The thermal denaturation of nonpolymerizable αα‐tropomyosin and its segments as a function of ionic strength

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Abstract
Nonpolymerizable tropomyosin (NPTm) is found to unfold thermally at high ionic strength almost exactly as the parent protein, but it does not aggregate at low ionic strength. Thus, NPTm can be used as a tropomyosin surrogate whose coiled‐coil structural stability can be probed by varying the ionic strength. Studies of NPTm by CD show that increasing ionic strength stabilizes the coiled‐coil structure. CD spectra over a wide range of helix content, obtained by varying either temperature or ionic strength, show an isodichroic point at 203 nm, suggesting a local, residue‐level, two‐state model. At given temperature, such a local helix ⇆ random equilibrium suggests In [Φh/(l − Φh)] = A1 + A2In, wherein Φh is the fraction helix, and A1, A2 and n are constants. In the low ionic strength region, theoretical limiting laws for ionic strength mediated charge–charge, dipole–dipole, and apolar–apolar (salting out) interactions give, respectively, n = 0.5, 1.0, and 1.0. Our experimental values for 40°C, where the data span a wide range of helix content, show n = 1.0, suggesting that ionic strength stabilizes either by reducing dipole–dipole repulsions or by enhancing hydrophobic interactions, both probably interhelix in nature. Two segments of tropomyosin, 11Tm127 and 142Tm281, neither of which aggregate at low ionic strength, give results similar to those for NPTm, i.e., n = 0.96 and 0.84, respectively.