SOLVENT EFFECTS ON THE TAUTOMERIZATION OF 5'-DEOXYPYRIDOXAL - A PHOTOPHYSICAL STUDY

Absorption and fluorescence spectra of 5'-deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range 10-65-degrees-C. The areas under the absorption bands were analyzed to obtain the mole fraction (f(N), f(Z)) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (DELTAnu), fluorescence quantum yield of the neutral form (Q(N)), fluorescence ratio of the neutral to the zwitterionic form (phi(N)/phi(Z)) and the rate constant of tautomerization (k(l)) from Z to N in the excited state. Some of these parameters (f(N), DELTAnu, Q(N), k(l)) were found to depend on the proton donor character of the solvent, whereas others (phi(N)/phi(Z)) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of -OH groups on its environment.