A comparative kinetic study on the singlet molecular oxygen‐mediated photoxidation of α‐ and β‐chymotrypsins

Abstract

Kinetic aspects of the sensitized photooxidation of a- and b-chymotrypsins have been studied at pH 6 and 8. The sensitization, employing classical O2(1Dg)-photogenerators, such as xanthene dyes, is a kinetically intricate process because of the presence of ground state dye?protein associations and to the simultaneous participation of superoxide ion and singlet molecular oxygen [O2(1Dg)].Both proteins, that possess the same distribution pattern of photooxidizable amino acids, suffer a pure O2(1Dg)- mediated photodynamic attack, using the carbonylic sensitizer Perinaphthenone. Overall and reactive rate constants for the O2(1Dg)-quenching (in the order of 108 and 107/M/s, respectively), and rates of oxygen consumption determined by time-resolved, spectroscopic and polarographic methods indicate that a- and b-chymotrypsins are less photooxidizable at pH 6, as a result of an enhancement of the O2(1Dg)-physical quenching component. In general terms, b-chymotrypsin exhibits the greater overall proclivity to interact with O2(1Dg), whereas structural factors, possibly evidenced by a higher exposure of the reactive tryptophan residues, impart an increased photooxidation degree to the proteins at pH 8, specially to the a-chymotrypsin.