| Abstract: | A theoretical study of effects of excluded volume intermolecular interactions on the sharpness of helix–coil transitions in solutions of polyamino acids or simple proteins indicates that the transition width may vary appreciably as a function of polymer concentration. The analysis is based on a second virial approximation for the excess free energy of mixing of a solution of polymers of varying degrees of helicity. The virial coefficients involved are roughly estimated on the basis of gross polymer geometry. For large N (degree of polymerization) the transition is found, typically to sharpen with increasing concentration, becoming second order and then first order at sufficiently high concentrations. The critical polymer concentration is found to be roughly of the order N?1.2 ??0?1 for an “all or none” model and of order σ1/2 N?0.2 ??0?1 for a model with continuously variable degree of helicity (??0 is the volume of a single helical molecule and σ1/2 the normalized statistical weight of a helix–coil interface). In the second case for N ～ 103 and σ ～ 10?2–10?4, the predicted critical concentration is in the range 10?1–10?3 g/cm.3 Comparison is made with experiments on solutions of poly(γ-benzyl-L glutamate). |
