SOLVENT EFFECT ON THE QUENCHING OF THE MONO-L-ASPARTYL CHLORIN E(6) TRIPLET-STATE BY METRONIDAZOLE

The quenching of the triplet state of mono-L-aspartyl chlorin e(6) ((3 )MACE) by the metronidazole ground state (METRO) has been investigate d in 14 different binary mixtures of water and solvent by 532 nm laser hash photolysis. This quenching is a diffusion-controlled process in water with a quenching constant of k(Q)(0) = 1.9 x 10(9) M-1 s(-1) . I n neat solvent, the quenching of (3)MACE by METRO is not observed. Th e quenching constant is reduced in binary mixtures. The dependence of k(Q)(0) in the mixture on the solvent molar fraction may be expressed by ln k(Q) = ln k(Q)(0) - gamma(solv)X(M.F.)(solv) The activation barr ier of quenching on going from water to water-solvent mixtures is unch anged and in the range 3-4 kcal mol(-1). The solvent effect is analyse d in terms of specific solute-water-solvent interactions. A mechanism for the quenching of (3)MACE by METRO is proposed. (C) 1997 Elsevier Science S.A.