Conformational exchange on the microsecond time scale in α-helix and β-hairpin peptides measured by ¹³C NMR transverse relaxation

¹³C-NMR relaxation experiments (T₁, T₂, T_1ρ, and NOE) were performed on selectively enriched residues in two peptides, one hydrophobic staple α-helix-forming peptide GFSKAELAKARAAKRGGY and one β-hairpin-forming peptide RGITVNGKTYGR, in water and in water/trifluoroethanol (TFE). Exchange contributions, R_ex, to spin-spin relaxation rates for ¹³C_α and ¹³C_β groups were derived and were ascribed to be mainly due to peptide folding-unfolding. To evaluate the exchange time, τ_ex, from R_ex, the chemical shift difference between folded and unfolded states, Δδ, and the populations of these states, p_i, were determined from the temperature dependence of ¹³C chemical shifts. For both peptides, values for τ_ex fell in the 1 μs to 10 μs range. Under conditions where the peptides are most folded (water/TFE, 5°C), τ_ex values for all residues in each respective peptide were essentially the same, supporting the presence of a global folding-unfolding exchange process. Rounded-up average τ_ex values were 4 μs for the helix peptide and 9 μs for the hairpin peptide. This 2-3-fold difference in exchange times between helix and hairpin peptides is consistent with that observed for folding-unfolding of other small peptides.