TRIPLET-SINGLET ENERGY TRANSFER IN PROTEINS

Triplet-singlet energy transfer was observed in a protein-chromophore complex. The system studied was a complex of proflavin and α-chymotrypsin. The tryptophan residues of chymotrypsin were the triplet energy donors, while proflavin bound at the active site served as the singlet acceptor. The occurrence of tripletsinglet energy transfer was revealed by a delayed fluorescence from bound proflavin and selective quenching of the tryptophan phosphorescence. The over-all transfer efficiency was higher than 80%. The kinetics of the delayed proflavin fluorescence and of the residual tryptophan phosphorescence revealed that there are at least two classes of tryptophan residues and that their rate constants for triplet-singlet transfer are 20 and 2.2 sec-1. Our observations suggest that triplet-singlet transfer can serve as a useful adjunct to singlet-singlet transfer inducing proximity relationships in the 15-60-Å range in biological macromolecules. Furthermore, triplet-singlet transfer can aid in elucidating processes involving the singlet and triplet excited states of proteins. The significant finding in this regard is that the major pathway from the excited singlet level of tryptophan residues in chymotrypsin is internal conversion rather than intersystem crossing. © 1969, American Chemical Society. All rights reserved.