BIOPOLY MEItS VOL. 10, PP. 2063-2070 (1971)

Investigations on Spectra and Orientation of Dipole Moments of Uracil and Thymine by Electrochromism

I<. SEIBOLD and H. LABHART, Phllsikalisch-chenzisches Institut der I'nii1er*silal Zurich, Zurich, Switxer~laizd

Synopsis ? . 1 he direction of llie grornid sttile dipole nioinelit with respect to h e trslisitioii i i i o -

nient t o the lowesl excited singlet stale as wel1:ts thedipole tnoinent in this excited stale could be determined from the influence of an electric field on the light absorption of uracil :ind thymine in solution. By making use of results on the orientation of the tmnsition moment in the molecular framework, the orientation of the ground- and excited-state di- pole moments can also be fixed and compared with theoretical predictions. The agree- ment is fair. The measurements show that in both compounds a weak band is hiddtm under the longest-wavelength absorption band.

INTRODUCTION

A great deal of work has been done with respect to the elucidation of the spectrum and electronic structure of purine and pyrimidine bases, the impor- tant constituents of DNA and RNA. Experimental investigations by hlason,l Voet et a1.,2 and Clark and T i n ~ c o , ~ together with model calcula- tions, led to a systematic survey of ultraviolet-spectra in solution. The optical extinction of some of these compounds has also been measured in the gas phase by Clark et al.4 Stewart and Davidson5 and Stewart and Jcnsen6 determined the direction of transition nioments of 9-methylade- nine and 1-methylthymine from the dichroism of single crystals. The ground-state dipole moments of adenine and 1,3-dirnethyluracil have been measured by DeVoe and T i n o ~ o . ~ On the other hand, there have been a number of quantumchcmical calculations with the use of different degrees of approxitnation. A most, valuable survey is given by A. Pullman* cover- ing the results from simple Huckel-type calculations up to noneinpirical c ~ o m p u t a t i o n ~ . ~ - ~ ~ There are still considerable discrepancies between the theoretical values of molccular properties, and only part of these could liitlicrto bc cornpared t,o cxpcrimcntal findings because of a lack of c.xpc\ri- mrntal data wnciw~ing hidden spectral transitions, dipole niomcnts, transi- tion momcwts :uid thoir directions with respwt to th r molecular skeleton. It is, however, possible to get such information from the investigation of tho influence of an electric field on t,h(i ultravioh~t spectra. Ifi This technique also allows the determination of dipolv momc.nts in excited states. Thus we considered it \\-orth\\-hile to invostigntc sonic' of thcsc biologicdly impor- tant heterocycles with this method.

2063

@ 1971 by John Wiley & Sons, Inc.

MOLECULAR PROPERTIES FROM THE CHANGE OF THE EXTINCTION UPON APPLICATION OF AN ELECTRIC FIELD

In an electric field, molecules having a permanent dipole moment are par- tially oriented and the absorption bands arc slightly shifted. If the field is perpendicular to the light beam passing the solution and x is the angle bc- t\\-cc.n the direction of its polarization :md the app1ic.d ficld F acting on thc molecule, the rc4ative c*harigc. of light, int.cwsity traiismittcd through tht: s:tIn~'lo

(1)

is proportiold to thtb o p t i d cl(wity 0 of tlio solul,ioii arid thv square oi' tliv field strength.

A / / I = ( I F - I ~ - " ) / I ~ - L I

On($ may thus clcfinc

which is independent of the field strength and optical density. AI/I , which usually is very small, can be measured with the help of an apparatus de- scribed earlier." The field acting on the molecule may in nonpolar sol- vents be approximated by the Lorentz-field F = F,(EDK + 2)/3, where EDK

is the dielectric constant and Fa the applied voltage divided by the distance between the electrodes. For a spectral region whcre there is no overlap between different electronic transitions, Liptay18 established the following connection between Lx and molecular properties:

CX = ~ ( A P ) ~ + (3 cos2x - 1)[3(mAp)2 - ( A V ) ~ ] (6)

