Thermodynamic Analysis of β-Hairpin-Forming Peptides from the Thermal Dependence of1H NMR Chemical Shifts

Publisher Website
Google Scholar
Abstract
The temperature dependence of the 1H chemical shifts of six designed peptides previously shown to adopt β-hairpin structures in aqueous solution has been analyzed in terms of two-state (β-hairpin ⇆ coil) equilibrium. The stability of the β-hairpins formed by these peptides, as derived from their Tm (midpoint transition temperature) values, parallels in general their ability to adopt those structures as deduced from independent NMR parameters: NOEs, ΔδCαH, ΔδCα, and ΔδCβ values. The observed Tm values are dependent on the particular position within the β-hairpin that is probed, indicating that their folding to a β-hairpin conformation deviates from a “true" two-state transition. To obtain individual Tm values for each hairpin region in each peptide, a simplified model of a successive uncoupled two-state equilibrium covering the entire process has been applied. The distribution of Tm values obtained for the different β-hairpin regions (turn, strands, backbone, side chains) in the six analyzed peptides reveals a similar pattern. A model for β-hairpin folding is proposed on the basis of this pattern and the reasonable assumption that regions showing higher Tm values are the last ones to unfold and, presumably, the first to form. With this assumption, the analysis suggests that turn formation is the first event in β-hairpin folding. This is consistent with previous results on the essential role of the turn sequence in β-hairpin folding.