It has only recently been widely recognized that phosphorescence from tryptophan can be observed in proteins at room temperature. In combination with fluorescence, dynamics processes in proteins can be measured on time scales from picoseconds to seconds. By virtue of the long lifetime of the triplet excited state, it is reactive to many molecules. We have characterized some of these molecules and have suggested that an electron transfer or exchange is the primary mechanism of quenching. A perplexing feature of tryptophan triplet emission at room temperature is that the lifetime is so variable from protein to protein. Perhaps an electron transfer reaction to a moiety internal to the protein is responsible for this variability. Conformational changes in the protein might also affect this electron transfer. The dependence of the transient absorption and emission of tryptophan on the extent of protein (parvalbumin) denaturation has been investigated. This work has been published (Biochemistry 34, 1355 (1995)).
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