Influence of load on order-disorder transition in native collagen fibres

The influence of external stress on the hydrothermal and chemical (induced by water solutions of KCNS and HCl) contraction of native collagen fibres has been investigated. The experiments were made in the isometric and isotonic measurements modes. The coincidence of isometric and isotonic data permits the consideration of a certain portion of the isometric curve as a phase equilibrium curve similar to the pressure-temperature curves of phase equilibrium for the low molecular weight substances. The existence of a certain critical tension and of a corresponding critical temperature in the case of hydrothermal contraction and of a critical composition of environment in the case of chemical contraction was suggested, and their values were calculated for the systems under consideration. Thermodynamic calculation gives us an estimate of the enthalpy of melting Δ H u = 2.12 ± 0.3 kcal/mole of peptide units in the case of hydrothermal contraction and correspondingly of a certain “enrichment of salt” Δ ϵ = 4.6 · 10 − 2 moles KCNS/mole of aminoacid units in the case of KCNS-contraction. The cooperativity parameters for both processes were estimated, and their order was found to be 10⁻⁴ The nature of the transition occurring is discussed in terms of the general theory of order-disorder transitions in unidimensional ordered systems. The conclusions drawn are confirmed by X-ray evidence.