pH-Related Fluorescence Quenching Mechanism of Pterin · to show favorable excited state proton transfer (ESPT) abilities in acidic conditions which induce the experimentally observed

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pH-Related Fluorescence Quenching Mechanism of Pterin

Derivatives and the Effects of 6-Substituents

Journal: Canadian Journal of Chemistry

Manuscript ID cjc-2017-0644.R1

Manuscript Type: Article

Date Submitted by the Author: 30-Nov-2017

Complete List of Authors: Liu, Lei; Anhui Science and Technology University, College of Chemistry and Materials Engineering Sun, Bingqing; Anhui Science and Technology University, College of Resource and Environment

Is the invited manuscript for consideration in a Special

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https://mc06.manuscriptcentral.com/cjc-pubs

Canadian Journal of Chemistry

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pH-Related Fluorescence Quenching Mechanism of

Pterin Derivatives and the Effects of 6-Substituents

Lei Liu*a, Bingqing Sunb

aCollege of Chemical and Materials Engineering, Anhui Science and Technology University,

Fengyang, 233100, China

bCollege of Resource and Environment, Anhui Science and Technology University, Fengyang,

233100, China

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Abstract: 2-amino-4-hydroxypteridine (pterin) and its derivatives serve as photo-oxidants and

exhibit strong fluorescence. When they interact with hydrogen acceptors such as acetate and

phosphate, their fluorescence are significantly quenched in acidic condition (pH 4.9-5.5) whereas

retained in basic condition (pH 10.0-10.5). This pH-related fluorescence quenching mechanism

of pterin and its derivatives are fully investigated by using density functional theory (DFT) and

time-dependent density functional theory (TD-DFT). Pterin and its derivatives are demonstrated

to show favorable excited state proton transfer (ESPT) abilities in acidic conditions which induce

the experimentally observed fluorescence quenching. In contrast, the ESPT processes are found

to be retarded due to the lack of strong hydrogen-bonding interactions in basic environments

which sustain their fluorescence. Interestingly, these ESPT processes are found to show different

site specificities depending on the 6-site substituents. The introduction of electron-donating

substituent activates the N1 site, making it the preferable ESPT site. By contrast, the introduction

of electron-withdrawing substituent activates the N5 site, making it the favorable ESPT site. The

substitutions of different functional groups are found to affect the locations of acidic centers

during the excitation and relaxation processes. This further affects the hydrogen-bonding patterns

and ultimately brings site specificity to the ESPT process.

KEYWORDS: pH-related fluorescence quenching, site-specific excited-state proton-transfer,

time-dependent density functional theory, substituent-effect

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INTRODUCTION

As a group of heterocyclic compounds, pterin and its derivatives (pterins) exist extensively in

living organism and are generally believed to bring photodamages to bio-molecules including

DNAs,1,2 enzymes3,4 and nucleotides.5,6 These are usually triggered by UV-A (320-420nm)

irradiation and many efforts have been made to study their photo-physical and photo-chemical

processes. However, most of these works focus on the electron transfer processes between triplet

pterins and guest molecules.1-6 Theoretical studies into the optic properties of singlet excited

state pterins, especially the origin of fluorescence changes when they interact with guest

molecules are still quite rare.

Scheme 1. Molecule diagrams of pterin and the 6-site substituted derivatives in acidic and basic

aqueous solutions.

As shown in Scheme 1, pterins usually exist with different structures depending on the pH

values. They usually exhibit different fluorescence phenomena when interacting with hydrogen-

accepting agents such as acetate anion and phosphate anion. The fluorescence of pterins in acidic

condition (pH 4.9-5.5) is quenched significantly with the addition of these ions whereas that in

basic condition (pH 10.0-10.5) is not quenched.7-9 In our previous study, by using time-resolved

fluorescence decay spectroscopy and quantum chemical calculations, we have demonstrated that

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the fluorescence quenching of pterin in acidic condition stems from the excited state proton

transfer (ESPT) process whereas the absence of ESPT in basic condition sustains pterin’s

fluorescence.10 Hydrogen-bonding patterns are found to change with pH values which not only

govern the occurrence of ESPT but also control the site specificity of this ESPT process. This

kind of pH-related and site-specific proton-transfer feature may also exist in non-conjugated

pterin derivatives namely 6-formylpterin (FPT), neopterin (NPT) and bio-pterin (BPT).

