J. Phys. Chem. 1981, 85,3851-3855
involves yet a higher electronic excitation in the aryl ring. The very low I* yield suggests an internal conversion mechanism and dissociation predominantly from the ground state or a triplet state. However, the -10% I* yields at 193 nm in both benzyl iodide and phenyl iodide indicate that the dissociation mechanism is not just a simple decomposition of the lowest electronic state but that a substantial fraction of the dissociation occurs through higher electronic states as well. Additional spectroscopic
385 1
and photofragmentation work will be necessary to determine the nature of the electronic states and the dissociation processes in these complex molecules.
Acknowledgment. We gratefully acknowledge the National Science Foundation for the support of this work and the National Aeronautics and Space Administration for a recent grant (NAG-1-170) for future support of this research.
Ultraviolet Photoelectron Studies of Volatile Nucleoside Models. Vertical Ionization Potential Measurements of Methylated Uridine, Thymidine, Cytidine, and Adenosine C. Yu, T. J. ODonnell, and P. R. LeBteton” Department of Chemistty, University of Illinois at Chicago Circle, Chicago, Illinois 60680 (Received: August 4, 1981)
Ultraviolet photoelectron spectroscopy has been employed to obtain vertical ionization potentials for 0-methyl derivatives of uridine, 3-methylthymidine, cytidine, and adenosine. The interpretation of the spectra has been aided by results from CNDO/S molecular orbital calculations. For each of the molecules studied, the band arising from the highest occupied molecular orbital is resolved and is associated with an orbital which is similar in electron distribution to that of the highest occupied ?r orbital of the free nucleotide base. In all of the nucleoside models studied, it is found that glycosidic bond formation causes a reduction of the ionization potential of the free nucleotide base. In some cases this perturbation exceeds 0.6 eV. The photoelectron results reported here for the nucleoside models and earlier results for free nucleotide bases have been used to obtain ionization potential values of 9.0,8.7,8.6,8.4, and 8.0 eV for the underivatized ribonucleosides, uridine, thymidine, cytidine, adenosine, and guanosine, respectively. These ribonucleoside ionization potentials have been compared with previously reported association constants for the formation of stacked complexes which occur during the self-association of nucleosides and during the association of the nucleosides with riboflavin. In both types of complexes, it is found that association constants increase as the ionization potentials of the nucleosides decrease.
Introduction Recently UV photoelectron spectroscopy has been employed by this group and others to provide gas-phase ionization potentials of those nucleotide bases which occur most frequently in DNA and RNA.1-9 In the present study this technique has been used to obtain ionization potentials of the corresponding ribonucleosides. After nucleotide bases, nucleosides, which consist of bases linked to sugars, are the most simple molecular units which make up polynucleotides. The ionization potential data reported here provide a quantitative measurement of the electrondonating properties of these important biological molecules. In any attempt to measure the physical properties of nucleosides in the gas phase, problems associated with the (1) A. Padva, P. R. LeBreton, R. J. Dinerstein, and J. N. A. Ridyard, Biochem. Biophys. Res. Commun., 60, 1262 (1974). (2) A. Padva, T.J. O’Donnell,and P. R. LeBreton, Chem. Phys. Lett., 41, 278 (1976). (3) S.Peng, A. Padva, and P. R. LeBreton, Proc. Natl. Acad. Sci. U.S.A., 73, 2966 (1976). (4) C. Yu, S.Peng, I. Akiyama, J. Lin, and P. R. LeBreton, J. Am. Chem. Soc., 100, 2303 (1978). (5) J. Lin, C. Yu, S.Peng, I. Akiyama, K. C. Li, Li Kao Lee, and P. R. LeBreton, J . Am. Chem. Soc., 102, 4627 (1980). (6) J. Lin, C. Yu, S.Peng, I. Akiyama, K. C. Li, Li Kao Lee, and P. R. LeBreton, J. Phys. Chem., 84, 1006 (1980). (7) G. Lauer, W. Schafer, and A. Schweig, Tetrahedron Lett., 3939 (1975). (8) D. Dougherty, K. Wittel, J. Meeks, and S. P. McGlynn, J. Am. Chem. SOC.,98, 3815 (1976). (9) D. Dougherty and S.P. McGlynn, J. Chem. Phys., 67,1289 (1977). 0022-3654/81/2085-3851$01.25/0
