Hydrogen exchange and hydration dynamics in gelatin gels

Gelatin, derived from the collagen triple helix, is the most widely used functional biopolymer and a prototype for studies of physical gels. Gelatin gels have also served as models for soft biological tissue in efforts to elucidate the molecular basis of the magnetic relaxation phenomena that govern magnetic resonance image contrast. Yet, the microstructure, hydration, and magnetic relaxation behavior of gelatin gels are not well understood. To address these issues, we report here the water 2H and 17O magnetic relaxation dispersion (MRD) profiles from gelatin gels over wide ranges of resonance frequency and pH. For the global analysis of this extensive data set, we use a generalized relaxation theory that remains valid for arbitrarily slow molecular dynamics. The strong pH dependence in the 2H profiles can be rationalized quantitatively as the result of exchange with bulk water of labile hydrogens in gelatin side chains. The global analysis of the MRD data yields hydrogen-exchange rate constants, acid dissociation constants, and orientational order parameters in agreement with independent structural, thermodynamic, and kinetic data. The MRD analysis reveals a highly mobile hydration layer at the surface of the gelatin triple helix and a small number of trapped water molecules with residence times on the order of 10(-8) s, presumably associated with structural defects and branch points in the gel. The MRD data also indicate that approximately 20% of the gelatin residues belong to flexible polypeptide chains, rather than to rigid triple-helical segments. By identifying the molecular species and motions responsible for the 2H and 17O dispersion profiles, this study takes a significant step toward a quantitative understanding of water relaxation in aqueous gels and biological tissue.