However, the cases may be quite different when pterin’s 6-site is substituted. As previously

reported, the substitution of hydrogen atom on 6-site to formyl group lowers the quenching

magnitude of the ions considerably.9 The substitution on 6-site seems to coordinate the proton-

transfer ability of pterin derivatives and may also be able to affect the proton-transfer site. This

may originate from the change of the charge distribution and hydrogen-bonding pattern after

being substituted, which still remains unrevealed. As is known, photodamages are often coupled

with ESPT processes.11-14 Pterins-induced photodamages to DNAs and enzymes may also be

coupled with proton-transfer processes as these biomolecules usually contain hydrogen accepting

groups. Thus, having a detailed understanding of the ESPT nature of pterins will be of some

biologic significance.

In this paper, DFT and TD-DFT are applied to investigate the quenching mechanism of pterins

as well as the effects of the 6-site substituents on ESPT site specificity. FPT is studied and

compared with the unsubstituted pterin to give an explanation on the effect of ESPT process

induced by electron-donating substituent. BPT and NPT, which contains electron-withdrawing

substituents, are investigated to illustrate the effect of electron-withdrawing substituent on the

ESPT site specificity.

METHODS

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The ground state (GS) and excited state (ES) geometries are explored by DFT and TD-DFT

calculations at B3LYP/6-31+G(d,p) theory level. All the local minima are confirmed by no

imaginary mode from vibrational analyses. Then, a series of relaxed scans are performed by

scanning the energy profiles along the proton-transfer coordinates to generate potential energy

curves (PEC) for different proton-transfer pathways at both GSs (S0) and first ESs (S1). Bi-

dimensional ESs (S1 state) potential energy surfaces (PES) are generated by performing a series

of constrained optimizations on O-H and H-N distances while other atoms are relaxed. The GS

self-consistent field (SCF) energy of acidic model FPT-acetate complex is chosen as zero point

in all cases in order to give a clear comparison between the energy barriers. All the single-point

energy profiles, namely, energy corrections, natural bond orbital (NBO) analyses, and bond order

analyses (Wiberg bond index15) are calculated at the B3LYP/6-311++G(d,p) theory level. The

individual nitrogen atom’s contributions to frontier molecular orbitals are obtained using Ros-

Schuit (SCPA) partition.16 Solvent effects are considered for all the above calculations (except

for basis set superposition error correction) with the self-consistent reaction field (SCRF) method

in a polarizable continuum model17,18 (PCM) with water as solvent in order to provide a direct

comparison with experimental data.9 Hydrogen bond energy are obtained with equation 1 (EHB

represents the hydrogen bond energy, E1 represents energy of FPT, E2 represents energy of

acetate ion, E3 represents energy of the FPT-acetate complex, EBSSE represents the BSSE energy).

These above theory levels have been successfully applied in studying the ESPT properties of

pterin10 which are used in this contribution and regenerate precise results comparing to the

experimental data9 (see details in Table S1, Supporting Information). All the calculations are

performed using Gaussian 09 program suite.19

EHB = E3-E1-E2+EBSSE (1)

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RESULTS AND DISCUSSION

pH-Related Quenching Mechanism of FPT and the Proton-Transfer Site Specificity

The pH-related quenching mechanism of FPT and the proton-transfer site specificity is

investigated first. As revealed in our previous work,10 different hydrogen-bonding interactions

between unsubstituted pterin and acetate in different pH regions is the major factor that induces

the pH-related fluorescence quenching. Thus, the hydrogen-bonding patterns of FPT in both pH

regions should be fully investigated first.

