Time-Resolved Temperature-Jump Ir-Measurements on Beta-Hairpin Peptides with Alternate Cross-Strand Interactions

Abstract

The formation of β-sheets plays an important role in protein folding. We studied the folding dynamics of tryptophan zipper (Trpzip) peptides being ideal simple model systems of β-hairpins stabilized by the hydrophobic interactions of two cross-strand Trp-Trp pairs. Rapid heating of the solvent is induced by a Raman-shifted Nd:YAG pulse (ΔT ∼ 10°C in 10 ns) and ns-to-µs peptide dynamics were monitored at single wavelengths using a quantum cascade laser tunable in the amide I region. Our isotope-edited kinetics support a multistate dynamic behavior being consistent with a hydrophobic collapse hypothesis for the folding process of Trpzip2 β-hairpin variants. In order to analyze hydrophobic interactions, the tryptophans in the Trpzip2 sequence (SWTWENGKWTWK-NH2) were replaced pairwise by either valines or tyrosines. Equilibrium CD and FTIR measurements show lower transition temperatures for all mutants. While the Trp-Trp stabilized peptides have different relaxation rates for the loss of sheet and gain of disordered structure, our mutant studies indicate that the aromatic substitution of Trp→Tyr results in kinetics being faster than for Trpzip2, but essentially the same for the sheet and disordered components, with similar activation energies. However with Val substitutions, different relaxation kinetics were found. Our time-resolved temperature-jump measurements reveal the impact of cross-strand interactions on the folding mechanism of peptides, in particular illustrate different effects of hydrophobic versus aromatic interactions on stability and dynamics of structure formation.