.116n2(O15) 1.9789 p⁄(C13AO14) 0.2477 49.30 0.34 0.118n2(O17) 1.9775 p⁄(C18AO19) 0.0998 31.07 0.64 0.128

p⁄(N1AC2) 0.0153 17.19 0.27 0.064n2(O8) 1.8308 r⁄(O29AH30) 0.0093 13.79 0.32 0.004n2(O15) 1.9789 r⁄(C13@O14) (Monomer) 0.0206 6.32 1.24 0.079

r⁄(C13@O14) (Dimer) 0.0193 47.53 0.35 0.116p⁄(C13@O14) (Monomer) 0.2400 6.72 1.22 0.081p⁄(C13@O14) (Dimer) 0.2477 49.30 0.34 0.118

a E(2) means energy of hyperconjugative interactions.b Energy difference between donor and acceptor i and j NBO orbitals.c F(i,j) is the Fock matrix element between i and j NBO orbitals.

Table 3Composition of H-bonded NBOs (in terms of natural atomic hybrids).

Bond (AAB) Monomer Dimer

EDA % EDB % spn A B EDA % EDB % spn A B

s% p% s% p% s% p% s% p%

C7@O8 65.4 34.5 sp1.95 33 65 41 – 65.5 34.4 sp1.90 34 41 41 58O9AH10 25.4 74.5 sp3.63 74 78 100 – 25.4 74.5 sp3.96 21 78.3 100 –C13@O14 65.0 34.9 sp1.95 33 66 40 59 65.0 34.9 sp1.94 33 65 40 59O15AH16 25.5 74.4 sp3.65 21 78 100 – 25.8 74.2 sp3.97 20 79 100 –

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vibration [38–40]. In the present work, the CAH stretching vibra-tions are observed at 3098 and 3072 cm�1 in Raman and at 3097and 3080 cm�1 in IR with the scaled values at 3096, 3092, 3077and 3072 cm�1. As indicated by PED, these modes involve 99% con-tribution suggesting that they are pure stretching modes.

CAH in-plane and out of-plane bending vibrations normallytake place as a number of strong to weak intensity sharp bandsin the region of 1300–1000 and 1000–750 cm�1 [41–43] respec-tively. CAH in-plane bending vibrations scaled at 1459, 1464,1241 and 1242 cm�1 show excellent agreement with the strongIR bands observed at 1445 and 1226 cm�1 and the medium Ramanbands observed at 1463 cm�1 as such these bending vibrations arecoupled with ring CAC and CAN stretching modes. CAH out ofplane bending vibrations are observed as strong bands at 936and 835 cm�1 in IR and as a weak band in Raman at 835 cm�1

showing good agreement with the theoretically computed valueswith the 51% contribution of PED. The other vibrations such aswagging, twisting and torsions of CAH are also identified and pre-sented in Table 4. The CAC stretching modes of the aromatic groupare expected to occur in the range 1650–1200 cm�1 and thesebands are observed in IR at 1619 and 1445 cm�1 and in Ramanat 1463 cm�1 with the corresponding scaled values at 1623,1621, 1438 and 1437 cm�1.

The C@N stretching skeletal bands [44–46] which are expectedto occur in the range 1650–1550 cm�1 are observed at 1543 cm�1

in IR and at 1541 cm�1 in the Raman. Scaled values of these modes

at 1544 and 1541 cm�1 reveals that these modes are not puremodes but contain significant contribution from other modes. Nor-mally CAN stretching vibrations of aromatic rings are observed inthe region 1382–1266 cm�1 [45]. The bands observed at 1264,1183 and 1161 in IR and at 1261 and 1182 cm�1 in Raman areassigned to CAN stretching vibrations with the calculated valuesat 1242, 1241, 1173, 1160 and 1158 cm�1. PED of these modes alsosuggest that they are not pure modes and are evident from last col-umn of Table 4.