low volatility of these molecules arise. When encountered in mass spectrometry, these problems are often overcome by preparing derivatives which are stable at the temperatures required to produce significant vapor pressures.1° Such derivatives have been employed here. Molecules for which photoelectron spectra are reported include uracil (I), tris(trimethylsily1)-D-ribose (11), 2’,3’,5’-tri-O-methyluridine(111), 3-methyl-3’,5’-di-0methylthymidine (IV), 2’,3’,5’-tri-O-methylcytidine (V), N,N-dimethyl-2’,3’,5’-tri-O-methylcytidine (VI), and 2’,3’,5’-tri-O-methyladenosine (VII). The structures of these molecules are shown in Figures 1 and 2. Considering the large number of valence orbitals which occur in nucleosides, an unambiguous assignment of the lowest-energy photoelectron band to a specific molecular orbital seems unlikely. However, earlier studies of the free nucleotide bases indicate that in all cases the first photoelectron band arises from a a orbital (al)which has a vertical ionization potential that is at least 0.5 eV less than that of the second highest occupied orbital.l+ This observation suggests that the highest occupied orbital in each of the nucleosides also gives rise to a resolved band. Similar studies of simple nucleoside models also indicate that glycosidic bond formation may sometimes cause a preferential destabilization of the alorbital of the bases. For example the alionization potential of the nucleoside model, 1-methyluracil, is 0.4 eV lower than that of the ?rl (10) D. L. von Minden and J. A. McCloskey, J. Am. Chem. SOC.,95, 7480 (1973).
0 1981 American Chemical Society
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1
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I
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Yu et al.
I I I I I L I 14.0 16.0 18.0 20.C
IONIZATION POTENTIAL (eV)
Figure 1. He(1) photoelectron spectra of uracil, tris(trimethylsilyl)-oribose, and 2’,3’,5‘-tri-O-methyluridine. For uracil, vertical ionization potentials and assignments are given for each of the five highest occupied orbitals. For 2’,3‘,5’-tri-O-methyIuridine (111) similar information is given for the highest occupied orbbl. Vertlcal bars appearing below the spectrum of I11 represent the pattern of energy levels associated with the ten highest occupied orbitals as predicted by CNDO/S. The theoretical results have been scaled so that the energy of the highest occupied orbital predicted by CNDOlS coincides with the first ionization potential obtained experimentally.
orbital in uracil.2 On the other hand, the second highest occupied orbital in 1-methyluracil is destabilized only 0.15 eV compared to that in uracil. Such observations suggest that the photoelectron spectra of nucleosides contain resolved bands associated with the highest occupied T orbitals of the nucleotide bases. The ionization potentials of nucleosides reflect the chemical behavior of these molecules in ways which have significant biological implications. One area in which T ionization potentials of nucleosides are expected to be related to important chemical properties involves physical binding interactions which lead to stacked molecular complexes. In the present study the ionization potentials of ribonucleosides, which occur most frequently in polynucleotides, are compared to the binding properties of these molecules in complexes involving various stacking partners. Experimental Section Photoelectron spectra were measured with a PerkinElmer PS 18 photoelectron spectrometer equipped with a He(1) lamp and a heated probe. The temperatures at which the spectra of molecules I-VI1 were measured are given in Figures 1 and 2. For each molecule the sample temperature was constant within f l “ C during a spectroscopic run. Ionization potentials were calibrated by using the 2PSl2and 2P,,, bands of Xe and Ar. For all compounds spectra obtained from a single sample over a period of 1h were identical, indicating that no decomposition occurred. The values of vertical ionization potentials reported in Figures 1 and 2 represent the mean of five
IONIZATION POTENTIAL ( e V )
Flgure 2. He(1) photoelectron spectra of 3-methyl-3‘,5’-di-0methylthymidine, 2’,3’,5’-tri-O-methykytidine, N,N-dimethyl-2‘,3’,5’tri-0-methylcytidine, and 2’,3’,5’-tri-O-methyIadenosine along with vertical ionization potentials and assignments for the highest cooccupied orbitals. Vertical bars appearing below the spectra represent the pattern of energy levels predicted by CNCOIS. For each molecule the CNDO/S results have been scaled so that the energy of the HOMO coincides wlth the first ionization potential.
