On How a Myosin Tryptophan May be Perturbed

A well-known indication that a nucleotide has bound to myosin is the enhancement of the fluorescence of a specific tryptophan in the "subfragment 1" segment of the protein. Empirically the effect has been enormously useful in myosin enzymology. But beyond an early suggestion that it arises from a purine-tryptophan charge-transfer complex, the mechanism of the effect has not been considered. Here we consider the alternative that it arises from an ionizable group (either another residue or the phosphate of the nucleotide) whose proximity to the tryptophan is altered by substrate binding. We study this possibility by studying the interaction of an ionizable residue and tryptophan when both are incorporated in a diketopiperazine structure. The geometry of the situation is inferred from molecular mechanics simulations. Unexpectedly, the best explanation seems to be that the field of the imposed charge, acting across space, affects events in the excited state of the indole.