Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals.

Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr-Val, Val-Tyr and Val-Tyr-Val) mediated by singlet molecular oxygen [O(2)((1)Delta(g))], phosphate (HPO(4)(*-) and PO(4)(*2-)) and sulfate (SO(4)(*-)) radicals was studied, employing time-resolved O(2)((1)Delta(g)) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O(2)((1)Delta(g))-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O(2)((1)Delta(g)) by Tyr-derivatives (k(t)/k(r) values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr-Val, 2.8 for Val-Tyr and 1.5 for Val-Tyr-Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O(2)((1)Delta(g)) quenching process. Products analysis by LC-MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO(4)(*-) and PO(4)(*2-), generated by photolysis of the P(2)O(8)(4-) at different pH) and sulfate radicals (SO(4)(*-), produced by photolysis of the S(2)O(8)(2-)) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO(4)(*-) > or = HPO(4)(*-) > or = PO(4)(*2-). Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO(4)(*-).

Photophysical properties of corrphycenes.

The photophysical properties of the Zn salt of octaethylcorrphycene (compound 1) and the doubly protonated octaethylcorrphycene (compound 2) were determined in benzene solutions. Fluorescence spectra and fluorescence quantum yields of phiF (1) = 0.03+/-0.02 and PF (2) = 0.06+/-0.02 were measured. The triplet-triplet absorption spectra were obtained by means of flash-photolysis experiments. The triplet quantum yield values phiT (1) = 0.79+/-0.08 and phiT (2) = 0.82+/-0.08 were obtained by using laser-induced optoacoustic spectroscopy. The quantum yield of singlet molecular oxygen generation in air-saturated solutions, phidelta (1) = 0.55+/-0.07 and phidelta (2) = 0.38+/-0.05, were also measured using time resolved NIR luminescence.