Photoemission properties of methyl-substituted guanines: Photoelectron and fluorescence investigations of 1,9-dimethylguanine, O6, 9-dimethylguanine, and 9-methylguanine

Article abstract of DOI:10.1021/ja00162a015

Photoelectron and fluorescence emission properties of 1,9-dimethylguanine (1,9-DMG), O⁶,9-dimethylguanine (O⁶,9-DMG), and 9-methylguanine (9-MG) have been examined. Gas-phase HeI photoelectron spectra have been compared with theoretical ionization potentials provided by HAM/3 semiempirical quantum mechanical calculations, and by SCF ab initio calculations carried out with 3-21G and 4-31G basis sets. Geometrics of 1,9-DMG and O⁶,9-DMG used in the calculation were based on X-ray crystallographic data measured in this investigation. For 1,9-DMG, which occurs in the amino keto form of guanine that participates in Watson-Crick base pairing, and for O⁶,9-DMG, which is a model compound for mutagenic and carcinogenic products formed in DNA alkylation reactions, ionization potentials have been measured for 8 and 10 of the highest occupied orbitals, respectively. For 9-MG, which can occur in either an amino keto or an amino enol form, the present results suggest that both tautomers occur in the gas phase, but that the amino enol tautomer predominates. Measurements of fluorescence emission spectra indicate that protonated 0⁶,9-DMG has a higher fluorescence quantum yield that protonated 1,9-DMG or 9-MG. At a pH of 2.0 and at an excitation wavelength of 270 nm the relative emission intensities of O⁶,9-DMG, 1,9-DMG, and 9-MG are 11.0, 1.0 and 1.2 respectively, and the fluorescence lifetimes are 5.4, 0.4 and 0.4 ns. Results from HAM/3 and CNDO/S CI calculations on N7-protonated O⁶,9-DMG, 1,9-DMG, and 9-MG are consistent with the conclusion that the low fluorescence intensities of 1,9-DMG and 9-MG are related to the vibronic coupling of low-lying singlet nπ* and ππ* states, and to the occurrence of efficient intersystem crossing.