Molecular Theory of the Helix–Coil Transition in Polyamino Acids. III. Evaluation and Analysis of s and σ for Polyglycine and Poly-l-alanine in Water

Abstract

Previously, the Zimm–Bragg parameters s and σ for the helix–coil transition in polyglycine and poly-l-alanine were calculated in terms of molecular quantities. These calculations are extended here to take into account the effects of water as the solvent and are analyzed to deduce the relative importance of the various interaction terms for the helix–coil transition. The screening effect of the electrostatic interactions by the water molecules which lie between or near two charged atoms is taken into account by using a dielectric constant D = 4.0 for short-range interactions, and by cutting off all interactions of longer range than about 6.0 Å. The calculated values of the parameter σ, which are very sensitive to the value of D, agree well with experimental ones. The origin of the greater stability of the α-helical conformation of poly-l-alanine compared to that of polyglycine is analyzed. The effect on the value of the parameter s, arising from the binding of water molecules to free CO and NH groups of residues in the coil state, is considered. Parameters which express the strength of the binding of water molecules are adjusted so as to make the calculated values of s for poly-l-alanine in water as a function of temperature fit best with experimental values. When the same parameters are used in the calculation of s for polyglycine in water, the theory predicts that polyglycine should be in the random coil form in water in the temperature range of 0–100°C, which agrees with experiment. The values obtained for these parameters are compared with experimental data for binding water molecules to a small-model peptide molecule. Finally, a simple but realistic model of the helix–coil transition, deduced from the calculation of s and σ in terms of molecular quantities, is proposed as a first approximation not only for homopolymers but also for co-polymers.