The UV photoelectron spectra of adenine, 9-methyladenine, and 6-methylaminopurine contain highly resolved bands arising from the six highest occupied molecular orbitals. The spectra have been analyzed using UV absorption data, photoelectron data from previous studies of heterocyclic compounds, and results from both semi-empirical and ab initio molecular orbital calculations. The analysis indicates that the first, third, and fifth photoelectron bands in adenine and the two methyl substituted derivatives arise from 7r orbitals. The second, fourth, and sixth bands arise from nitrogen atom lonepair orbitals. Compared to adenine, the six uppermost orbitals of 9-methyladenine and 6-methylaminopurine have lower ionization potentials. This destabilization of the valence electrons is expected to play an important role in causing the increase in base stacking forces observed in methyl substituted adenines.The binding characteristics of biological purines are thought to be strongly influenced by charge transfer interactions (1-3) and polarization forces (3, 4) involving these molecules. Because electrons residing in the highest occupied and lone-pair orbitals of purines are easily removed, such electrons play an important role in determining binding characteristics. The energy levels of valence electrons in biological purines have been extensively investigated in both semi-empirical and ab initio molecular orbital calculations (5-8). To date, UV absorption measurements have provided virtually all of the experimental information available about these molecules (6, 7, 9, 10). It is found, however, that the absorption spectra of purines contain strongly overlapping bands which preclude a simple interpretation. More highly resolved experimental information about the occupied valence orbitals of biological purines will greatly facilitate an understanding of their ground-state electronic structure. In recent studies of biological pyrimidines it has been found that UV photoelectron spectroscopy can provide this information (11-13). The present study of adenine, 9-methyladenine, and 6-methylaminopurine is a detailed photoelectron examination of biological purines. EXPERIMENTAL Photoelectron spectra were measured with a Perkin Elmer PS-18 spectrometer equipped with a heated probe. Adenine and 6-methylaminopurine were obtained from Sigma Chemical Co. 9-Methyladenine was obtained from Fox Chemical Co. All compounds were used without further purification. The spectra were run in the temperature range 126-185o. Ionization potentials were calibrated using the 2P3/2 and 2P1/2 bands of Xe and Ar. For all three molecules, spectra measured from the same sample over a 1-to 2-hr period were identical, indicating that no decomposition occurred. Fig. 1 shows He I photoelectron spectra of the molecules stud- RESULTS AND DISCUSSIONied. An examination of the spectra indicates that adenine exhibits six widely separated bands in the energy region 8.5-13.2 eV and that the ordering of these bands is identical in all three of the molecules inve...

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SynopsisUltraviolet photoelectron spectroscopy has been employed to examine the valence electronic structure of 5-fluorouracil, 5-chlorouracil, 5-bromouracil, and 5-iodouracil. Photoelectron bands associated with the three highest x orbitals and the two oxygen atom lone-pair orbitals were assigned by a comparison to similar bands observed in the photoelectron spectrum of uracil. Bands arising from the halogen atom lone-pair orbitals were assigned by comparing the present results with photoelectron spectra measured for halobenzenes, and by considering the linear dependence of halogen atom lone-pair ionization potentials upon halogen atom electronegativities. The present spectroscopic results have been compared with results from studies of association constants of 5-halouracil-adenine complexes. This examination indicates that the complex association constants increase as the ionization potentials of the highest occupied K orbital and the halogen atom lone-pair orbitals of the halouracils decrease.

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