A conformational study of porcine thyrocalcitonin.

The structure of porcine thyrocalcitonin has been evaluated by circular dichroism, optical rotatory dispersion, infrared spectroscopy, and fluorescence. The degree of helical structure was estimated by circular dichroism (222 mp) and optical rotatory dispersion (233 mp) at the n + r* transition of the Q: helix. The optical activity at this transition suggests that thyrocalcitonin contains approximately 10% a-helical structure in aqueous solution. The spectrum of thyrocalcitonin in 6 M guanidine was similar to that of a randomly coiled polypeptide. In Z-chloroethanol, thyrocalcitonin formed a structure containing approximately 50% (Y helix. The near ultraviolet circular dichroic spectrum of thyrocalcitonin revealed a major band at 288 rnp, indicating that the tryptophanyl chromophore had restricted rotational freedom. Reduction and alkylation of the amino-terminal heptapeptide ring of thyrocalcitonin produced no significant change in the circular dichroic or optical rotatory spectra. In addition, the effects of alkaline pH and temperature on tryptophanyl emission were similar to the unmodified hormone. These findings preclude a high degree of organized structure in the heptapeptide ring, which is in marked contrast to the aminoterminal hexapeptide ring of oxytocin. These studies indicate that porcine thyrocalcitonin exists predominately in a random coil in aqueous solution. The polypeptide, however, does not appear to possess a rigid conformation and a coil ti helix equilibrium may exist in water with guanidine shifting the equilibrium in favor of the coil and 2-chloroethanol in favor of the helix. The latter type of transition may occur at the receptor site of the hormone in lipid layers or cellular membranes where the water concentration is significantly reduced.