E is the decadic extinction coefficient, pfl and pa are the dipole moments in the ground state and in the excited state respectively, A p = pa - pg. m is the unit vector in the direction of the transition moment. h, k, T , and c have their usual meaning. In eqs. (4) and (5 ) small terms tak- ing account of the anisotropy of polarizability, of the change of polarizabil- ity upon excitation, and of the field-induced change of transition moments have been omitted. As the third term in eq. (3) is a small correction in the case of the molecules dcalt with in this papcr, one can get first-order esti- mates for Ax and Bx by plotting Lx against cl I n (e/P)/dP.

* mpr, refers to the scalar product of the two vectors m and pu.

SPECTRA AND DIPOLE MOAIENT ORIENTATION 2063

If Ax and Bx are plotted against (3 coszx - l), first-order estimates for (3(mpg)2 - pO2), (pgAp), and (mp,)(mAp) are obtained.

In a second step, the valucs for Ax and Bx can be improved by introducing first order estimates for (map) and I Apl into the third term of (3) and plot- ting

RESULTS

Uracil

lGgurc 1 slzo\vs thc extinction as well as thc measured va1uc.s of Lx. Thc cvaluatiori described in the prececding paragraph yields

3(mp,)2 - pr2 = 7.6 f 0.7 X esu (7)

p,Ap = 3.8 f 0.4 X esu (8)

(mp,)(mAp) = 7.S f 0.7 X esu (9)

Thcw three equations contain four unlinomis, namely Ips], I Apl, arid thc :~nglcs betwccn m and p, and m and Av. Therefore we havc to substitute onc of them from other mcasuremcnts. The dipole moment of 1,3-di- incthyluracil has bcen measured' to be 3.9 I). Adopting the samc value for

7" AL, 10'0 I 10

' o , - - = v n x - 0 x x - = 5 4 8 0 352" 00 LI u

a x i 90-

- 5 1

Fig. 1. Uracil in dioxiliie: (-) optical deiisity (scale a t right); (0, V, 0, A) Lx at various angles x.

2066 SEIBOLD AND LABHAltT

H I I

( 6 )

Pig. 2. Orientations of (u ) fig compatible with eq. (7) and data of Stewart et ill.5.6 iii uracil: ( b ) fig and fi0 with reference to molecular skeleton of uracil.

SPECTRA AND DIPOLE MOMENT ORIENTATION 2067

uracil, we find from eq. (7) that the angle between the transition moment m and the ground-state dipole moment pLp must be either 45" or 135" or -45" or - 135". (Positive angles are counted counterclockwise if the structure is mitten in the usual manner; Fig. 2. ) The uncertainty of these angles is only f 3" if an uncertainty of * 10% in the absolute value of the ground- state dipole moment is admitted. The transition moment to the lowest excited state of 1-methylthymine (1,9dimethyluracil) has been determined from single-crystal observations to lie a t -19" from the N1-G axk5y6 In uracil the direction of the transition moment cannot differ much from the direction in its dimcthylated derivative. We thus put the direction of the transition moment m at the same angle from the Nl-C, axis. The four dirrctions of vu compatible with eq. (7) can then be fixed with reference to the molecular skeleton as shown in Figure 2a. The main contribution to the molecular dipole moment comes no doubt from the two carbonyl groups. Only solution pgl is therefore physically acceptable and considered in the subsequent evaluation. From eqs. (9) and (7) it follows that the projcc- tion of Ap on m is 2.84 D, whereas from cq. (8) one may conclude that the projcotion of Ap on pp is 0.98 D. Graphical solution yields IApl = 3.3 f 0.4 D at a direction -27.5 + 3.0" from m. Addition to pgleads to pa, the dipole moment in the lowest excited singlet state. The result is shown in Figure 2b. In Table I the available theoretical predictions are comparcd with the present experimental results. No theoretical values for pa have been published so far.

