Chemical Shifts Provide Fold Populations and Register of β Hairpins and β Sheets

A detailed analysis of peptide backbone amide (HN) and Hα chemical shifts reveals a consistent pattern for β hairpins and three-stranded β sheets. The Hα's at non-hydrogen-bonded strand positions are inwardly directed and shifted downfield ∼1 ppm due largely to an anisotropy contribution from the cross-strand amide function. The secondary structure associated Hα shift deviations for the H-bonded strand positions are also positive but much smaller (0.1-0.3 ppm) and the turn residues display negative Hα chemical shift deviations (CSDs). The pattern of (HN) shift deviations is an even better indicator of both hairpin formation and register, with the cross-strand H-bonded sites shifted downfield (also by ∼1 ppm) and with diagnostic values for the first turn residue and the first strand position following the turn. These empirical observations, initially made for [2:2]/[2:4]-type-I' and -II' hairpins, are rationalized and can be extended to the analysis of other turns, hairpin classes ([3:5], [4:4]/[4:6]), and three-stranded peptide β-sheet models. The Hα's at non-hydrogen-bonded sites and (HN)'s in the intervening H-bonded sites provide the largest and most dependable measures of hairpin structuring and can be used for melting studies; however the intrinsic temperature dependence of (HN) shifts deviations needs to reflect the extent of solvent sequestration in the folded state. Several observations made in the course of this study provide insights into β-sheet folding mechanisms: (1) The magnitude of the (HN) shifts suggests that the cross-strand H-bonds in peptide hairpins are as short as those in protein β sheets. (2) Even L-Pro-Gly turns, which are frequently used in unfolded controls for hairpin peptides, can support hairpin populations in aqueous fluoroalcohol media. (3) The good correlation between hairpin population estimates from cross-strand H-bonded (HN) shift deviations, Hα shift deviations, and structuring shifts at the turn locus implies that hairpin folding transitions approximate two-state behavior