Conformations and Dynamics of Polypeptide Chains Revealed By Tryptophan-Cysteine Contact Formation Kinetics

Abstract

Spectroscopic probes sensitive to intra-chain contact formation events in polypeptides are increasingly used to study the conformational and dynamical properties of different amino acid sequences. Quenching of the triplet state of tryptophan by close contact with cysteine enables the measure of contact formation rates without the need of extrinsic probes, thus being suitable for the study of natural proteins and peptides. We illustrate the use of this method to investigate the unfolded state of small proteins in conditions close to native and the kinetics of weakly structured protein fragments. The coexistence of different conformational states can be revealed from the non-exponential relaxation of the excited triplet, enabling the characterization of both the chain dynamics for each state and the transition kinetics. Moreover, the rate of contact formation measured for the least structured states is compared with those observed for model disordered peptides, allowing to estimate the strength of electrostatic and hydrophobic interactions between residues other than the probes. We test this approach with the widely studied GB1 15-residue C-terminal, which folds into a beta-hairpin structure. The kinetics of elementary conformational steps leading to the folded state is outlined, revealing the presence of misfolded states as proposed in recent computational works.