A theoretical model simulating the anomalous concentration dependence of the equilibrium thermal unfolding curve of noncrosslinked tropomyosin

Thermal unfolding curves of tropomyosin have so far been fit only semi-quantitatively by the statistical-mechanical theory of the helix-coil transition. The calculated values of helix content are a bit too small for the most dilute solutions and a bit too large for the most concentrated ones. The theory, as hitherto used, assumes a uniform helix-helix interaction, whereas evidence from studies on molecular segments suggests otherwise. A theoretical model incorporating such non-uniformity in helix-helix interaction is used to produce simulated thermal unfolding curves. These simulated curves, when fit to the theory using the assumption of uniformity, reveal precisely the same discrepancies seen with the experimental data. We conclude that non-uniformity in helix-helix interaction along the tropomyosin molecule is responsible for the small discrepancy between experimental data and the uniform-model theory previously employed.