Table 1. Calculated structure and energy information for the acidic model and the basic model.a

Basic Model Acidic Model

S0 S1 S0 S1

HB1 —— —— 1.653 1.620

HB2 1.709 3.115 1.712 1.732

N1-H1 —— —— 1.062 1.068

(N1-H1)vib —— —— 2716 2606

N5-H2 1.047 1.010 1.045 1.041

(N5-H2)vib 2951 3589 2969 3028

φH1-N1-N5-H2 —— —— 0.43 1.12

HB energy -28.7 -14.7 -53.3 -54.7

aHB1 stands for the H1 atom involved hydrogen bond, HB2 stands for the H2 atom involved hydrogen bond. (N-H)vib stands for the vibrational frequency of the N-H bond (cm-1). All the bond lengths are given in angstrom (Å) and dihedral angles in degree (°). BSSE corrected hydrogen-bonding energies (HB energy) are given in kcal/mol.

The hydrogen-bonding patterns of acidic form FPT (AFP) and basic form FPT (BFP) are

investigated by adding one acetate anion to the corresponding FPT molecule at different sites.

Six different starting geometries are built (see detailed discussions on Page 3 and Page 4 in

Supporting Information). Then, geometry optimizations are performed to get the most

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thermodynamically stable structures. Only two binding patterns are stable, one for the acidic

environment FPT-acetate complex (AFP-Ac), the other for the basic environment FPT-acetate

complex (BFP-Ac). Based on the GS (S0) structures, first ES (S1) structures are then fully

optimized to investigate the change of hydrogen bonds in S1 state. Important structural

information data for these structures are listed in Table 1. Also, basis set superposition error

(BSSE) corrected total hydrogen-bonding energies are reported.

Figure 1. Hydrogen-bonding patterns in S0 (a) and S1 (b) states for the basic model (BFP-Ac).

PECs for (c) GSPT (d) and ESPT processes (energy differentials are given beside).

Herein, we first analyze the basic model to look into the mechanism for the absence of

fluorescence quenching. As plotted in Figure 1a, one moderate hydrogen bond forms in the basic

model with the bond length of 1.709 Å and the bonding energy of -28.7 kcal/mol (Table 1).

Energy profile for the proton-transfer from N5 site in S0 state is then obtained by scanning along

the O-H distances to check the occurrence of proton-transfer. As given in Figure 1c, the energy

keeps rising as H2 approaches O2 which indicates the absence of GSPT from this site. As

hydrogen bonds affect not only the GSPT processes but also the ESPT processes,20-26,36-40 the

hydrogen-bonding pattern in the S1 state is investigated. As shown in Figure 1b, the S1 state basic

model has a very weak hydrogen bond with the bond length of 3.115 Å and the bonding energy

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of -14.7 kcal/mol. Changes of the stretching vibrational frequencies of the bonds involved in the

hydrogen-bonding interactions can be used as measurements to probe the change of hydrogen-

bonding strengths.27 Herein, the stretching vibrational frequency for bond N5-H2 in S1 state is

obtained which is significantly blue-shifted by about 638 cm-1 compare to that of the S0 state

(Table 1). This further demonstrates the weakening of hydrogen bond in the S1 state. The

significantly weakening of the hydrogen-bonding interaction in S1 state will strongly retard the

proton-transfer process. This is directly confirmed by the ESPT potential energy curve (PEC)

which possesses an even larger energy barrier (19.07 kcal/mol) compared to that in S0 state (6.69

kcal/mol).

As is revealed, hydrogen-bonding interaction for the basic model is quite weak in S0 state

which is further weakened in the S1 state, which prohibits the ESPT process. Thus, the absence

of fluorescence quenching in the basic environment should stem from the absence of ESPT.

Figure 2. Hydrogen-bonding patterns in S0 state (a) and S1 state (d) for the acidic model (AFP-

Ac). PECs for the proton-transfer processes of the acidic model: (b) GSPT from N1 site; (c)

GSPT from N5 site; (e) ESPT from N1 site; (f) ESPT from N5 site. Energy barriers for proton-

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transfer processes are given in orange fonts, reverse transfer energy barriers are given in magenta

fonts.

