ON THE O2(1-DELTA-G)-MEDIATED PHOTOOXIDATIVE BEHAVIOR OF TRIPEPTIDE GLYCYL-TYROSYL-ALANINE IN ALKALINE-MEDIUM - A KINETIC-STUDY

The type II singlet molecular oxygen [O2(1DELTA(g))]-mediated photooxi dation of the tripeptide gly-tyr-ala was studied. It has two non-oxidi zable amino-acids (gly and ala) bonded to the oxidizable one, tyr. Ove rall (k(t)) and reactive (k(r)) rate constants for the interaction wer e determined by time-resolved methods (IR emission of O2(1DELTA(g))) a nd stationary photolysis, in water at pH 11.5 as well as in alkaline n on-aqueous etOH-MeCN (80:20, v/v, 10 mM in KOH) solutions. An importan t solvent polarity effect on k(t) was detected; the rate constant incr easing one order of magnitude in going from the organic mixture to wat er (k(t) H2O = 2 x 10(9) M-1 s-1). Nevertheless, k(r) does not paralle l this trend; gly-tyr-ala being less photooxidizable in a more polar e nvironment. The effective quantum yield (empty set(r)) for TPE photoox idation is much higher in etOH-MeCN (empty set(r) = 0.056) than in wat er (empty set(r) = 0.023). Results are discussed on the basis of the f ormation of an exciplex with polar character between the TPE and O2(1D ELTA(g)). Two remarkable points should be taken into account: a) the r ate costants for the interaction of O2(1DELTA(g)) with gly-tyr-ala are practically the same as for free tyr. b) New -NH2 groups are generate d upon sensitized irradiation. Both findings indicate that the peptide bonds in the TPE break as a result of the photooxidation. A thorough analysis with data for tyrosine and related dipeptides is undertaken.