Photooxidation of tryptophan: O-2((1)Delta(g)) versus electron-transfer pathway

Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy](2+)(3)) free in solution and adsorbed onto antimony-doped SnO2 colloidal particles was used as a photosensitizer for a comparison of the O-2((1) Delta(g)) and electron transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)(3)(2+) by O-2((3) Sigma(g)(-)) in an aerated aqueous solution leads only to the formation of O-2((1) Delta(g)) (phi(Delta) = 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)(3)(2+) adsorbed onto Sb/SnO2 results in a fast injection of an electron tron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)(3)(2+) and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)(3)(3+) is reduced by TRP with a bimolecular rate constant k(Q) = 5.9 x 10(8) M(-1) s(-1), while O-2((1) Delta(g)) is quenched by TRP with k(t) = 7.1 x 10(7) M(-1) s(-1) (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (k(c)) via both pathways were determined using fluorescence emission spectroscopy. With N-p, the rate of photons absorbed, being constant for both pathways we obtained k(c) = (372/N-p) M(-1) s(-1) for the O-2((1) Delta(g)) pathway and k(c) greater than or equal to (25013/N-p) M(-1) s(-1) for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O-2((1) Delta(g)).