A calorimetric study of the folding-unfolding of an α-helix with covalently closed N and C-terminal loops

The thermal melting of a dicyclic 29-residue peptide, having helix-stabilizing side-chain to side-chain covalent links at each terminal, has been studied by circular dichroism spectropolarimetry (CD) and differential scanning calorimetry (DSC). The CD spectra for this dicyclic peptide indicate that it is monomeric, almost fully α-helical at -10°C, and undergoes a reversible transition from the folded to the disordered state with increasing temperature. The temperature dependencies of the ellipticity at 222 nm and the excess heat capacity measured calorimetrically are well fit by a two-state model, which indicates a cooperative melting transition that is complete within the temperature ranges of these experiments (from -10°C to 100°C). This allows a complete analysis of the thermodynamics of helix formation. The helix unfolding is found to proceed with a small positive heat-capacity increment, consistent with the solvation of some non-polar groups upon helix unfolding. It follows that the hydrogen bonds are not the only factors responsible for the formation of the α-helix, and that hydrophobic interactions are also playing a role in its stabilization. At 30°C, the calorimetric enthalpy and entropy values are estimated to be 650(± 50) cal mol^-1 and 2.0(± 0.2) cal K^-1 mole^-1 respectively, per residue of this peptide. Comparison with the thermodynamic characteristics obtained for the unfolding of double-stranded α-helical coiled-coils shows that at that temperature the enthalpic contribution of non-polar groups to the stabilization of the α-helix is insignificant and the estimated transition enthalpy can be assigned to the hydrogen bonds. With increasing temperature, the increasing magnitude of the negative enthalpy of hydration of the exposed polar groups should decrease the helix-stabilizing enthalpy of the backbone hydrogen bonds. However, the helix-stabilizing negative entropy of hydration of these groups should also increase in magnitude with increasing temperature, offsetting this effect.