Triplet-triplet energy transfer in alpha-trypsin.

Experiments are reported that demonstrate that light absorbed by ionized tyrosinyl sensitizes the phosphorescence of tryptophanyl residues of native alpha-trypsin. The sensitization effect is abolished when alpha-trypsin is unfolded in guanidine hydrochloride. Under the experimental conditions used, the tryptophan phosphorescence could only have been induced by an electron-exchange interaction. These results, therefore, require that there be at least one ionized tyrosinyl-tryptophanyl pair in the native enzyme and that the distance between the two side chains be sufficiently short to permit electron exchange.