Ultraviolet photoelectron studies of the ground-state electronic structure and gas-phase tautomerism of purine and adenine

Ultraviolet photoelectron spectroscopy has been employed to study electronic structure and tautomerism in adenine and purine and in various methyl-substituted derivatives of these molecules. The assignment of the photoelectron spectra has been aided by results from HAM^ molecular orbital calculations which are empirically parameterized to provide orbital ionization potentials. The calculations predict that the lone-pair orbitals in purine and adenine are similar in spacial distribution and have similar ionization potentials. This conclusion is supported by the spectroscopic data which indicate that in purine the three highest occupied lone-pair orbitals have ionization potentials of -9.6, 10.6, and 11.7 eV. In adenine these orbitals have ionization potentials of -9.6, 10.5, and 11.39 eV. The planar exocyclic amino group in adenine causes the A structure of this molecule to be significantly different than that in purine. In adenine the ionization potentials of the five highest occupied A orbitals are 8.48, -9.6, -10.5, 12.10, and 13.21 eV. In purine the ionization potentials of the four highest occupied a orbitals are -9.6, -10.2, -11.9, and 13.10 eV. The band arising from the fifth highest occupied A orbital in purine has an ionization potential greater than 13.5 eV and is not resolved. A comparison of the photoelectron spectra of purine, adenine, and their 7and 9-methyl-substituted derivatives indicates that the spectra of purine and adenine are much more similar to the spectra of their 9-methyl derivatives than to the spectra of their 7-methyl derivatives. This observation supports the conclusion that in an isolated environment the N(9)H tautomers of both purine and adenine are more stable than the N(7)H tautomers. Introduction Prior to photoelectron studies most investigations of electronic structure in nucleotide bases were based primarily upon results from molecular orbital calculation^.'-^ As might be expected, predictions associated with the ordering and spacing of energy levels in these molecules vary greatly depending on the theoretical procedure employed. Previous spectroscopic investigations of these molecules have involved UV absorption measurementsb9 as well as studies involving circular and magnetic circular d i c h r o i ~ m . ~ ~ , ~ ~ In these previous spectroscopic studies the fact that a a* transitions strongly overlap n a* transitions has often made assignments difficult to obtain.I2 It has recently been demonstrated that UV photoelectron spectroscopy when used in conjunction with molecular orbital calculations can yield detailed information about the ordering and spacing of energy levels associated with valence electrons in nucleotide bases.l4-I8 Most of the previous work in this area has focused on the biological pyrimidines: uracil,"" thymine,I5-l7 and cytosine.'* The assignment of spectra and characterization (1) A. Pullman, Ann. N.Y. Acad. Sci. 155,65 (1969). (2) W. Hug and I. Tinoco, Jr., J . Am. Chem. SOC., 96, 665 (1974). (3) W. Hug and I. Tinoco, Jr., J . Am. Chem. SOC., 95, 2803 (1973). (4) E. Clementi, J. M. Andre, M. 0. Andre, D. Klint, and D. Hahn, Acta ( 5 ) J. S . Kwiatkowski and B. Pullman, Int. J. Qualum Chem. 15, 499 (6) D. Voet, W. B. Gratzer, R. A. Cox, and P. Doty, Biopolymers, 1, 193 (7) L. B. Clark, G. G. Peschel, and I. Tinoco, Jr., J . Phys. Chem., 69,3615 (8) L. B. Clark and I. Tinoco, Jr., J . Am. Chem. SOC., 87, 1 1 (1965). (9) T. Yamada and H. Fukutome, Biopolymers, 6, 43 (1968). (10) D. W. Miles, R. K. Robins, and H. Eyring, J. Phys. Chem., 71, 3931 (1 1) C. A. Sprecher and W. C. Johnson, Jr., Biopolymers, 16,2243 (1977). (12) W. Voelter, R. Records, E. Bunnenberg, and C Djerassi, J . Am. (13) R. E. Linder, H. Weiler-Feilchenfeld, G. Barth, E. Bunnenberg, and (14) A. Padva, P. R. LeBreton, R. J. Dinerstein, and J. N. A. Ridyard, (15) A. Padva, T. J. O'Donnell, and P. R. LeBreton, Chem. Phys. Lett., (16) G. Lauer, W. Schafer, and A. Schweig, Tetrahedron Letf., 45,3939 Phys. Acad. Sci. Hung., 27, 493 (1969).