Micro-CT dilatometry measures of molecular collagen hydration using bovine extensor tendon

Purpose: This article introduces a new method to study macromolecular hydration using micro-CT dilatometry. The complexity of hydration dependence on solvent temperature, pH, ionic charge, ionic activity, and ionic radii are barriers to comprehensive understanding of protein function. The crystalline character of collagen-tendon suggests that tendon dilatometry may give direct access to measures of molecular tropocollagen solvation response. Methods: The molecular basis of the stoichiometric hydration model (SHM) provides tools to validate bovine tendon as a model to study protein-solvent shape response by micro-CT measures of tendon diameter, length, and mass during dehydration. The SHM relates macroscopic properties to molecular properties of water interacting with the surface of collagen molecules. There are marked changes at critical SHM hydration levels h=0.0653, 0.262, and 0.724 g water/g dry weight. Results: Micro-CT analysis of the length, diameter, and volume combined with gravimetric measures of tendon mass as a function of hydration h(g water/g dry solid) shows asymmetric changes in length, diameter, and density as predicted by SHM. The collagen molecules perturb water properties of polar hydration N=11 waters per tripeptide unit or h∼0.724 g/g to confirm MDS prediction of elevated hydration density 20%-50% higher than bulk water. Conclusions: Results validate the use of tendon dilatometry amplification factors of 106-108 as an effective model to investigate protein molecule shape change response to solvent molecules. The tendon model for the first time allows direct study of protein hydration and functional response under physiological conditions.