We have recorded high-resolution 13C NMR spectra of silk fibroins in the solid state from Bombyx mori and Philosamia cynthia ricini and appropriate model peptides and polypeptides to gain insight into the dynamic features of side-chain groups of Tyr and Ser residues. First, 13C NMR peaks of Tyr residues were identified, except for the peak of Cβ, by comparing 13C NMR peaks of the B. mori fibroin with those of the crystalline fraction whose Tyr residue is reduced to less than 2%. It turned out that the phenolic ring of Tyr residues undergoes a flip-flop motion with a rate constant of 10-8 s, as inferred from the coalesced single peaks of (T1 C). The aromatic side chains of (T1 C) and (Phe)n, however, undergo rotational diffusion with a correlation time on the order of 8 s, as manifested from selective reduction of the 13C spin-lattice relaxation time of the laboratory frame (Tf). The shortened T1 C values (<1 s) of Ala Cs in (Al)n and fibroins were ascribed to the presence of C3 rotation. The T1 C of Ser Cβ in (Al)n and fibroins appears to arise from two different relaxation processes, a shorter (0.6-1 s) and a longer (10-40 s) component. The former and latter were obviously ascribed to the presence of free hydroxymethyl groups and those hydrogen bonded to either carbonyl or other hydroxyl groups, respectively. © 1990, American Chemical Society. All rights reserved.