spectroscopic runs. The data were reproducible to f0.03 eV. Samples of uracil and tris(trimethylsily1)-mibose were obtained from Sigma Chemical Co. and Pierce Chemical Co., respectively. Synthesis of Nucleoside Models. The volatile 0-methyl nucleoside derivatives employed in this study were synthesized from the appropriate parent nucleosides by using exhaustive methylation procedures.ll For 2‘,3’,5’-tri-Omethyluridine (III), 3-methyl-3’,5’-di-O-methylthymidine (IV), 2’,3’,5’-tri-O-methylcytidine (V), and 2’,3‘,5‘-tri-Omethyladenosiae (VII) the syntheses and purification procedures used here were the same as those reported in the literature-l1 A sample of N,N-dimethyl-2’,3‘,5’-tri-O-methylcytidine (VI) was synthesized in four steps. In the first N,N-dimethyl-2’,3’,5’-tri-O-benzoylcytidine was converted to 4chlor0-2’,3‘,5’-tri-O-benzoyluridine.~~ This was carried out by treating 33.3 g of N,N-dimethyl-2’,3’,5’-tri-O-benzoylcytidine with 48 mL of thionyl choride and 4.5 mL of N,N-dimethylformamide in the presence of dimethylchloromethyleneammonium chloride. In the second step excess sodium in anhydrous methanol was added to 4~~
~~~~
~
~
(11) J. T. Kusmierek, J. Giziewicz, and D. Shugar, Biochemistry, 12,
194 (1973);Z. Kazimierczuk, E.Darzynkiewicz, and D. Shugar, ibid., 15, 2735 (1976). (12) J. Zemlicka and F. Sorm, Collect. Czech. Chem. Cmmun.,30,2052 (1965).
UV Photoelectron Studies of Volatile Nucleosides
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chloro-2f,3f,5’-tri-O-benzoylcytidine to yield 4-methoxycytidine.13 Next, N,N-dimethylcytidine was obtained via the aminolysis of 4-methoxycytidine by the addition of dimethylamine (40% by weight) in methan01.l~ Finally, the exhaustive methylation of N,N-dimethylcytidine yielded VI. Mass spectra of compounds 111-VI were measured and compared to those previously reported for corresponding deuteratd derivatives.1° In each case the mass spectrum of the material used on the present studies exhibited the expected fragmentation pattern. For compound VII, mass-spectral and melting-point measurements indicated the presence of a small amount (-2%) of N,N-dimethyl-2’,3’,5’-tri-O-methyladenosine. When photoelectron spectra of VI1 were measured, the sample purity was improved by heating the material at 120 “C for 0.5 h in the spectrometer before measurements were taken. This procedure removed N,N-dimethyl-2’,3’,5’tri-0-methyladenosine, which is more volatile than VII.
tion with Koopmans’ theorem16 are a useful tool for the interpretation of photoelectron data. In addition to molecules for which spectra are reported here, the calculations were also extended to the nucleoside model N,N-dimethyladenosine and to the underivatized ribonucleosides, uridine, thymidine, cytidine, adenosine, and guanosine. Finally CNDO/S calculations were carried out on 2’,3’,5’-tri-O-methylribose and on the bases uracil, thymine, 3-methylthymine, cytosine, N,N-dimethylcytosine, adenine, N,N-dimethyladenine, and guanine. Molecular geometries of uridine, thymidine, and cytidine, and of the nucleotide bases uracil, thymine, and cytosine, were obtained directly from crystallographic data.17-20 The geometry of guanosine was derived from the crystal structure of N,N-dimethylguanosine.21Geometries employed to describe adenine, N,N-dimethyladenine, and guanine were the same as those used in earlier studies.l* The geometry of 3-methylthymine was obtained by combining the coordinates for thyminezzwith coordinates for the methyl group in l - m e t h ~ l t h y m i n e . ~ ~ Results and Discussion The geometry of 2’,3’,5’-tri-O-methylribose was derived from that of the ribose ring in uridine crysta1s.l’ The Figure 1 shows photoelectron spectra measured for exocyclic 0-CH3 bond length and the CH, geometry used uracil, tris(trimethylsily1)-D-ribose,and 2’,3’,5’-tri-Ofor the methoxy groups were taken from tables of standard methyluridine. The assignment of the uracil spectrum bond lengths and bond angles.24 given in Figure 1 is the same as that provided in earlier The geometries of the nucleoside models I11 and V-VI1 studies.lP8 Like the UV photoelectron spectra of other were obtained by using the