Enthalpy distribution for the alpha-helix/random coil transition in a model peptide: a study of two-state behavior

We use heat capacity data of Taylor et al. to calculate the enthalpy distribution of a model peptide using the moments/maximum-entropy method. The peptide was designed with small covalent loops at both ends of the molecule to nucleate alpha-helix thus giving a system that would be expected to show a helix-coil transition that is very close to being two state. If we subtract a background contribution from the heat capacity data, then the enthalpy distribution we obtain shows two distinct peaks representing helix and coil. The difference in the peak enthalpy values agrees closely with the DeltaH obtained from the two-state analysis. On the other hand, if we use the complete heat capacity without subtracting background we then obtain an enthalpy distribution that has only a single peak at all temperatures. We show that this result can be consistent with the existence of two states, helix and coil, but only if the range of variation of the enthalpy of each species is so large as to make the notion of a species fairly meaningless.