The effect of sulfhydryl blocking groups on the thermal unfolding of alpha alpha tropomyosin coiled coils.

Equilibrium thermal unfolding curves from circular dichroism are given for alpha alpha tropomyosin and for alpha alpha tropomyosin blocked at C190 by a) carboxyamidomethylation; b) carboxymethylation. Although commonly assumed to be benign, these blocks in fact produce some weakening. All three substances are virtually completely alpha-helical at low T. Fraction helix vs T for parent protein is apparently monophasic (single inflection point). The curve for carboxyamidomethylated protein is very close to that of the parent, but is biphasic, with a small "pretransition". The curve for carboxymethylated protein is prominently biphasic, with a much larger pretransition. Some implications for the molecular model of these equilibria are discussed.