Cases for the acidic model are then studied using the same methodology to investigate the

origination of fluorescence quenching in acidic environment. As shown in Figure 2, hydrogen-

bonding pattern in the acidic environment is very different from that in the basic environment. In

both S0 state and S1 state, two hydrogen bonds form between acetate and AFP with quite short

bond lengths and total hydrogen-bonding energies as large as -53.3 kcal/mol and -54.7 kcal/mol,

respectively (Table 1). These bonding energies are much larger than those in basic environment,

which indicates the acidic model possesses stronger hydrogen-bonding interactions in both S0

and S1 states. Thus, we can expect that proton-transfer processes in acidic environment will be

facilitated by these hydrogen-bonding patterns and may occur much more smoothly. Moreover,

unlike the case in basic environment, there are two potential proton-transfer sites which may be

all or partially involved in proton-transfer processes. HB1 is enhanced whereas HB2 is weakened

in S1 state. These are confirmed by both the changes of hydrogen bond lengths and the changes

of N-H bond vibrational frequencies (Table 1). As is shown in Table 1, after excitation and

relaxation the bond length of N1-H1 changes from 1.653 Å to 1.620 Å and stretching vibrational

frequency of this bond shifts from 2716 cm-1 to 2606 cm-1, which means HB1 is enhanced in S1

state. As for HB2, the changes of hydrogen bond lengths and the changes of N-H bond

vibrational frequencies suggest the weakening of HB2 in S1 state. Thus, proton-transfer may

show some site specificity. Herein, PECs are obtained to determine the occurrence of proton-

transfer in both states as well as to check the possibly existing proton-transfer site specificity. As

plotted in Figure 2b and Figure 2c, energy profiles from both sites in the GS rise in the processes

of proton-transfer. Also, the backward proton-transfer processes are much easier as both

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backward energy profiles feature almost spontaneous processes (structures of the product and

maximum point for H1 transfer in the ground state are given in the Supporting Information as

Figure ). All these mean that proton-transfer cannot take place from S0 state. As for S1 state,

ESPT from N1 site has a quite small energy barrier of 2.31 kcal/mol and leads to a quite stable

product (Figure 2e). In contrast, PEC for ESPT from N5 site has a much larger barrier (Figure

2f). Bi-dimensional PESs for ESPT from the two sites are then built to provide further insights

into the proton-transfer site specificity. As shown in Figure 3a, proton-transfer pathways from

N1 site have very large barriers when stepwise changes one of the two distances (see purple and

gray arrows). The only possible pathway of the ESPT process (black arrows), which is almost a

barrier-less process, features simultaneous changes of O-H and H-N distances. Figure 3b shows

the PES for ESPT process from N5 site. As shown in this picture, all the proton-transfer

pathways have quite large barriers compared to that from N1 site and lead to an unstable product.

This further confirms that the N5 site is not the favorable ESPT site.

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Figure 3. Bi-dimensional PESs for the ESPT from N1 (a) and N5 (b) sites.

As confirmed by the above analyses, the fluorescence quenching of FPT in acidic environment

is originated from a site-specific ESPT process. N1 site is proved to be the favorable ESPT site.

It is generally believed that the weakening and enhancing of hydrogen bond in electronic ESs

can affect the ESPT processes and tune the photophysical and photochemical processes of small

organic dyes as well as complicated metal-organic clusters.28-32 Therefore, the site-specific ESPT

from N1 site in case of APT-Ac probably stems from the enhancement of HB1 in the ES.

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Hydrogen-bonding pattern not only controls the fluorescent properties of FPT but also governs

the priority of the proton-transfer site.

Figure 4. Energy diagrams for ESPT from N1 and N5 sites of (a) acidic form pterin and (b)

acidic form FPT. Green arrows and red arrows represent favorable and unfavorable ESPT

pathways, respectively.

Substitution Effects on the Proton-Transfer Processes

As revealed in our previous work for the unsubstituted pterin,10 N5 site is the preferable ESPT

site over N1 site. However, in the case of FPT, the favorable ESPT site shifts from N5 to N1.

This is probably due to the introduction of the electron-withdrawing formyl group on the 6-site.

