NMR Diffusion Studies of Water in Natural Biopolymers

The chapter presents the results of one-dimensional NMR methods in studying molecular anisotropy and microstructure of collagen fibers from two connective tissues with different cross-linking level (fibers from adult steer and young calf) at humidity level (HL) of 0.6 g water per g dry mass. The apparent diffusion coefficients (Dapp) have been studied in oriented collagen fibers (along the static magnetic field B0) for two experimental cases: (1) the gradient was applied along the static magnetic field B0 and (2) the gradient was switched on perpendicular to the magnetic field B0. The dependences of Dapp on diffusion time discovered a restriction diffusion of water for both gradient directions. A model of equally spaced parallel planes with permeable barriers, has been used to estimate a restricted distance and permeability coefficient. For both types of collagen fibers (adult and young) anisotropic diffusion of water has been discovered. Moreover, self-diffusion of water in fibers of natural silk (Bombyx mori) with HL=0.18 g H2O per g dry mass has been studied by pulsed field gradient NMR stimulated echo at various diffusion times Δ between 10 and 200 ms. The analysis showed that the decrease in Dapp with the increasing Δ due to the restricted diffusion. The results obtained were compared with published data on restricted diffusion in natural macromolecular systems with low water content.

Keywords: Apparent diffusion coefficient Dapp, Free induction decay (FID), Natural Biopolymers, Pulsed Field Gradient (PFG) NMR, Spin-echo (SE), Spinlattice (longitudinal) relaxation time T1, Spin-spin (transverse) relaxation time T2, Stimulated echo (STE).