same conformations occurring trimethylsilyl-substituted molecules,14 the spectrum of in the corresponding unmethylated ribonucleosides. The tris(trimethylsily1)-D-riboseis poorly resolved. The broad geometries of the methoxy groups were derived in the same region of photoelectron emission occurring between 9.0 and manner described above. For N,N-dimethyl-2’,3’,5’-tri-O12.0 eV arises from a series of overlapping bands. These methylcytidine (VI) the geometry employed in the calcuare associated with oxygen-atom lone-pair orbitals and Q lation was obtained by combining the coordinates emorbitals of the Si(CH3)3group which have ionization poployed for molecule V with those previously employed in tentials in the region around 10 eV.14J5 a study of N,N-l-trimethyl~ytosine.~ A comparison of the spectra for uracil and tris(triThe geometry employed to describe 3-methyl-3’,5’-dimethylsily1)-D-riboseindicates that the lowest-energy band 0-methoxythymidine (IV) was derived from the crystal in the spectrum of uracil occurs at 9.59 eV. In this energy data for thymidine@and the geometry of 3-methylthymine region the spectrum of tris(trimethylsily1)-D-riboseexhibits described above. low photoelectron intensity. As expected from this obThe geometry of N,N-dimethyladenosine was obtained servation, the spectrum of 2’,3’,5’-tri-O-methyluridine (III), by combining crystal data for adenosinez0with the geomshown in the lower panel of Figure 1, exhibits a well-reetry of N,N-dimethyladenine described in an earlier study? solved band at 8.96 eV. This band arises from the highest CNDO/S results in conjunction with Koopmans’ theooccupied molecular orbital (HOMO) of uracil. rem16yield a value of 10.63 eV for the ionization potential At higher energies the spectrum of I11 is poorly resolved. of 2’,3’,5’-tri-O-methylribose and values of 10.35,9.60,9.44, The broad unstructured character of the emission occurand 9.01 eV for the ionization potentials of the nucleoside ring in the energy region 9.0-11.0 eV is expected in view models 111-VII, respectively. While the absolute values of the large number of bands occurring in this energy of the predicted ionization potentials are not accurate, the region of the spectra of uracil and tris(trimethylsilyl)-DCNDO/S calculations describe other features of molecular ribose. orbital structure in nucleosides which appear to be valid. Figure 2 shows photoelectron spectra measured for 3For example, computational results indicate that the methy1-3’,5’-di-O-methoxythymidine(IV), 2’,3’,5’-tri-OHOMO of ribose has an ionization potential which is in methylcytidine (V), N,N-dimethyl-2’,3’,5’-tri-O-methylall cases greater than those of the nucleoside models. This cytidine (VI), and 2’,3’,5’-trimethyladenosine(VII). All is consistent with the observation that in tris(trimethy1of the spectra of Figure 2, like the spectrum of molecule silyl)-D-ribosethe first vertical ionization potential is above 111, exhibit a broad region of high photoelectron intensity 9.0 eV, while the first vertical ionization potentials of all lying in the energy region 9.0-11.0 eV. This arises from several overlapping bands. Also, like the spectrum of molecule 111,the spectra of Figure 2 all exhibit one resolved (16) T. Koopmans, Phsica, 1, 104 (1934). low-energy band below 9.0 eV. (17) E. A. Green, R. D. Rosenstein, R. Shiono, and D. J. Abraham, Acta Crystallogr., Sect. E, 31, 102 (1974). Molecular Orbital Calculations. As an aid in inter(18) D. W. Young, P. Tollin, and H. R. Wilson, Acta Crystallogr., Sec. preting the spectra of molecules 111-VII, CNDO/S moB , 25, 1423 (1969). lecular orbital calculations were carried out on these nu(19) S. Furberg, C. S. Peterson, and C. Romming, Acta Crystallogr., 18, 313 (1964). cleoside models. In previous of nucleotide bases, (20) T. F. Lai and R. E. Marsh, Acta Crystallogr., Sect. B , 28, 1982 it was found that CNDO/S calculations used in conjunc(1971). ~~
(13) M. J. Robins and S. R. Naik, Biochemistry, 10, 3591 (1971). (14) E. Heilbronner, V. Hornung, H. Bock, and H. Alt, Angew. Chem., Int. Ed. Engl. 8, 524 (1969). (15) A. D. Baker, D. P. May, and D. W. Turner, J. Chem. SOC.B, 22 (1968).
