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HETEROGENEITY IN THE THERMALLY‐INDUCED QUENCHING OF THE PHOSPHORESCENCE OF MULTI‐TRYPTOPHAN PROTEINS

Abstract— The steady‐state intensity and lifetime of the tryptophan phosphorescence from a number of globular proteins in 2:1 (v/v) glycerol buffer were monitored as a function of temperature. The phosphorescence lifetimes are essentially independent of the tryptophan local environment in rigid solution at temperatures < 170K, but vary markedly between proteins at temperatures at which the solutions become fluid. The ratio of steady‐state intensity to lifetime P/τ was found to be temperature independent despite the quenching for free tryptophan and the lone residue in myelin basic protein. Heterogeneity in the triplet quenching of the tryptophans in liver alcohol dehydrogenase and alkaline phosphatase were revealed as step‐like decreases in the ratio of P/T followed by plateau regions characterizing the homogeneous behavior of the more resistent tryptophans in the proteins. This heterogeneity exists not only between solvent‐exposed and buried residues, but reflects variations in the flexibility of the structure surrounding distinct buried tryptophans in the globular proteins.