Melting of a beta-hairpin peptide using isotope-edited 2D IR spectroscopy and simulations

Isotope-edited two-dimensional infrared spectroscopy has been used to characterize the conformational heterogeneity of the beta-hairpin peptide TrpZip2 (TZ2) across its thermal unfolding transition. Four isotopologues were synthesized to probe hydrogen bonding and solvent exposure of the beta-turn (K8), the N-terminus (S1), and the midstrand region (T10 and T3T10). Isotope-shifts, 2D lineshapes, and other spectral changes to the amide I 2D IR spectra of labeled TZ2 isotopologues were observed as a function of temperature. Data were interpreted on the basis of structure-based spectroscopic modeling of conformers obtained from extensive molecular dynamics simulations. The K8 spectra reveal two unique turn geometries, the type I' beta-turn observed in the NMR structure, and a less populated disordered or bulged loop. The data indicate that structures at low temperature resemble the folded NMR structure with typical cross-strand hydrogen bonds, although with a subpopulation of misformed turns. As the temperature is raised from 25 to 85 degrees C, the fraction of population with a type I' turn increases, but the termini also fray. Hydrogen bonding contacts in the midstrand region remain at all temperatures although with increasing thermal disorder. Our data show no evidence of an extended chain or random coil state for the TZ2 peptide at any temperature. The methods demonstrated here offer an approach to characterizing conformational variation within the folded or unfolded states of proteins and peptides.