Carboxylic acid vibrationsVibrational analysis of carboxylic acid was carried out giving

preferences to carbonyl and hydroxyl groups which are significantfor the antimicrobial activity of PDCA. The most important spectralfeature of COOH group is a broad AOH hydroxyl group that con-nects the molecules, with the observed IR frequency around3500 cm�1 [47]. OH group vibrations are likely to be more sensitiveto the environment which causes pronounced shifts in the hydro-gen-bonded species. Weak and very strong bands observed at 3306and 3265 cm�1 in IR are assigned to the OAH stretching vibration.Broadening and red shifting by �194, �235 cm�1 depicts the levelof intermolecular OAH� � �O hydrogen bonding. Theoretically pre-dicted wavenumbers (3306, 3305 and 3292 cm�1) coincide exactlywith the experimental wavenumbers and these modes are purestretching modes as evident from the PED value (97–99%). OAHin plane bending mode is expected to appear as a strong band in

Table 4Vibrational assignment of PDCA dimer by normal coordinate analysis based on SQM force field calculations.

Observed fundamentals/cm�1 Selective scaled B3PW91 with 6-311G (d) force field

mIR mRaman mcal cm�1 Assignment with PED (P10%)

3306w – 3306 tOH II (98)3306w – 3305 tOH I (99)3265vs – 3292 tOH II (97)3265vs – 3292 tOH I (97)3097w 3098s 3096 tCH I (99)3097w 3098s 3092 tCH II (99)3080w 3072w 3077 tCH I (99)3080w 3072w 3074 tCH II (99)1753vs – 1759 tas C@O II (38), tas C@O II (19), tss C@O II (15)1753vs – 1753 tasC@O I (38), tas C@O II (19), tss C@O I (15)1716vs 1724vs 1725 tasC@O II (44), tss C@O I (16), tas C@O II (10)1716vs 1724vs 1722 tas C@O I (45), tss C@O I (17), tas C@O II (11)1619vw – 1623 mCC I (47), mCN I (18), bCH I (17), RASYD I (11)1619vw – 1621 mCC II (47), mCN II (18), bCH II (17)1543m 1541vs 1544 mCN I (65), RASYDO I (17), bCH I (13)1543m 1541vs 1541 mCN II (67), bCH II (15), RASYD II (11)1445vs 1463m 1464 bCH I (50), mCN I (28)1445vs 1463m 1459 bCH II (52), mCN II (31)

– 1438 mCC I (43), bCH I (24), mCC I (11)– 1437 mCC II (42), bCH II (23), mCC II (11)

1394s – 1384 bOH II (58), tas C@O II (13)1394s – 1372 bOH I (48), tas C@O I (17)1357vs 1356vw 1358 bOH II (56), tas C@O II (14)1357vs 1356vw 1357 bOH I (55), tas C@O I (14)1264vs 1261m 1290 RTRID I (35), b OH I (3), mCN I (12), bCH I (11), mCC I (11)1264vs 1261m 1284 RTRID II(34), bOH II (22), mCC II(13), mCN II (13), bCH II (11)1226vs – 1242 mCN II (20), bCH II (19), bOH II (14), mCC II (13), mCC II (12)1226vs – 1241 mCN I (22), bCH I (19), m CC I (15), mCC I (12), OH I (12)1183s 1182s 1173 mCN I (23), mC@O I (12)1183s 1182s 1173 mCN II (17), mC@O II (17)1161m – 1160 mCN I (66), mCC I (19)1161m – 1158 mCN II (63), mCC II (23)1128w – 1125 tss C@O II (26), mCN II (16), tasC@O II (16)1128w – 1120 tssC@O I (26), tasC@O I (16), mCN I (11)1097vs 1096w 1101 tssC@O II (31), RTRID II (19), tasC@O II (16),1097vs 1096w 1100 tssC@O I (33), mC@OASY I (18), RTRID I (16)1066vw 1066vs 1074 mCN I (19), mCN II (16), mCC I (12), mC@O I (11)1066vw 1066vs 1071 mCN II(20), m CN I(13), m CC II (11), mC@O II (10), mC@O I (10)936s – 939 gCH II (83)936s – 937 gCH I (84)881s – 883 RASYD II (33), mCC II (21), RTRID II (12), RASYDO II (11), mC@O II (10)881s – 878 RAYDO I (38), mCC I (21), RTRID I (16), mC@O I (11)869vs – 861 RPUCK II (38), gCC II (32), gCH II (16)869vs – 860 RPUCK I (37), gCC I (32), gCH I (19)835vs 835w 843 gCH II (51), RPUCK II (25), gCC II (20)835vs 835w 839 gCH I (47), RPUCK I (26), gCC I (22)768m 759m 761 xOC I (55), sCO I (13), bCC I (13)768m 759m 759 xOC II (59), bCC II (14), sCO II (14)738m – 742 xOC I (34), RPUCK I (31), sCO I (14),738m – 737 xOC II (30), RPUCK II (20), sCO II (14), s H� � �O II (13)724w – 714 RASYD I (31), mCC I (12), mCC I (10)724w – 714 RASYDO II (24), mCC II (12), mCC II (10), RASYD II (10)643m 642w 642 RASYD I (24), COSCI I (24),643m 642w 641 dCO II (26), RASYDO II (18),– – 629 sCOI (54),– – 623 dCO I (39), RAYDO I (10)– – 621 dCO II (45)– – 599 sCO I (37), sCO II (20), b H� � �O (15)583m – 597 sCO II (39), b H� � �O (31)583m – 593 sCO II (67)545s – 574 gCC II (29), RASYT II (25), RPUCK II (16)545s – 563 gCC I (30), RASYTO I (15), RPUCK I (14), COROCK I(13), gCH I (10)– – 472 bCC I (27), COROCKI (26), dC@O II (11– – 468 bCC II (30), COROCK II (30), dC@O II (11), mCC II (10)– 412m 429 RASYT I (43), gCC I (24), RAYTO I (10)– 412m 425 RASYTO II (42), gCC II (19), b H� � �O (11)– – 378 dC@O I (24), mCC I (20)– – 374 dC@O II (27), mCC II (22)– – 325 mCC I (20), s H� � �O (19), s H� � �O (15), dCO I (12), RASYDO I (10)– – 321 mCC II (20), RASYT II (19), C@OROCK II (14), dCO II (11)– – 311 RASYT II (33), gCC II (14), C@OROCK (14)– – 307 COROCK (25), RAYTO I (22), gCC I (17), RASYT I (15)– 212m 215 bCC I (43), COROCK (29), xOC I (13)

