Temperature-dependent optical rotatory dispersion properties of helical muscle proteins and homopolymers

Thermally induced helix–coil transitions of myosin rod, light meromyosin, and tropomyosin were studied by optical rotatory dispersion (ORD). Fractional helicity was calculated from both the Moffitt‐Yang parameter, b0, and the corrected mean residue rotation [m′] at 231.4 nm. Between 3 and 30°C, [m′] increases linearly with a slope of 59/°C, whereas b0 is virtually constant, indicating apparently different thermal melting behavior. Poly(L‐lysine) and poly(L‐glutamic acid) in their helical forms and myoglobin also show a nearly linear temperature dependence of [m′]231.4. Muscle proteins in 6M guanidine hydrochloride and the random‐coil forms of the homopolymers exhibit temperature‐dependent values of [m′]231.4 and b0. We conclude from these observations that ORD properties of both α‐helices and random‐coil polypeptides have significant intrinsic temperature dependencies. A new method of estimating fractional helicity as a function of temperature is proposed.