(21) T. Brennan, C. Weeks, E. Shefter, S. T. Rao, and M. Sundaralingam, J. Am. Chem. SOC.,94, 8548 (1972). (22) K. Ozeki, N. Sakabe, and J. Tanaka, Acta Crystallogr., Sect. B ,
25, 1038 (1969). (23) K. Hoogsteen, Acta Crystallogr., 16, 28 (1963). (24) “Table of Interatomic Distances and Configurationsin Molecules and Ions”, The Chemical Society, Burlington House, London, 1965.
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TABLE I: Comoarison of Ionization Potentials of Nucleoside Models and Nucleotide Basesn 2’ ,3’,5’4ri-0-methyluridine(111)
3-methy1-3’,6’-di-O-methylthymidine (IV) 2‘,3‘,5‘-tri-O-methylcytidine (V) N,N-dimethyl-2’,3’,5’-tri-O-methylcytidine (VI) 2’,3‘,5’-tri-O-methyladenosine (VII)
8.96 8.58 8.46 8.25 8.35
9.59 9.18b 8.94‘ 8.68 8.48
0.63 0.60 0.48 0.43 0.13
0.42 0.45 0.29 0.25 0.07
a All energies given in eV. The ionization potential of 3-methylthymine is estimated t o be 0.15 eV less than that of thymine consistent with the observation that the ionization potential of 3-methyluracil is 0.15 eV less than that of uracil. See ref 2. The ionization potential of N,N-dimethylcytosine is estimated t o be 0.29 eV greater than that of N,N,1trimethylcytosine in agreement with experimentally determined differences in the ionization potentials of cytosine and 1-methylcytosine. See ref 4.
of the nucleoside modlels are less than 9.0 eV. Furthermore, the CNDO/S calculations predict that in all nucleoside models the energy gap between the HOMO and the second highest occupied orbital is more than 0.6 eV and that levels associated with the second through fifth highest occupied orbitals differ in energy by no more than 1.2 eV. These predictions are consistent with the spectral data which indicates that in each of the models a significant energy gap occurs between the first band, which is resolved, and a broad region of high photoelectron intensity which arises from a series of unresolved bands. This correlation between experiment and theory is demonstrated in Figures 1and 2. These figures show the pattern of orbital energies predicted by CNDO/S calculations for the nucleoside models. In order to characterize the highest occupied orbitals of the nucleoside models, we have examined electron distributions predicted by the calculations for the upper occupied orbitals. These have been compared with electron distributions predicted for the upper occupied orbital of the free bases. In all cases it is found that the highest occupied orbital of the nucleoside model is very similar to the ?rl orbital of the corresponding free base. The assignments given in Figures 1and 2 for the first band appearing in the spectra of molecules 111-VI1 are based on this observation. The calculations predict that, of the more stable orbitals in the nucleoside models, some are localized on the base, some on the sugar, and some are distributed over both moieties. For example in 2/,3/,5’-tri-O-methyluridine the calculations predict that the second highest occupied orbital is localized mostly on the sugar group, the third highest orbital is a lone-pair orbital associated with the oxygen atom at the 4 position of uracil, and the fourth orbital is diffused over both the sugar and base moieties. In order to examine the effects of glycosidic bond formation upon the ?r-electron-donatingability of nucleotide bases, we have compared the ionization potentials of the nucleoside models (111-VII) to those of the free bases in which an H atom replaces the sugar group. For this comparison ionization potentials of the nucleoside models and of the free bases uraci1,l cytosine: and adenine5were taken directly from photoelectron data. For the remaining free bases, ionization potentials have been obtained from photoelectron data on related molecules. The ionization potential of 3-methylthymine was estimated from the ionization potentials of thymine, uracil, and 3 - m e t h y l u r a ~ i l .The ~ ~ ~ionization potential of N,Ndimethylcytosine was estimated from data on cytosine, 1-methylcytosine, and N,N,l-trimethyl~ytosine.~ Table I lists vertical ionization potentials for the nucleoside models (111-VII) and for the corresponding free purines and pyrimidines. The results indicate that in each case formation of the glycosidic bond in the nucleoside model causes a reduction in the ionization potential of the ?rl orbital of the base. In some cases this perturbation is
