SUBSTITUENT AND SOLVENT EFFECTS ON THE PHOTOTAUTOMERISM OF ALLOXAZINES

Some selected results of semiempirical calculations (AM1, INDO/S-CI-1, Modified INDO) related to observed in the presence of acetic acid all oxazine-isoalloxazine phototautomerism are demonstrated. Good correlat ion of experimentally observed phototautomeric efficiencies with calcu lated electron density changes on N-1 and N-10 atoms are shown for sev eral methyl- and cyanoalloxazines. Strong solvent- induced charge redi stribution between both nitrogens is correctly predicted theoretically as being a cause of apparent instability of the isoalloxazinic tautom er in its ground state. The experimental study revealed also the compl ex dependence of phototautomeric effect on: (i) polarity of the medium expressed by Onsager function and (ii) solvent-acetic acid interactio n reflected by the equisolvation points.