Helix reversible arrow coil transitions in dilute aqueous collagen solutions

The transformation Of rat-tail tendon collagen from the native helical to the random coil form, and the process of reversion of the latter to the former, have been followed viscometrically and polarimetrically in dilute aqueous solutions, both temperature and concentration being varied. The rate Of the former process increases ca. 130-fold from 35 to 40 degrees. Final values of the intrinsic viscosity and specific rotation after long periods at temperatures from 35 to 38 degrees show incomplete transformation within this range. The 3 degrees breadth of the transformation equilibrium is attributed to minor variations among the native protofibrillar population. Reversion from coil to helix is first order in the concentration over the range c = 0.066 to 0.41 g./100 ml. The reversion rate displays a large negative temperature coefficient, the half-time increasing ca. 135-fold from 5 to 23 degrees. Inasmuch as the transition temperature T. for the reverted Collagen matches that of the native material and the optical rotation of the former approaches that of the latter, their structures evidently are equivalent. The apparent unimolecularity of the reversion process can be reconciled with the generally accepted three-strand coiled-coil model for the protofibril of the native form by postulating a transitory intermediate consisting of a single chain helix, possibly of the type attributed to synthetic poly-L-proline II, the formation of this intermediate being rate determining. This hypothesis succeeds further in explaining the large negative temperature coefficient of the reversion process. Consideration of the increase in the minimum helix length required for thermodynamic stability at a given degree of supercooling Delta T = T-m - T, where Tm is the equilibrium transformation temperature, leads to an expression of the form Const. exp(-A/kT Delta T) for the specific rate where A is a constant. The observed reversion rates are compatible with this relationship.