THERMAL-STABILITY OF THE TRIPLE HELIX OF TYPE-I PROCOLLAGEN AND COLLAGEN - PRECAUTIONS FOR MINIMIZING ULTRAVIOLET DAMAGE TO PROTEINS DURING CIRCULAR-DICHROISM STUDIES

The helical stability of type I procollagen and type I collagen was examined by circular dichroism (CD) under a variety of conditions. The temperature at midpoint of the helix-to-coil transition (Tm) of both collagen and procollagen was reduced by prolonged exposure of the proteins to the ultraviolet beam of the CD instrument. The reduction in Tm was apparently explained by random scission of the polypeptide chains, since peptide fragments of varying size were generated by the ultraviolet irradiation. Although the phenomenon was not explored in detail, the observations raised the possibility that ultraviolet damage may be encountered in CD studies on other proteins. With collagen and procollagen irradiation damage was minimized by interrupting the ultraviolet beam or carrying out the thermal transition rapidly. The same Tm values were obtained by increasing the temperature at rates varying from 0.8 to 65°C/h. When the effects of ultraviolet irradiation were minimized, it was possible to examine the thermal transitions of procollagen and collagen in neutral buffer and at concentrations low enough to avoid precipitation. Under these conditions the Tm of type I collagen from chick embryo tendons was 42 °C. Type I procollagen from the same source had the same Tm. The results demonstrated therefore that the propeptides found at both the N and C terminals of type I procollagen have no effect on the thermal stability of a triple-helical domain of the molecule. Partially purified procollagen from chick embryo fibroblasts had the same Tm as purified procollagen from the same source, an observation which does not support the hypothesis that the protein is secreted as an aggregate in which lateral association of monomers increases their thermal stability. © 1979, American Chemical Society. All rights reserved.