Phenylalanine ring dynamics by solid-state 13C NMR

Abstract The partial collapse of dipolar and chemical-shift tensors for carbons, both on and off the ring C 2 symmetry axis of l -phenylalanine crystallized from two different solvent systems, confirms that a fraction of sites for one of the crystalline modifications permits high-frequency 180° ring flips. Analysis of the same tensors indicates the presence of other motions for both materials. These motions are reasonably well represented by a model involving restricted isotropic rotational reorientation, a motion which can be associated with looseness of packing in the crystals.