Figure 4 gives a detailed description of the substituent-effect on the proton-transfer priority. For

the case of unsubstituted pterin (Figure 4a), N5 is the favorable ESPT site with an energy barrier

of 0.9 kcal/mol. For FPT (Figure 4b), N1 serves as the favorable ESPT site with an energy

barrier of 2.3 kcal/mol. The ESPT barrier for the case of FPT is larger than that of the

unsubstituted pterin which indicates that ESPT is more difficult to take place in the case of

formyl-substituted pterin. This should be the reason for the experimentally9 observed lower

quenching magnitude of FPT with acetate anions in acidic environment. Herein, we can conclude

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that the introduction of electron-withdrawing group formyl changes both the ESPT ability and

ESPT site-specificity. To give a more intuitive study into the origination of this site specificity,

the excitation and ESs relaxation processes are investigated.

Figure 5. Excitation and relaxation processes of (a) FPT-Ac and (b) pterin-Ac in acidic

environment. Individual nitrogen atom’s contributions to the HOMO and LUMO are reported in

the form of percentage. NBO charge values are also given beside (orange fonts).

Origination of the Site Specificity

Photoexcitation can, sometimes, affect the charge distribution all over the molecule and

coordinate its acid/base properties.33-35 This would influence the hydrogen-donating abilities of

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the acidic sites and consequently affect the hydrogen-bonding patterns. Thus, the excitation

process of AFP-Ac is analyzed and compared to that of AP-Ac (pterin-Ac complex in acidic

environment). As reported in Table 2, excitation at the first main absorption peak populates AFP-

Ac to the S2 state which corresponds to the HOMO→LUMO transition. Figure 5a plots the

frontier molecular orbitals which indicate an intramolecular charge transfer (ICT) process occurs

from N5 site to N1 site. This excitation from the GS to the S2 Franck-Condon state (FC) may

primarily change the molecule’s acidic center. N1 and N5 atoms’ contributions for the molecular

orbitals are obtained to give a quantitative description of the ICT process. As is shown, the

contribution of N5 to the corresponding frontier molecular orbital drops sharply (from 18.9% to

3.3%) soon after the photoexcitation while that of N1 increases to some extent (from 0.3% to

0.5%). This indicates N5 site becomes electron-deficient after photoexcitation. To give further

quantitative information, NBO charges on the two ESPT sites are investigated. It is clearly

shown that, upon photoexcitation, the NBO charge on N5 become much less negative (from -

0.757 to -0.667) whereas that on N1 site does not change much. All these mean that N5 site

becomes more acidic immediately after excitation. This kind of acidic center shifting upon photo

excitation is very similar to the case of AP-Ac (Figure 5b). However, this seems to conflict with

the above PEC results as N1 site is the favorable proton-transfer site and should be the acidic

center. This may due to the relaxation of the molecule structure which further influences the

charge distribution all over the molecule. The relaxation process, during which the hydrogen-

bonding patterns adjust to the new electronic state, is thus investigated. After photoexcitation, the

molecule primarily locates at the S2 FC state which has the largest oscillator strength. As shown

in Table 2, energy of the S2 FC state is very close to that of S1 FC state which means that internal

conversion (IC) between the two states should be ultrafast. Proton-transfer cannot happen in such

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short time scales which will lead to the S1 state minimum (minS1) before ESPT takes place. As

shown in Figure 5a, NBO charges on N1 site do not change much during the relaxation process

whereas that on N5 site drops back from -0.667 to -0.774. This means that the acidity of N5 site

weakens during the relaxation of the molecular bone and N1 site resume its acidity and leads to

the enhancement of HB1 in S1 state. Also, Wiberg bond order analyses are performed to provide

further evidences. As given in Table S3 (Supporting Information), bond order of N5-H2

increases to 0.7043 after relaxation. This is larger than the bond order of N1-H1 in S1 state

(0.6270), indicating N1-H1 possesses smaller bond strength. Herein, the enhanced hydrogen

bond (HB1) as well as the weakened covalent bond (N1-H1) further triggers the site-specific

proton-transfer, which ultimately quenches FPT’s fluorescence. For the case of AP-Ac, as

discussed comprehensively in the previous work,10 the acidic center remains at the N5 site which

does not change much during the relaxation of the molecular bone. This makes N5 the preferable

ESPT site.

