Proton nuclear magnetic resonance lineshapes and transverse relaxation in a hydrated barley protein

In many hydrated biopolymer systems proton NMR lineshapes consist of a broad component and a narrow component. It is often assumed that the narrow component arises from water and that its lineshape arises from nuclei in the motionally narrowed regime. For the model cereal protein system, C-hordein–water, the effects of magnetic field strength, magic-angle spinning and 90–90 and 90–180 pulse sequences are examined. It is shown that the narrow components contain contributions from both static dipolar interactions and field-dependent terms. It is therefore concluded that simplistic interpretations of the origins of the relaxation mechanisms in terms of motionally narrowed models are not suitable.