TABLE I Experimental and Theoretical Results for Uracil (Lowest Energy Transition)

Theory

Theor. Experiment PPPs IEHCb CNDOc SCF-CMd

lP.lt D 3.9" 4 .0 7.63 4.61 Angle of m with

Angle of p1 with respect to N1-Ca axis - 19"' -9"

respect to N1-Cd axis 26 zt 3" 43 " 35.7" 36"

IPal, 1) 5 .7 f 0 . 3

a Data of Pullman.* b Data of A. Pullman et al.13 c Data of Giessner-Prettre and A. Pullmaii.*2

c Values from DeVoe and Tinoco7 for 1,3-dimethyluracil. f Values from Stewart et a1.6-6 for l,.i-dimethyluracil.

Data of Berthod et al.l*

As seen from Figure 1 there is a distinct irregularity in the wavelength- dtqxndence of IJx a t wavelengths shorter than 255 nm. This is a strong indication that in this region t.he long-wavelength band of uracil overlaps with a second transition which is hidden in the absorption spectrum. This

8068 SEIBOLL, AND LABHART

is a nice confirmation of a theoretical prediction by Mason' and it agrees with single crystal observations by Stewart and Davidson5 as well as with some more indirect evidence for a hidden transition from the circular dichroism of uridine and thymidine.lS The transition has neit.her been pre- dicted by Bailcy*O in uracil nor by Tanaka arid Nagaliura2' in thymine.

Thymine

for various v;du(hs of x is shown in k igurc 3. 'Thv spwtrum i i i t l w rtgion ol' t,h(h first :kbsorptioii b : d togetlicr with L x

Evaluat,ioii of Lx lcads to

S(mt1,)2 - pLy2 = 5.2 f 0.5 X 10-3'j wu

pgAp = 4.7 f 0.5 X csu

(mp,)(mAp) = 4.3 f 0.4 X csu

M'o agaiii put thr dipole moment at pr = 3.9 f 0.4 D. Thon thc angle bc- twm m and pg is found to be 48 f 2". Ap becomes 1.65 f 0.3 D and is nrarly parallel to the transition moment. p, is then 5.25 f 0.4 D. If we again fix m at the same directionwith respect to the NI-Cd axis as determined for 1-methylthymine, p, is 29 f 2" and pa is 16 f 4". The comparison with theoretical results is madc in Table 11, and the experimental results arc givcn in Figure 4.

L, '1 o x = O' v X = 35.2'

0 x = 54.8'

A X = 90'

Fig. 3. Thyiiiiiie in dioxaiie: (-) optical density (scale at right); (0, 0, 0, A) Lx at various angles x.

SPECTRA AND DIPOLE MOMENT ORIENTATION 2069

I H

I I

Fig. 4. Orientations of pv aiid pa with reference to iiiolecular skelehii of thyniiiic.

Thc increase of thc dipole moment upon excitation of uracil and thymine is in accordance with the observed bathchromic shift of the first absorption peak with increasing dielectric constant of the solvent.

Thc regular shape of the dependence of Lx upon the wavelength is per- t,urbed bclow about 275 nm. This again indicates the existence of a second clcctronic transition which is hidden under the absorption maximum. It is shifted to longer wavelength compared to the corresponding band in uracil.

TABLE I1 Experimental and Theoretical Results for Thymine (Lowest Energy Transition)

Theory

I P u L 1) 3.9" 4.3311 4 . F 4 . 0 1)

Aiigle of m with respect to

Angle of P, with respect to

IPal, D 5.25 f 0 . 4

3.30

N1-Ch axis -1'J"' -24'

Nl-C, axis 29 * 2" 39O 5ODd

* Data of Giessner-Prettre and A. Pullman.22

I' Values from L)eVoe and Tiiioco' for 1,3-dimethyluracil.

c Data of Clementi e t al.24

Data of Denis and A. P~l l rnan.2~

Data of MBly and A. Pullrna1i.1~

Values from Stewart et aL6S6 for l,.~-dii~iethyliiracil.