Table 2. Calculated low-lying ESs excitation energies of AFP-Ac and AP-Ac.a

Transition abs. (nm) f contrib. AFP-Ac S0→S1 387 0.0012 H-3→L

S0→S2 368 0.3594 H→L S0→S3 354 0.0004 H-5→L

AP-Ac S0→S1 348 0.1304 H→L S0→S2 347 0.0027 H-2→L S0→S3 302 0.0002 H-1→L

aAbsorption wavelengths (abs.), corresponding oscillator strengths (f), and orbital contributions (contrib.). Only the first three low-lying state properties are listed for clarity, detailed information are given in Table S2 (Supporting Information).

Thus, the different ESPT site specificities for the above mentioned two cases stem from the

shift of acidic center after photoexcitation and relaxation. Substitution of the 6-site with the

electron-withdrawing formyl group inactivates the N5 site, making N1 site the favorable ESPT

site. To check the substitution effects of electron-donating groups, BPT and NPT are investigated.

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Figure 6. Structural information of BPT in (a) GS and (b) the first ES. Structural information of

NPT in (c) GS and (d) the first ES. Hydrogen bond lengths are given in orange fonts (Å). N-H

bond stretching vibrational frequencies are given in magenta fonts (cm-1).

Effects of Electron-Donating Groups on the ESPT Site Specificity.

The molecules’ structural properties for BPT and NPT in both GSs and first ESs are obtained

to give a quick view of the changes of hydrogen-bonding strengths after excitation and relaxation

processes. The case of BPT is investigated first. As is shown in Figure 6a and Figure 6b, HB1

length is slightly lengthened from 1.674 Å to 1.683 Å and stretching vibrational frequency of

N1-H1 is blue-shifted from 2785 cm-1 to 2843 cm-1 in S1 state. This demonstrates that HB1 is

weakened in S1 state. In contrast, HB2 length is significantly shortened from 1.742 Å to 1.584 Å

with N5-H2 stretching vibrational frequency red-shifts considerably from 3050 cm-1 to 2524 cm-1.

This indicates that HB2 is strengthened greatly in S1 state. This hydrogen bond strengthening of

HB2 in S1 state will make N5 the favorable ESPT site. The case of NPT is much like that of BPT

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as revealed in Figure 6c and Figure 6d. Using the same methodology, one can conclude that N5

site is also the preferable ESPT site in the case of NPT.

Figure 7. Excitation and relaxation processes for (a) BPT-Ac and (b) NPT-Ac in acidic



The excitation and relaxation processes of BPT and NPT are then investigated and plotted in

Figure 7. Wiberg bond order analyses are performed to provide further insights into the change

of acidic center during excitation and relaxation processes (Table 3). For the case of BPT, the

absorption peak locates at the S1 state which corresponds to the HOMO→LUMO transition

(Table S2, Supporting Information). As revealed by the HOMO and LUMO diagram and

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nitrogen atoms’ contribution for frontier molecular orbitals, this transition features an ICT

process. NBO charge on N5 increases considerably from -0.779 to -0.660 whereas that on N1

site becomes a little more negative. Bond order of N1-H1 does not change much while that of

H5-H2 drops a little bit from 0.7070 to 0.6949. All these mean that N5 site becomes more acidic

immediately after excitation. After the relaxation process, unlike the case of FPT-Ac, the NBO

charges do not change much both on N1 and N5 sites. In contrast, bond order of N5-H2

significantly decreases from 0.6949 to 0.6513. These demonstrate that the acidity of N5 site

further increases after the relaxation process. Thus, in the case of BPT, N5 is the acidic center

and will be the preferable ESPT site. For the case of NPT-Ac, both the excitation and relaxation

processes are similar to those of BPT-Ac, which indicates the ESPT processes for both BPT-Ac

and NPT-Ac have the similar feature. Therefore, the substitution of 6-site with electron-donating

group will activate the N5 site, making it the favorable ESPT site.

