Studies on the Denaturation of Tropomyosin and Light Meromyosin

Optical rotatory dispersion has been used to study the heat denaturation of tropomyosin and light meromyosin fraction 1 (LMMFr I) as a function of pH and salt concentration. The unfolding of these highly a‐helical molecules takes place in stages indicating regions of varying stability along the molecule. At all pH values NaCl and KCl increase the stability and this is attributed to electrostatic screening of charged groups. There is a decrease in stability to heat at alkaline pH values and the molecules are most stable on the acid side of the iso‐electric point. Guanidine hydrochloride is more effective than urea as a denaturant. In the case of tropomyosin almost complete reversibility of the optical rotation changes by these denaturants can be achieved provided the ‐SH groups are prevented from forming disulphide bonds. For LMMFr I the rotation changes and reversibility are complicated by depolymerization into smaller units. Molecular weight determinations by sedimentation equilibrium indicate a critical urea concentration for depolymerization which is not evident from the optical rotation changes. Although guanidine hydrochloride is more effective than urea in abolishing the helical structure of LMM Fr 1, it is not as effective as a depolymerizing agent.