remarkably large. For example, addition of the sugar group reduces the ionization potentials of uracil and 3-methylthymine by more than 0.6 eV. The relative magnitudes of the perturbations observed for the five bases of Table I vary in a manner which is consistent with earlier photoelectron results. For example, the ionization potentials of the pyrimidines uracil, 3methylthymine, and cytosine exhibit a greater sensitivity to glycosidic bond formation than does the ionization potential of adenine. This is expected since methyl substitution a t the 1position of uracil,2 thymine,2 and cytosine4 causes a greater perturbation of the ?rl orbital than similar methyl substitution at the 9 position of adenine. Furthermore, it is found that the ?rl orbitals of uracil and 3-methyluracil are perturbed more by the sugar group than the ?rl orbital of cytosine. This is consistent with the observation that the ?rl orbitals of uracil and thymine have greater electron density at the 1position than does the ?rl orbital of c y t o ~ i n e . ~ ? ~ For each of the nucleoside models studied, Table I lists the reduction in the base ionization potential which accompanies glycosidic bond formation as predicted by CNDO/S. An examination of the theoretical results in Table I indicates that, for all of the nucleoside models studied, the experimental lowering of the first ionization potential of the base due to glycosidic bond formation is 1.72-1.33 times greater than that predicted by the CNDO/S calculations. On the average the theoretically determined perturbation is 1.53 times smaller than the experimental perturbation. The magnitude of this error is consistent with that obtained in other studies in which photoelectron results were compared to results from CNDO/S calculations.8i26 In these studies experimentally measured reductions in the ?rl ionization potential brought about by methyl and cyclohexyl substitution to nucleotide bases are also -1.5 times larger than those predicted by CNDO/S. CNDO/S predictions of the perturbation of alorbital energies have been used to estimate the ionization potentials of the unmethylated ribonucleosides, uridine, thymidine, cytidine, adenosine, and guanosine. The ionization potentials were obtained from the relationships IPnucleoside = IPbase - A(lp)xaled A(IPhcaled= ~ . ~ ~ [ A ( I P ) C N D O / S ] Here IPnucleosideand IPbaseare the ionization potentials of the nucleoside and its corresponding free base, while I P c m is the difference between the ionization potentials of the Lase and the nucleoside as predicted by CNDO/S. The ionization potentials of the ribonucleosides as well as values for A(IP)& and A ( I P ) c N D ~are / ~ listed in Table 11. These values for the ribonucleoside ionization po(25) T. J. O’Donnell, Ph.D Thesis, University of Illinois at Chicago Circle, Chicago, IL, 1980.
The Journal of Physical Chemistty, Vol. 85,No. 25, 1981 3855
UV Photoelectron Studies of Volatile Nucleosides
I
TABLE 11: Nucleoside Ionization Potentialsa uridine thymidine cytidine adenosine guanosine
A (IP)CNDO/S
A (Ip)scaled
0.35 0.32 0.25 0.06 0.15
0.54 0.49 0.38 0.09 0.23
1
1
IPb 9.0 8.7 8.6 8.4
I 20
r
8.0c
All energies in eV. Estimated errors in the riboside ionization potentials listed here are 0.1 eV. Obtained by employing ionization potential values for uracil, thymine, cytosine, and adenine reported in ref 1, 2, 4, and 5 . Obtained by estimating the ionization potential of the 9-H tautomer of guanine t o be 0.14 eV less than that of the more stable 7-H tautomer which has an ionization potential of 8.28 eV. This estimate is consistent with the observation that the ionization potential of 9-methylguanine is 0.14 eV less than that of 7-methylguanine. See ref 6. a
I
I
80
85
90
IONIZATION POTENTIAL (EV)
80
85
90
IONIZATION POTENTIAL (eV)
Figure 3. Comparison of nucleoside ionization potentials and blnding constants for (0)uridine, (I thymidine, ) (A)cytidine, (0)adenosine, (0)guanosine, and (A)N,N-dimethyladenosine. Panel A shows association constants for interactions of nucleosideswffh riboflavin. Panel B shows association constants for interactions involving the self-association of nucleosides. The ionization potential of N,Ndimethyladenosine was obtained in a manner analogous to that used to obtaln the ionization potentials of the unmethylated ribonucleosides. The ionization potential of N ,Ndimethyladenine employed in this procedure is that reported in ref 5.