Table 3. Calculated N-H bond orders for BPT-Ac and NPT-Ac in different electronic states.a

GS FC minS1

BPT-Ac (N1-H1)BO 0.6485 0.6494 0.6525

(N5-H2)BO 0.7070 0.6949 0.6513

NPT-Ac (N1-H1)BO 0.6489 0.6496 0.6528

(N5-H2)BO 0.7077 0.6947 0.6502

aGS represents ground state, FC represents the Franck-Condon state, minS1 represents the S1 state minimum. (N-H)BO represents Wiberg bond order of N-H bond. See detailed information in Table S3, Supporting Information.

CONCLUSION

In conclusion, we have investigated the pH-related quenching mechanism of 6-formylpterin with

acetate anion by using DFT and TD-DFT methods. The experimentally observed fluorescence

quenching in acidic environment is originated from the site-specific ESPT from N1 atom of

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acidic form FPT to O1 atom of acetate anion. The strengthening of hydrogen bonds in S1 state

facilitates proton transfer. By contrast, the ESPT process in basic environment is strongly

prohibited by the weakening of the intermolecular hydrogen bond in S1 state, which sustains

FPT’s fluorescence. Hydrogen bonds are found to play dominant roles which not only govern the

occurrence of ESPT but also determine the proton-transfer site specificity. Moreover, this kind of

ESPT site specificity is found to be closely related to the 6-site substituents. The introduction of

electron-withdrawing group into the 6-site inactivates the N5 site, making N1 the preferable

ESPT site. On the contrary, the substitution of 6-site with electron-donating group activates the

N5 site which makes it the favorable ESPT site. As revealed in this contribution, acetate ion

quenches the fluorescence of pterin and its derivatives in acidic environment via ESPT processes.

This indicates that pterin and its derivatives serve as facile proton donors which are highly

possible to transfer their protons to biologic molecules with hydrogen-accepting sites after

photoexcitation. As pterin and its derivatives exist extensively in living organisms and act as

photo-oxidants in many biologic processes, this kind of ESPT may be a possible mechanism for

pterins-induced photodamages which, as far as we know, still remains unrevealed. This

perspective needs to be further investigated and subsequent researches are underway.

ACKNOWLEDGMENT

This work is supported by the Materials Science and Engineering Key Discipline Foundation

(Grant No. AKZDXK2015A01).

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Figure Captions

Figure 1. Hydrogen-bonding patterns in S0 (a) and S1 (b) states for the basic model (BFP-Ac).

PECs for (c) GSPT (d) and ESPT processes (energy differentials are given beside).

Figure 2. Hydrogen-bonding patterns in S0 state (a) and S1 state (d) for the acidic model (AFP-

Ac). PECs for the proton-transfer processes of the acidic model: (b) GSPT from N1 site; (c)

GSPT from N5 site; (e) ESPT from N1 site; (f) ESPT from N5 site. Energy barriers for proton-

transfer processes are given in orange fonts, reverse transfer energy barriers are given in magenta

fonts.

Figure 3. Bi-dimensional PESs for the ESPT from N1 (a) and N5 (b) sites.

Figure 4. Energy diagrams for ESPT from N1 and N5 sites of (a) acidic form pterin and (b)

acidic form FPT. Green arrows and red arrows represent favorable and unfavorable ESPT

pathways, respectively.

Figure 5. Excitation and relaxation processes of (a) FPT-Ac and (b) pterin-Ac in acidic



Figure 6. Structural information of BPT in (a) GS and (b) the first ES. Structural information of

NPT in (c) GS and (d) the first ES. Hydrogen bond lengths are given in orange fonts (Å). N-H

bond stretching vibrational frequencies are given in magenta fonts (cm-1).

Figure 7. Excitation and relaxation processes for (a) BPT-Ac and (b) NPT-Ac in acidic



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Graphical Abstracts

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Documents

pH-Related Fluorescence Quenching Mechanism of Pterin · to show favorable excited state proton transfer (ESPT) abilities in acidic conditions which induce the experimentally observed