Solid-state deuterium NMR (H-2 NMR) was used to study the dynamics of the tyrosine (Tyr) residue in silk fibroin from Bombyx mori (B. mori) and Samia cynthia ricini (S. c. ricini). Specifically deuterated cocoon silk was obtained by feeding silk worms with Tyr, labeled either at the C-beta carbon ([3,3-H-2(2)]Tyr) or at the aromatic ring ([3＇,5＇-H-2(2)]Tyr). The H-2 NMR spectra of the [3,3-H-2(2)]Tyr-labeled silk fibroins showed typical rigid powder patterns, indicating that there is essentially no motion about the C-alpha-C-beta bond axis, both in B. mori and S. c. ricini. In contrast, the H-2 NMR spectra of the [3＇,5＇-H-2(2)]Tyr-labeled silk fibroins consisted of two dynamic components each: a rigid powder contribution, plus a motionally averaged contribution. Hence, some of the Tyr side chains are mobile at the phenolic ring. This motion was characterized to be a pi-flip as is typical of aromatic rings. The corresponding H-2 NMR line-shape of the B. mori sample could be successfully simulated by attributing 20% of the signal to a motionally averaged component with a fast rate (10(6) Hz) and the remaining 80% to a much slower component (<10(3) Hz). Likewise, the simulation of S. c. ricini silk fibroin indicated that 60% of the rings are engaged in fast motional averaging (10(7) Hz), while 40% undergo slow motion (10(4) Hz). Thus, the fraction of the fast component is considerably higher for S. c. ricini silk fibroin than for B. mori, which must be a consequence of their different amino acid sequences. It appears that the side-chain mobility depends on the local packing density around the Tyr residue. We conclude that the Gly-Ala repeats in B. mori silk fibroin are relatively tightly packed. In contrast, a large part of the Gly-rich regions in S. c. ricini are comparatively loosely packed.