QUENCHING OF THE EXCITED SINGLET-STATE OF THE N-(9-METHYLPURIN-6-YL)PYRIDINIUM CATION BY SULFUR-CONTAINING AMINO-ACIDS AND CARBOXYLIC-ACIDSIN AQUEOUS-SOLUTION

Quenching of the lowest excited singlet state of the N-(9-methylpurin- 6-yl) pyridinium cation (Pyr(+)) by sulphur-containing organic compoun ds in neutral aqueous solution was studied by steady state and time-re solved fluorescence techniques. The quenching rate constants k(q) were determined from the Stern-Volmer plots for several sulphur-containing amino acids and carboxylic acids (Q(-ZB)) in their zwitterionic (Z(B) = 0) and anionic (Z(B) = -1, -2 and -3) forms on the basis of lifetim e and fluorescence intensity measurements at constant ionic strength ( mu). The rate constants (extrapolated to mu = 0) for the dynamic quenc hing process were found to be in the range (0.4-4) X 10(10) M(-1) s(-1 ) depending on the ionic charge Z(B), and were close to the diffusion- controlled limit. For comparison, analogous rate constants measured fo r alanine, glycine and proline (non-sulphur amino acids) were found to be at least four times lower, k(q) = 1 X 10(9) M(-1) s(-1). The ion p air association constants, corresponding to the static quenching const ants obtained by combining the intensity and lifetime quenching data, were in the range 5-11 M(-1), indicating that the observed quenching w as mainly due to a dynamic process. The mechanism of quenching was dis cussed in terms of electron transfer from the sulphur atom of Q(-ZB) t o the Pyr(+) singlet state, and was compared with the quenching of the Pyr(+) triplet state recently shown to occur via the electron transfe r process (J. Am. Chem. Sec., 117 (1995) 127).