SOLVENT AND METHYL SUBSTITUENT EFFECT ON PHOTOTAUTOMERISM AND IONIZATION OF ALLOXAZINES

Abstract— The absorption and emission spectra of alloxazine and its methyl derivatives have been measured in solvents of different polarity. From roughly linear correlation between two long wavelength absorption maxima and the emission maxima positions vs solvent polarities. systematic deviations have been observed for alloxazines in pyridine, acetic acid and water. The deviations depend on the methylation site of alloxazine in a way suggesting formation of hydrogen bond in the ground state and phototautomerism in the excited state. As phototautomerism of alloxazines in the presence of pyridine and water may involve ionic equilibria, the pKa values of alloxazines have been measured (8.11–8.58) and the pKB* estimated from the Förster cycle (2.3–6.6 for N‐1 monoanion and 7.4–7.8 for N‐3 monoanion). The phototautomeric efficiency of alloxazines in pyridine, acetic acid and water depend on the acidity of the N‐1 proton and on the steric effects of methyl substituents, mainly at C‐9. The anionic forms of alloxazines differ from the phototautomeric species in emission maxima positions, radiative lifetimes and they are dissimilarly influenced by the methylation site and medium viscosity. Copyright © 1979, Wiley Blackwell. All rights reserved