Isotopically Site-Selected Dynamics of a Three-Stranded β-Sheet Peptide Detected with Temperature-Jump Infrared-Spectroscopy.

Infrared detected temperature-jump (T-jump) spectroscopy and site-specific isotopic labeling were applied to study a model three-stranded β-sheet peptide with the goal of individually probing the dynamics of strand and turn structural elements. This peptide had two Pro-Gly (pG) turn sequences to stabilize the two component hairpins, which were labeled with C═O on each of the Gly residues to resolve them spectroscopically. Labeling the second turn on the amide preceding the Pro (Xxx-Pro amide) provided an alternate turn label as a control. Placing C═O labels on specific in-strand residues gave shifted modes that overlap the Xxx-Pro amide I' modes. Their impact could be separated from the turn dynamics by a novel difference transient analysis approach. Fourier-transform infrared spectra were modeled with density functional theory-computations which showed the local, isotope-selected vibrations were effectively uncoupled from the other amide I modes. Our T-jump dynamics results, combined with nuclear magnetic resonance structures and equilibrium spectral measurements, showed the first turn to be most stable and best formed with the slowest dynamics, whereas the second turn and first strand (N-terminus) had similar dynamics, and the third strand (C-terminus) had the fastest dynamics and was the least structured. The relative dynamics of the strands, Xxx-Pro amides, and C-labeled Gly residues on the turns also qualitatively corresponded to molecular dynamics (MD) simulations of turn and strand fluctuations. MD trajectories indicated the turns to be bistable, with the first turn being Type I' and the second turn flipping from I' to II'. The differences in relaxation times for each turn and the separate strands revealed that the folding process of this turn-stabilized β-sheet structure proceeds in a multistep process.