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Table 4 (continued)

Observed fundamentals/cm�1 Selective scaled B3PW91 with 6-311G (d) force field

mIR mRaman mcal cm�1 Assignment with PED (P10%)

– 212m 211 bCC II (42), COROCK II (32), xOC II (13)– – 174 s H� � �O (21), s H� � �O (21), gCC I (15), COROCK I (11)– – 167 gCC II (22), s H� � �O (13), gCC I (11), b H� � �O (10)– 155vs 146 bCC I (57)– 155vs 144 bCC II (62), C@OROCK II (13)– 120vs 120 s H� � �O (42), s H� � �O (34)– 120vs 114 s H� � �O (34), sCC II (25), s H� � �O (10), s H� � �O (10)– 82s 95 sCC I (30), RAYTO I (25), s H� � �O (11), sCC II (10)– 82s 89 sCC II (40), RASYT II (19), s H� � �O (15)– 66s 56 s H� � �O (59), sCC II (26)– – 51 s H� � �O (42), s H� � �O (33)– – 45 sCC II (38), m H� � �O (17), s H� � �O (14), s H� � �O (12)– – 29 sCC I (43), m H� � �O (24)– – 26 sCC I (39), sCC II (24), b H� � �O (19)– – 18 sCC II (25), s H� � �O (24), m H� � �O (13), sCC I (13)– – 16 m H� � �O (43), b H� � �O (41)– – 10 bHO (37), sCC II (30), sCC I (18)

vw – very weak; w – weak; m – medium; vs – very strong; s – strong; sh – shoulder; t – stretching; tss – symmetric stretching; tas – asymmetric stretching; b – in-planebending; c – out-of plane bending; x – wagging; t – twisting; d – scissoring; s – torsion; sASYTO – out-of plane asymmetric torsion; sASYT – asymmetric torsion; g – gauche;ROCK – rocking; PUCK – puckering; TRID – trigonal deformation; ASYD – asymmetric deformation; ASYDO – out of plane asymmetric deformation; R – ring.

Fig. 2. (A) Experimental, and (B) simulated FT-IR spectra of PDCA.Fig. 3. (A) Experimental, and (B) simulated FT-Raman spectra of PDCA.

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the region 1440–1260 cm�1 [44,48] and is observed as very strongbands at 1357 and 1394 cm�1 in IR with its Raman counterpart at1356 cm�1 which are coupled with C@O stretching modes. ScaledOAH vibrations show good agreement with the experimentalresults.

