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INFLUENCE OF THE PEPTIDE BOND ON THE SINGLET MOLECULAR OXYGEN‐MEDIATED (O 2 [ g ]) PHOTOOXIDATION OF HISTIDINE and METHIONINE DIPEPTIDES. A KINETIC STUDY
Author(s) -
Miskoski Sandra,
García Norman A.
Publication year - 1993
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1993.tb02317.x
Subject(s) - chemistry , quenching (fluorescence) , reaction rate constant , acetonitrile , singlet oxygen , photochemistry , phosphorescence , histidine , solvent , singlet state , kinetics , excited state , oxygen , fluorescence , organic chemistry , enzyme , physics , quantum mechanics , nuclear physics
The dye‐sensitized photooxidation of l ‐histidine (His) and l ‐methionine (Met) and their simplest dipeptides with glycine (Gly) (His‐Gly, Gly‐His, Gly‐Met) and Met‐methyl ester (Met‐ME) mediated by singlet molecular oxygen (O 2 [ g ]) was studied. The overall rate constants in acetonitrile‐H 2 O ( K t ) for O 2 ( 1 g ) quenching were measured by time‐resolved phosphorescence detection. In H 2 O a competitive kinetic method was employed. In both solvents the reactive rate constants ( K t ,) were determined to discriminate between the overall and physical contributions to the quenching. The kinetic and mechanistic aspects of the interaction are discussed. For His‐Gly, the peptide bond has practically no effect on the kinetics of photooxidation. For Gly‐His the overall rate constant is much higher than that for His and His‐Gly, in both H 2 O and acetonitrile‐H 2 O. The main contribution to k 1 (for Gly‐His) is the physical quenching of O 2 ( 1 g )‐ In water the k t /k r ratio for free His and His‐Gly is 1.0, reaching a value of 2.0 in the organic solvent‐H 2 O mixture. The rates of‐NH 2 loss upon sensitized photooxidation in all cases parallel the trend of k r values. The main results for the His series indicate that: (1) a polar environment favors autoprotection ( i.e. an increase in the contribution of physical quenching) against photodynamic effects; (2) only the rate constant for reactive interaction with O 2 [ g ] does not depend on the location of the peptide bond involving His. For Met derivatives the k t , values are higher in both solvents than that for free Met. Only for the free amino acid in H 2 O is the interaction with O 2 ( 1 g ) totally reactive. For Gly‐Met and Met‐ME the physical quenching prevails: k t , is, in both solvents, about one order of magnitude higher than k r . According to our results on ‐NH 2 loss and on the basis of previous investigations by others, the photooxidative products distribution in the Met series indicates that Gly‐Met yields only dehydroMet, whereas Met and Met‐ME produce a mixture of Met‐sulfoxide and the Met‐dehydro compound.