Tryptophan phosphorescence at room temperature as a tool to study protein structure and dynamics.

Fluorescence and phosphorescence resemble each other and in many ways can give the same type of information. Both originate from a dipolar interaction between light and the molecule. In this regard, both are polarized and subject to the same type of quenching phenomena. In other respects the information which they divulge are complementary. The fluorescence quantum yield is higher for exposed tryptophans and this is expressed in longer lifetime (Grinvald and Steinberg, 1976); in contrast long lifetime of phosphorescence appears to correlate with burial. Phosphorescence, spin-disallowed, is much longer lived than fluorescence. This allows the structural/dynamic characterization of proteins to be studied on a new time regime. A really remarkable finding of studies of protein phosphorescence is that there is such variability both in phosphorescence lifetime and quenchability. We would interpret this to indicate that the tryptophan environment can range from essentially a crystal, almost comparable in rigidity as found at 77 K, to tryptophans in a flexible environment, almost as flexible as free in solution. An interesting task will be to examine the relationship between the yield and lifetime of phosphorescence and details of the tryptophan environment in terms of rigidity and adjacent amino acids among the proteins with known three dimensional structure.