C@O stretching band appears as strong bands in the region1870–1540 cm�1 in which the position of C@O stretching banddepends on the physical state, electronic and mass effects of theneighboring substituents, conjugations, intramolecular and inter-molecular hydrogen bonding [44,49–51]. The dimeric form of car-boxylic acids has two vibrations, symmetric and asymmetricwhere the asymmetric stretch is usually observed at a higherwavenumber than the symmetric stretch [52]. On this basis, thevery strong band observed at 1724 in Raman with the very strongbands observed at 1753 and 1716 cm�1 in IR have been assigned toC@O asymmetric stretching vibration. The computed wavenum-bers (1759, 1753, 1725 and 1722 cm�1) agree well with theobserved wavenumbers. The band appearing at 1097 cm�1 as avery very strong band along with a weak band at 1128 cm�1 inIR whose counterpart appears at 1096 cm�1 in Raman as weakband are assigned to the C@O symmetric stretching vibrations.Due to the hydrogen bonding effect through the carboxyl groups,

scaled C@O stretching mode in dimer conformation are at 1125,1120, 1101 and 1100 cm�1. The wave numbers computed by NCAmethod for COOH wagging, twisting and rocking vibrations are ingood agreement with observed spectral values and are presentedin Table 4.

Low-wavenumber hydrogen-bond vibrationsHydrogen bonding modes [53] occur at low wavenumbers in

the region 50–300 cm�1 and H-bond strains usually appear in therange 100–150 cm�1 as intense bands [54]. The interesting featurein PDCA is the occurrence of intense Raman bands at 66, 82 and120 cm�1 where the calculated frequency correlates well withthe experimental data.

Electronic properties

3D plots of highest occupied molecular orbitals (HOMO) andlowest unoccupied molecular orbitals (LUMO) are shown inFig. S1. HOMO is located on the second molecule where as theLUMO is located over the entire dimer except carboxylic acid indi-cating that the first molecule is highly susceptible to nucleophilic

730 T.J. Beaula et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735

attack. LUMO as an electron acceptor represents the ability toobtain an electron and HOMO represents the ability to donate anelectron. The HOMO ? LUMO transition implies that electron den-sity transfer takes place to the first molecule from the second mol-ecule. Calculated energy values of PDCA in solvent and gas phaseare tabulated in Table 5. Calculated values of HOMO energy, LUMOenergy and HOMO–LUMO energy gap are �0.26121 eV, is�0.08298 eV and �0.17823 eV respectively. Lowering of theHOMO–LUMO energy gap explains eventual charge transfer inter-actions taking place within the molecule and the biological activityof the molecule.

The values of electro negativity, chemical hardness, softnessand electrophilicity index in gas phase are 4.6829, 2.4249, 0.2061and 4.5217 eV respectively. These results reveal that this moleculehas less hardness and high softness due to the low value of HOMO–LUMO gap. The calculated value of electrophilicity index describesthe biological activity of PDCA.

Fig. 4. Experimental UV–vis spectrum of PDCA.

UV absorption spectra

The UV–Vis absorption spectrum of PDCA recorded in water isshown in Fig. 4. Molecules allow strong p ? p⁄ transition in theUV–visible region with high extinction coefficients. NBO analysisindicates that molecular orbitals are mainly composed of p andr atomic orbital. TD-DFT/B3PW91/6-311G (d) calculations havebeen used to determine the low-lying excited states of PDCA. Thecalculated excitation energies, absorbance and oscillator strength(f) for the title molecule were compared with the experimental val-ues and are tabulated in Table 6. The maximum absorption peak(kmax) in the UV–vis spectrum predicts electronic transition at299 nm with an oscillator strength f = 0.0089 showing good agree-ment with the measured experimental data, which is the verystrong band observed at 277 nm, the characteristic peak that arisesin the pyrazine system due to n ? p⁄ transition [55]. The wave-lengths obtained with B3PW91/6-311G (d) computations are327, 321 and 299 nm (in water). Both HOMO and LUMO are themain orbitals that take part in chemical stability. In view of calcu-lated absorption spectra, the wavelength 327, 321 and 299 nm cor-responds to the electronic transition from the HOMO�1 to LUMOwith 93% contribution, from HOMO to LUMO+1 with 92% contribu-tion and HOMO to LUMO with 51% contribution respectively.

