Proton exchange and molecular orientation of water in hydrated collagen fibers. An NMR study of H2O and D2O

Abstract

An experimental study of proton and deuteron magnetic resonance of the hydration of collagen as a function of water content, temperature, and the addition of salts has been made. From the temperature dependency of linewidths, correlation times for molecular rotation of water molecules and proton exchange times have been determined. For a water content of 45 g H₂O per 100 g collagen the rotational correlation time is ${\text{3× 10}}^{\text{−8}}\sec$ at 25 °C, with an activation energy of 4.8 kcal/mole, and the proton exchange time is ${\text{1.3 × 10}}^{\text{−4}}\sec$ with an activation energy of 10 kcal/mole. Ammonium ions increase the proton exchange rate. The proton exchange most probably occurs between proton donating and proton accepting groups on the macromolecule, intermediated by hydrogen‐bonded water molecules. It is shown that the exchange theory of Gutowsky, McCall, and Slichter is applicable to dipole coupling in anisotropic systems. From an analysis of the anisotropy of H₂O and D₂O rotation in terms of Saupe's parameters, is is concluded that a model with two specific water binding sites is consistent with experimental results.