Selective Excitation of the L State of Tryptophan in Collagen-like Peptides Can Reveal the Formation of a Heterotrimer.

Fluorescence emission from tryptophan residues has been often used to probe the protein structure due to its transition dipole moment sensitivity to the local environment. We report the fluorescence study of a collagen-like peptide heterotrimer modified with the tryptophan in the X position (X-Y-Gly) that shows the diminished fluorescence in a homotrimer versus a heterotrimer when the L state is selectively excited. This behavior is only observed in folded peptides, below the helix-to-coil transition temperature, and can be explained by long-range interactions between the tryptophans located on different strands within the triple helix, not by the change in the local environment. Our results suggest that tryptophan homotransfer is possible at distances much longer than the (0.5-0.7 nm) previously estimated. These observations imply that the energy transfer between the L states of proximal tryptophans can be facilitated by constraining their rotation by the helix and, thus, can be employed as a reporter of heterotrimer formation in biosensors.