A 2H NMR Study of [Ser-3,3-2H2]- and [Ala-3,3,3-2H3]- Silk Fibroins in the Solid State. Role of Side-Chain Moiety in Stabilization of Secondary Structure

An attempt was made to prepare [Ser-3,3-2H2]- and [Ala-3,3,3-2H3]-fibroins by feeding respective amino acids Dl-[3,3-2H2]-serine (Ser) and [3,3,3-2H3]-alanine (Ala), respectively, to silkworms, Bombyx mori and Philosamia cynthia ricini together with daily artificial diet. We recorded 2H powder-pattern NMR spectra of [Ser-3,3-2H2]- and [Ala-3,3,3-2H3]-cocoons and fibroins (10–20% deuteration) by means of the quadrupole echo method. It was found that separation of the 2H quadrupole splittings is 125 and 39 kHz, respectively, for [Ser-3,3-2H2]- and [Ala-3,3,3-2H3]-cocoons and fibroins, respectively. Accordingly, it was concluded that side-cain reorientation of Ser residue is virtually frozen irrespective of differences in conformations of silk I and silk II, suggesting that the hydroxyl groups in these cases might be hydrogen-bonded to C=O or NH groups in intra or interchain. On the other hand, side-chain methyl group of Ala residue in [Ala-3,3,3-2H3]-fibroin turned out to undergo rapid three-fold rotation about the Cα–Cβ axis, as viewed from data of the quadrupole splittings.