title: Relaxation and quenching of the excited states of tryptophan in keratin.
abstract: 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.
authors.affiliation: DSIR Physical Sciences, Lower Hutt, New Zealand.
authors.forename: G J
authors.initials: GJ
authors.lastname: Smith
authors.forename: W H
authors.initials: WH
authors.lastname: Melhuish
mesh_terms: D000818:Animals; D007633:Keratins / Q000737:chemistry*; D008163:Luminescent Measurements; D011789:Quantum Theory; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D014364:Tryptophan; D014935:Wool
chemical_list: D007633:Keratins; D014364:Tryptophan