NMR analysis

The experimental 13C and 1H NMR spectrum are shown in Fig. 5.The calculated 13C NMR and 1H NMR chemical shifts for the PDCA

Table 5Calculated energy values of PDCA in solvent and gas phase.

DFT/B3PW91/6-311G (p) Solvent (Water) Gas

Etotal (Hartrees) �1282.737 �1282.70EHOMO (eV) �7.4499 �7.1078ELUMO (eV) �2.5445 �2.2580EHOMO–LUMO gap (eV) �4.9054 �4.8498EHOMO�1 (eV) �7.4515 �7.1454ELUMO+1 (eV) �2.5143 �2.2125EHOMO�1–LUMO+1gap (eV) �5.2415 �4.9329EHOMO�2 (eV) �8.0730 �7.7748ELUMO+2 (eV) �1.9072 �1.6683EHOMO�2–LUMO+2gap (eV) �6.1658 �6.1065Electronegativity v (eV) 4.9972 4.6829Chemical hardness g (eV) 2.4527 2.4249Softness 1 (eV)�1 0.2038 0.2061Electrophillicity index w (eV) 5.0907 4.5217Dipole moment (D) 1.0082 0.4977

together with the corresponding experimental values and theassignments are presented in Table 7 as values relative to TMS.

1C atom is mostly localized on the periphery of the moleculesand its chemical shifts would be more susceptible to intermolecu-lar interactions in the aqueous solutions as compared to that forother heavier atoms. Aromatic carbons give signals in overlappedareas of the spectrum with chemical shift values from 100 to150 ppm [56,57]. The two acid carbons appear at d167.7 ppm assinglet and the two methyl carbons of the ring appear at d144.5and 146 ppm. Due to the influence of electronegative nitrogenatom, the chemical shift value of C5 and C6 significantly differ inthe shift positions. The higher chemical shift of C5 and C6 are dueto the attachment of electron withdrawing nitro group and carbox-ylic acid.

1H NMR spectrum exhibits two proton signals indicating thepresence of two different proton environments. The identical acidprotons appear at d8.7 ppm and the two ring CAH methyl protonsappear at d4.7 ppm. Peak at d8.7 ppm is due to the high de shield-ing nature of the OAH proton. The Correlation Spectroscopy (COSY)spectrum of PDCA shown in Fig. 5C also confirms the occurrence of1H NMR spectrum.

Molecular electrostatic potential (MESP)

MESP mapping is very useful in the investigation of the molec-ular structure with its biological property relationships [58]. MESPof PDCA dimer is shown in Fig. 6. The value of point charges con-stituting the electrostatic potential are calculated and tabulatedin Table S6.

It is obviously seen from Fig. 6 that the region around oxygenatoms which are linked with carbon through double bond repre-sents the most negative potential region (red)1 and the regions hav-ing the positive potential are over the hydrogen atoms (blue).Regions of negative V(r) are usually associated with the lone pairof electronegative atoms. Thus it is seen from the MEP map thatregions having negative potential are over the electronegative atoms(oxygen and nitrogen atom) and regions having positive potentialare over hydrogen and carbon atoms. From these results, it can beinferred that H atoms indicate strongest attraction while O and Natom indicates strongest repulsion. The fitting point charge to theelectrostatic potential also indicates that, O15 atom is the most

1 For interpretation of color in Fig. 6, the reader is referred to the web version ofthis article.

Table 6The UV–Vis excitation energy and oscillator strength for PDCA calculated by TDDFT/B3PW91/6-311G (d) method.

No. Exp. wave length (nm) Energy (eV) Cal. wave length (nm) Osc. strength Symmetry Major contributes

1. 3.0563 327 0.0044 Singlet-A H-1–>LUMO (93%)2. 3.1103 321 0.0076 Singlet-A HOMO–>L + 1 (92%)3. 277 3.3404 299 0.0089 Singlet-A HOMO–>LUMO (51%), HOMO–>L + 2 (37%), HOMO–>L + 3 (16%)

Fig. 5. (A) 13C NMR, (B) 1H NMR, and (C) Cosy.

