Principles of Protein Structure - G. Schultz 1982

Statistical Mechanics of the Helix-Coil Transition
Comparison with Experimental Data: Helix-Coil Transition Curves and the Zimm-Bragg Model
Transition Temperature and Helix Propensity

Transition temperatures of synthetic polymers are converted into relative statistical weights at a given Temperature. The Relationship between s, T, and ∆H near the transition point is shown in Fig. A.2. To a first approximation, it is linear (Sec. A.4, p. 300). Let us compare these relationships for two different homopolymers with enthalpy differences ∆ H(¹) and ∆H(²⁾ and transition temperatures θ(¹) and θ(²). If The values of ∆H and θ are sufficiently close, both linear relationships can be combined on a single plot (Fig. A.2). The plot can be used to obtain the relative statistical weights s(¹) and s(²) for both types of homopolymers at a given temperature T₀. The statistical weights reflect the propensity of the polymers for a helical conformation (equations (A. 18) and (A.4)) at the given temperature.

Typically, the value of ∆H is unknown. However, it can be estimated from the corresponding values of θ using equations (A. 19). Thus, whenever θ can be measured for a homopolymer of a given residue*, its value can be used to roughly estimate the propensity for helical conformation.