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 αα tropomyosin and for αα 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 α-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. © 1990.