2070 SEIBOLD AND LABHART

CONCLUSION

The theoretical values for the relative directions of the ground-state di- pole moment and the transition moment to the lowest excited state are thus as a whole in satisfactory agreement with our experimental findings. The best fit is found with the CNDO predictions by

It would be interesting to have the theoretical values for excited-state di- pole moments for comparison with our experimental ones.

We thank the Swiss National Foundation for supportitig this work. i\'otu added aJter submisswn: W. A. Eaton atid T. 1'. Lewis [./. (,'/k(?ti. l'hys., 53, 'Jltifl

( 1!)70)) just determitied the longest-aaveleugth transition inonletit o f l-tne~tiylufiwil froin the absorption of single crystals it1 polarized light to lie approxiniately i t i the N, G dirertioti. In this paper we assumed it to lie it1 the same directioii i n uracil as in I-niel h- ylttymine, i.e., a t -19". On adopting the new value for uracil, the directiotis of P, and c. ought to be changed to 46" f 3" and 13" f 8" respectively. The former is i t i

still better agreement with theoretical calculations. For thymine no inodificiltion seeins to be indicated.

References 1. S. F. Mason, J. Chem. SOC., 1954,20; ibid., 1960,21!3. 2. 1). Voet, W. B. Gratzer, 11. A. Cox, and P. Doty, Biopolytruns, 1, lYd(1!)63), 3. L. B. Clark and I. Tinoco, Jr., J. Amer. Chem. Soc., 8 7 , l l (196.5). 4. L. B. Clark, G. G. Peschel, and I. Tinoco, Jr., J. Phys. Chem., 69,3615 (196.7). 5. R. F. Stewart and N. Davidson, J. Chem. Phys., 39,255 (1963). 6. It. F. Stewart and L. H. Jensen, J. Chem. Phys., 40,2071 (1964). 7. H. DeVoeand I. Tinoco, Jr., J. Mot?. Biol., 4,500 (1962). 8. A. Pullman in The Jerusalem Symposia on Quantum Chemistry and Biochemistry,

Vol. IZ, Quantum Aspects of Heterocyclic Compounds in Chemistry and Biochemistry, E. D. Bergmann and B. Pullman, Eds., Jerusalem, 1970.

9. B. Pullman and A. Pullman, Adv. Quantum Chem., 4,262 (1968). 10. H. Berthod and A. Pullman, J. Chim. Phys., 55,942 (1965). 11. H. Berthod, C. Giessner-Prettre, and A. Pullman, Theor. Chim. Actu, 5 , 33

(1966). 12. 11. Berthod, C. Giessner-Prettre, and A. Pullman, Int. J. Quantum Chcm., 1,183

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( 1968:. 14. A. Pullman, Znt. J. Quantum Chem., 2,187 (1968). 1.5 B. M d y and A. Pullman, Theor. Chim. Acta, 13,278 (1969). 16. H. Labhart, in Advances in Chemical Physics, Vol. 13, I. Prigogine, Ed., Intersci-

17. H. Labhart, Chimia, 15,20 (1961). 18. W. Liptay, Z . Naturforsch., 20a, 272 (1965). 19. W. Voelter, R. Records, E. Bunnenberg, and C. I)jerassi, J. Amw. chcm. SOC., 90,

20. M. L. Bailey, Theor. Chim. Acta, 16,309 (1970). 21. M. Tanaka and S. Nagakura, Theor. Chim. Acta, 6, :?LO (1966). 22. C. Giessner-Prettre and A. Pullman, Theor. Chim. Acta, 9,279 (1968). 23. A. Denis and A. Pullman, Theor. Chim. Actu, 7,110 (1967). 24. E. Clementi, J. M. Andr6, M. C1. Andr6, D. Klint, and D. Hahn, Hungarica Phys-

ence, New York-London, 1967, p. 179.

6163 (1968).

icu Acta, in press.

Received September 1, 1970 Revised November 4,1970