T.J. Beaula et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735 731

electronegative atom. The calculated point charge corresponding toN1 and N22 atom are �0.46888 and �0.39838 a.u respectively andthese regions appear nearly blue in color because of the fact that,

N1 and N22 atoms are surrounded by electropositive atoms. The mostnegative value around �0.6 a.u is associated with O9 and O15 whileH32 value is about �0.03 a.u. Thus, an electrophile can preferentially

Table 7The observed (in DMSO) and predicted 1H and 13C NMR isotropic chemical shifts(with respect to TMS, all values in ppm) for PDCA.

Atom (13C) Exp. Theo. Atom (1H) Exp. Theo.

C2 146.0 160.4 H10 4.7 6.2C3 144.5 157.8 H11 8.7 8.9C5 144.5 154.5 H12 8.7 8.9C6 146.0 164.7 H16 4.7 6.4C7 167.7 178.4C13 167.7 179.2

Absolute shielding value of TMS: 199.9853.Absolute shielding value of TMS: 32.5976.13C NMR chemical Shifts.1H NMR chemical Shifts.

Fig. 6. Electrostatic potential (ESP) surface of PDCA.

Table 8aAntibacterial activity of DMSO extracts 2,3-Pyrazine dicarboxylic acid.

S.No Bacterialpathogen

Zone of inhibition (mm)

125 g 250 g 500 g Streptomycin DMSOsolvent

1 Escherichia coli 10 13 15 20 –2 Staphylococcus

aureus11 12 15 20 –

Table 8bAntifungal activity of DMSO extracts 2,3-Pyrazine dicarboxylic acid.

S.No Fungal pathogen Zone of inhibition (mm)

125 g 250 g 500 g Fluconazole DMSOsolvent

1 Aspergillusochraceus 15 15 15 30 –2 Candida albicans 20 30 35 15 –

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attack PDCA molecule at O9 and O15 position. From MESP studies it isfound that the distribution of charge in the hydrogen atom of OAHbond is more positive than that of hydrogen in the CAH bond(+0.423909 a.u and +0.039456 a.u at H10 and H11 respectively). Thiscontributes to a cooperative effect between the donor and acceptorassociated with an evident contraction in OAH� � �O providing addi-tional stability to the system. This result supports the NBO resultsand the red shift of OAH stretching frequencies in IR, which are veryimportant in the enhancement of the biological activity of the titlecompound.

Antimicrobial activity

The synthesized compound was screened for its in vitro antimi-crobial activity against bacterial and fungal strains by agar welldiffusion method. The activity was determined by measuring theinhibition zone diameter values (mm) of the investigated com-pound. The antimicrobial and solvent sensitivity test for both bac-terial and fungal strains were measured and mentioned in theTables 8a and 8b respectively. It is noted that the solvent itselfhas no activity on the microbes.

PDCA dissolved at different concentrations were screened fortheir activity against Bacterial strains like E. coli and S. aureus. Afterincubation at 37 �C for 24 h, the inhibition zone diameter sur-rounding each disc was measured and is given in Table 8a. Activityof PDCA against bacterial pathogens is photographed and it isshown in Fig. 7(A). It is found that PDCA has high activity on E. coliand S. aureus with increase in concentration.

PDCA was also screened for antifungal activity against Aspergil-lusochraceus and C. albicans (Table 8b) and it was found to possessconsiderable activity against Aspergillusochraceus but it was com-paratively little low when compared to the pathogen C. albicans(Fig. 7(B)).

These results clearly indicate that the compound has antimicro-bial activities against the tested organisms. The inhibitory activityof the synthesized compound on the organism shows high activityin higher concentration which reveals that as the concentration ofthe compound increases, the inhibition of bacterial and fungalstrains also increases. The presence of the electron withdrawinggroup COOH in PDCA is responsible for the antimicrobial action.

On the whole, the compound PDCA shows higher antimicrobialactivity against Gram-positive and Gram-negative bacteria.

Molecular docking studies

Protein–ligand interactions play a critical role in the distribu-tion, metabolism and transport of small molecules in biologicalsystems and processes [59]. Molecular docking technique is anattractive scaffold to understand the ligand–protein interactionswhich can substantiate the experimental results.