tentials are reasonable when compared to the ionization potentials of the nucleoside models listed in Table I. For example, the ionization potentials of the models 111, V, and VI1 differ by less than 0.1 eV from the estimated ionization potentials of the corresponding ribonucleosides uridine, cytidine, and adenosine. This small difference is reasonable since methylation of the sugar group is not expected company decreases in the ribonucleoside ionization poto cause large perturbations of the slorbitals of the bases. tentials. On the other hand, Tables I and I1 indicate that a larger Panel B of Figure 3 shows results similar to those condifference (0.31 eV) occurs between the ionization potentained in Panel A. In this case association constants obtials of cytidine and N,N-dimethyl-2’,3’,5’-tri-O-methyltained in binding studies of the self-association of nucytidine (VI). This is consistent with earlier studies, where cleosidesZ8are compared with the nucleoside ionization it was found that the ionization potential of N,N,l-tripotentials. Here it is also found that for the most common methylcytosine is 0.29 eV lower than that of l-methylribonucleosides a reduction in ionization potential is gen~ytosine.~ erally accompanied by an increase in association constant. Finally, a large difference (0.36 eV) occurs between the The results in Figure 3 are interesting because different reported ionization potentials of uridine and thymidine, models have been employed to describe binding of nuwhile a small difference (0.11 eV) occurs between the cleosides to riboflavin and the self-association of nucleoionization potentials of thymidine and 3-methyl-3’,5’-disides. In the former case charge transfer forces are believed 0-methylthymidine (IV). These observations are conto be important.27 Here the nucleotide bases are thought sistent with previous data, which indicate that the alorto be electron donors and riboflavin is an electron acceptor. bital in uracil is very sensitive to methyl substitution at In the latter case it is believed that binding is due to the 5 position.238 The difference between the ionization transient polarization forces. It has been found that nupotentials of uracil and thymine is 0.4 eV.2i8 On the other cleoside self-association constants generally increase as the hand the slorbital of thymine, like that in uracil, is expolarizabilities of the individual nucleotide bases inpected to be less sensitive to methyl substitution at the c r e a ~ e . ~In, ~both ~ types of complexes, decreases in 7rl 3 position. The ionization potential of 3-methyluracil is ionization potentials accompany increases in stacking asonly 0.15 eV less than that of uracil.2 sociation constants. Stacking Interactions. It is interesting to consider The strong influence of the highest occupied T orbital possible aspects of chemical behavior which are reflected upon the stacking properties of the most common riboin the ionization potentials of nucleosides. In previous nucleosides also occurs in some other heterocyclic systems. studiesz6it was found that, as the ionization potentials For example, in recent ab initio studies of stacked comof free purines and pyrimidines decrease, nucleoside plexes containing substituted indoles, it was found that stacking association constants generally increase. This the largest single orbital contribution to polarization forces relationship is observed for complexes containing riboflavin arises from the highest occupied r orbital.29 and nucleosides formed from these purines and pyrimidines. Panel A of Figure 3 contains a plot of association Acknowledgment. Support of this work by the National constants for riboflavin complex formation2’ vs. the riboInstitutes of Health and by the Computer Center of the nucleoside ionization potentials reported here. As expected University of Illinois at Chicago Circle is gratefully acfrom the previous studies of free bases, the figure demknowledged. onstrates that increases in the association constants ac(26) N. S. Hush and A. S. Cheung, Chem. Phys. Lett., 34, 11 (1975). (27) P. 0. P. Ts’o in “Basic Principles in Nucleic Acid Chemistry”,P. 0. P. Ts’o, Ed., Academic Press, New York, 1974, pp 526-62.
(28) J. C. M. Tsibris, D. B. McCormick, and L. D. Wright, Biochemistry, 504 (1965). (29) H. Weinstein,P. Chou, S. Kang, C. L. Johnson, and J. P. Green, Int. J. Quantum Chem., Quantum Bid. Symp., 3, 135 (1976); H. Weinstein and R. Osman, ibid.,4, 253 (1977).
