Infrared Study of the Stability and Folding Kinetics of a 15-Residue β-Hairpin

Abstract

The thermal stability and folding kinetics of a 15-residue β-hairpin (SESYINPDGTWTVTE) have been studied by using infrared (IR) spectroscopy coupled with laser-induced temperature-jump (T-jump) technique for rapid folding−unfolding initiation. An alternative method based on analyzing IR difference spectra was also introduced to obtain thermodynamic properties of β-sheets, which complements the commonly used circular dichroism (CD) and fluorescence techniques. Equilibrium IR measurements indicate that the thermal unfolding of this β-hairpin is fairly broad. However, it can be described by a two-state transition with a thermal melting temperature of ∼29 °C. Time-resolved IR measurements following a T-jump, probed at 1634 cm^-1, indicate that the folding of this β-hairpin follows first-order kinetics and is amazingly fast. At 300 K, the folding time is approximately 0.8 μs, which is only 2−3 times slower than that of α-helix formation. Additionally, the energetic barrier for folding is small (∼2 kcal mol^-1). These results, in conjunction with results from other studies, support a view that the details of native contacts play a dominant role in the kinetics of β-hairpin folding.