DIKETONES AS PHOTOSENSITIZING AGENTS: APPLICATION TO αLAMINO ACIDS and ENZYMES

Abstract— 2,3-Butanedione (biacetyl), phenylglyoxal, 1,2-cyclohexanedione and similar carbonyl compounds have been used as specific arginyl probes in enzyme chemistry. Depending on irradiation conditions, these and other related carbonyl compounds may, however, cause extensive changes in the structure of an enzyme regardless of the essentiality or unessentiality of arginyl residue for enzyme activity. Particularly biacetyl and 2,3-pentanedione are powerful and general photoinactivators of enzymes in the visible region. A striking result of this process is the virtually total, time-dependent loss of tryptophan fluorescence of the enzyme, which is accompanied by other irreversible structural changes. Several ketones and ketone aldehydes photoinactivate enzymes in UV light as well; phenylglyoxal and 1,2-cyclohexanedione effectively inactivated enzymes at 254 nm and in the far UV region. In most cases, these photosensitized inactivations of an enzyme can be prevented by keeping the reaction mixture in anoxic conditions or adding singlet oxygen quenchers to the reaction mixture. 2-Thiol-L-histidine, 3-methyl-L-histidine and other imidazole derivatives are more effective protectors of enzymes against photodestruction than N_3-.