Docking is performed for the different receptors (PDB ID-1GSK,3PPS and 4A2E) and the ligand. The best docked conformationswere those which were found to have the lowest binding energyand the greatest number of members in the cluster, indicating con-vergence. Least energy represents the easy binding characters ofligand and receptor.

The binding mode as per amino acid residue is predicted to bethe part of the binding site in Fig. 8. Where LYS-288 and LYS-353can make hydrogen bonds with COOH of PDCA and SER-286 canmake hydrogen bonds with nitrogen.

The presence of carboxylic acid, two hydroxyl groups in orthoand meta positions of the pyrazine ring and also nitrogen atomin para position seems to play an important role in the study ofantimicrobial activity of PDCA [60]. Docking studies also providestrong evidence that the molecular basis for this activity is proba-bly due to 1GSK inhibitor.

Thermal analysis

TG and DTA are important techniques used to investigate thethermal behaviour of the materials. TG–DTA studies were carriedout in the nitrogen atmosphere at a heating rate of 10 �C min�1

from ambient temperature to 300 �C and are exposed in Fig. 9.From the thermogravimetry, it is observed that decompositioncan take place in two stages. The first stage decomposition isbetween 150 �C and 180 �C which is accompanied by 30.88% massfraction loss, due to the removal of CO2. The second stage

Fig. 7. (A) Antibacterial activity of PDCA. (B) Antifungal activity of PDCA (1) 500 lg (2) 250 lg (3) 125 lg (4) DMSO solvent (5) streptomycin.

Fig. 8. Docked conformation of ligand in the binding site of 1GSK along withdistances (in Å) between residues in the 1GSK binding pocket and the ligand.

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decomposition starts at 188 �C and ends at 300 �C where variousgaseous fragments are eliminated. The residue left out at the endis about 4% by weight which is due to the residual carbon massat the end of the decomposition reaction. The absence of weightloss around 100 �C confirms that there is no water of crystallisationin the molecular structure.

DTA reveals exactly the same changes shown by TG and thesharpness of this endothermic peak shows good degree of crystal-linity and purity of the material. From the DTA curve, it is observedthat the material is stable up to 150 �C, and then it undergoes irre-versible endothermic changes at 180 �C and 240 �C. These are dueto stepwise breaking up of the compound.

Conclusion

Optimized geometry reveals excellent agreement with theexperimental results. It clearly manifests the evidence for the devi-ation of distorted bond angles (exo angles) C18AC21AN22 and C18-

AC21AC26 from the expected trigonal angle. Hydrogen bondinginteractions have been thoroughly analyzed using NBO analysis.The transfer of ED from the lone pair oxygen to the anti-bondingorbital of OAH bond provides a strong evidence for hydrogenbonding. It gives the extraordinary basicity of PDCA which bringsabout most interesting biological properties. Vibrational analysisof PDCA has been carried out using the recorded FTIR and FTRaman spectra aided by NCA. The behaviour of carboxyl groupand pyrazine ring with the vibrational frequencies of the title com-pound has been discussed in detail. Less standard deviation

between theoretical and experimental wave numbers is confirmedby the qualitative agreement between the calculated and observedfrequencies. Red shift of OAH stretching frequencies in FT IR is aclear evidence for the occurrence of OAH� � �O interaction.

The relative stabilities, HOMO–LUMO gaps and implications ofthe electronic transitions have been examined and discussed thor-oughly. MESP map shows that the negative potential sites are on

Fig. 9. Thermal analysis of PDCA.

734 T.J. Beaula et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 723–735

oxygen atoms and the positive potential sites are around thehydrogen atoms. Thus the present investigation provides completevibrational assignments, structural informations and electronicproperties of the compounds. The intermolecular hydrogen bondformation in PDCA indicates that antimicrobial activity increaseswith the hydrogen bonding abilities.

The antimicrobial activity results of the viability assay haveproved PDCA to hold excellent antimicrobial nature since bacteriaand fungi cannot grow in the media containing PDCA. Thus, fromthe above investigations, it can be concluded that PDCA is a goodantimicrobial agent to treat diseases and further work can alsobe carried out to isolate the exact active moiety responsible forthe biological activity. Molecular docking study confirms the inter-action between PDCA and 1GSK inhibitor.

Acknowledgement

One of the authors T. Joselin Beaula thanks the IIT Chennai, forthe spectral measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2014.11.034.

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