Dynamics of Intramolecular Contact Formation in Polypeptides: Distance Dependence of Quenching Rates in a Room-Temperature Glass

Quenching of the triplet state of tryptophan by cysteine is an important new tool for measuring the rate of forming a specific contact between amino acids in a polypeptide chain. To determine the length scale associated with this contact, tryptophan was embedded in a room-temperature glass containing a high concentration of cysteine. The decay of the triplet population is extended in time, consistent with a rate coefficient that decreases exponentially with distance. Solving the diffusion equation with this distant-dependent rate reproduces the observed bimolecular rates in water and shows that quenching at low viscosities takes place $\lesssim 1Å$ from van der Waals contact between the tryptophan and cysteine.