Alpha-helix to random coil transitions of two-chain coiled coils: experiments on the thermal denaturation of isolated segments of alpha alpha-tropomyosin.

Circular dichroism (CD) experiments in the backbone (200-240 nm) region are reported for four isolated, excised two-chain, coiled-coil segments whose chains comprise, respectively, residues 11-127, 142-281, 1-189, and 190-284 of the rabbit alpha alpha-tropomyosin (Tm) sequence. The uv and CD spectra for the noncross-linked segments are very similar to those for parent Tm. At 3 degrees C, all have a helix content of 90% or more; moreover, all thermal denaturation curves depend on concentration, as required by mass action, and are completely reversible. At comparable concentrations, solutions show values of T1/2 (the temperature at which the helix content is 50%) following the order of 11Tm127 approximately 1Tm189 greater than 142Tm281 greater than 190Tm284. The thermal unfolding data for 11Tm127, 190Tm284, and 142Tm281 fall on apparently monophasic curves (single inflection point). However, curves for 1Tm189 show a heretofore unknown low temperature transition in which the helix content drops from approximately 90% at 2 degrees C to approximately 73% at 20 degrees C, indicating that this segment has one or more weak sections totaling approximately 50 residues per chain. Since thermal denaturation curves for noncross-linked 11Tm127, 142Tm281, and Tm have no such low temperature transition, i.e., the helix content is not additive, the weak region probably comprises the bulk of the residues between 127 and 189 in 1Tm189, but is somehow stabilized in 142Tm281 and in parent Tm.(ABSTRACT TRUNCATED AT 250 WORDS)