Relaxation and quenching of the excited states of tryptophan in keratin.

The temperature dependence of the fluorescence and phosphorescence originating from tryptophan residues in the solid protein keratin was studied. The position of the tryptophan fluorescence spectral maximum was measured as a function of temperature and was found to be substantially Stokes shifted even at 77 K. The activation energies for non-radiative relaxation of the singlet and triplet states of tryptophan in wool keratin, derived from Arrhenius plots of the fluorescence and phosphorescence yields, are 580 cm-1 and 995 cm-1 respectively. One possible fluorescence quenching mechanism which is consistent with these observations is a very effective, thermally induced mixing of the 1La and 1Lb excited singlet states which results in enhanced internal conversion apparent as a low fluorescent yield even at low temperatures. The phosphorescence decay from keratin in the absence of oxygen following excitation at 265 nm is non-single exponential. At room temperature it is dominated by a short-lived component with more than 80% of the decay completed within 3 microseconds of the flash. Longer living components with first-order lifetimes as long as 2-3 s were also observed.