Comparison of Isotopic Substitution Methods for Equilibrium and T-Jump Infrared Studies of β-Hairpin Peptide Conformation

Laser induced temperature jump (T-jump) relaxation kinetics were measured with infrared absorbance (IR) detection for a set of beta-hairpin peptides, related to the Trpzip2 hairpin, but containing single isotopic labels, C-13 on the amide C = O of selected residues both in the center of the strands and at the terminal regions of the hairpin. Variations in the behavior of single labeled peptides are compared to those previously reported for double labeled variants. Although single labels do not result in spectral intensity enhancement, as seen for cross-strand labeling, the IR frequency shifts are still diagnostic of hairpin unfolding. If C = O's in the beta-strand portion of the hairpin (between the Trp residues) are labeled, the dynamic behavior of the local modes is similar to the results obtained with double labels in terms of relaxation time and activation energy and closely tracks the kinetics of the beta-strand components. This implies that either property, local secondary structure (change of phi,psi), or cross-strand coupling enabled by strand formation and H-bonding relaxes with the same kinetic mechanism. Single labeled residues on the terminal positions have a different behavior and are less able to be detected due to overlap with the C-12 components, in contrast to double labels involving these positions, which are enhanced due to coupling. DFT-based spectral simulations that use the NMR structure of Trpzip2C indicate that the single labeled peptides should have roughly equivalent C-12 bands but the C-13 mode frequencies will vary with sequence position. Effective solvent corrections using COSMO yield significant changes in the frequencies but not in the relative isotope shifts obtained in our calculated spectra. Sequence positional dependence of labels is shown to be more discriminatory for kinetics changes than for thermodynamic variations.