Time-resolved temperature-jump infrared spectroscopy of peptides with well-defined secondary structure: A Trpzip β-hairpin variant as an example

Peptide folding dynamics were studied by time-resolved infrared spectroscopy after initiation with a nanosecond laser-excited temperature-jump. Relaxation kinetics are monitored using amide I′ absorption changes following rapid heating of the solvent by a Raman shifted Nd:YAG laser pulse. Lead salt laser diodes provide a tuneable IR source in the amide I′ region between 1600 cm−1 and 1700 cm−1 for probing structural unfolding at single wavelengths. The probe wavelengths are selected by using FTIR measurements that have been performed under thermal equilibrium conditions and indicate the amide I′ wavenumbers with the most significant absorption changes. Temperature-dependent spectral changes of water were separated from the spectral variation due to structural changes of the peptide, both in the equilibrium measurements and in the kinetic T-jump data. Polyglutamic acid has been used as a test system to demonstrate the capability of our instrument for acquiring microsecond peptide dynamics. In this work, we studied the relaxation dynamics of a 12-mer tryptophan zipper (Trpzip) peptide, a Trpzip2 variant, that adopts a stable β-hairpin structure in aqueous solution. Rate constants are a few microseconds depending on the sample temperature and are compared with published data indicating characteristics between those of Trpzip1 and Trpzip2.