Thermal unfolding of a beta hairpin studied with IR spectra enhanced by site-specific isotope labeling

Abstract

To model formation of beta-sheets in proteins it is essential to understand the interaction and dynamics between separate strands. The simplest model for such interactions are beta-hairpins, of which the TrpZip model of Cochran is one of the most stable in aqueous solution. We have modified the TrpZip2 sequence by substituting Ala on the 1,3, and 10 positions to destabilize the hairpin somewhat and to facilitate isotopic labeling. If C=O groups on opposite strands are labeled with 13C, their coupling can lead to distinctive amide I bands whose intensity, and sometimes frequency can reflect local cross-strand coupling, and thus hairpin formation at that position in the sequence. For a fully formed hairpin the A1A10 forms a labeled outer (14-atom) H-bonded ring and the A3K8 an inner one, while the A3A10 forms a central (10 atom) H-bonded ring. Equilibrium studies show the A1A10 to be less well formed (non-degenerate) than the A3K8, and the A3A10 has a different pattern with distinct disruptive effects on the 12C=O coupling. Laser excited T-jump measurements show fast relaxation kinetics that varies with wavelength probed and initial temperature. The differences in kinetics for different positions in the hairpin suggest a multistate process consistent with observations by others on related systems. Spectral simulations have been obtained using QM level force fields and structural results from extended MD calculations.