NMR RELAXATION AND THE ORIENTATIONAL STRUCTURE FACTOR

A class of NMR relaxation mechanisms is considered which is characteri zed by molecular reorientations mediated by translational displacement s. This particularly refers to systems in which molecules are confined to disordered structures determining the local preferential orientati ons of the incorporated molecules and providing translational degrees of freedom with a certain reduced dimensionality. Examples are solvent molecules adsorbed on surfaces of macromolecules, particle aggregates , or porous media. The orientation correlation function of molecules d iffusing along such confining structures therefore does not only refle ct the molecular dynamics but also the structural properties of the sy stem. An orientational structure factor is introduced analogous to the structure factor of scattering theory. A number of typical model situ ations is treated. Experimental relaxation data of the hydration shell s of lipid bilayers, proteins, and silica fine-particle aggregates are considered for comparison. The main measuring technique was field-cyc ling NMR relaxation spectroscopy permitting the record of frequency de pendences over many decades. The data for hydrated proteins can perfec tly be described by an equipartition of surface wave numbers in a cert ain range defined by the dimensions of the protein and, on the other h and, of the water molecules.