Structure of Bombyx mori Silk Fibroin Based on Solid-State NMR Orientational Constraints and Fiber Diffraction Unit Cell Parameters

Abstract

Isotopic labeling of Bombyx mori silk fibroin was achieved biosynthetically using two approaches. First, labeled fibroin was achieved by feeding silk worms [13C1]Gly and [13C1]Ala along with an artificial diet. Second, in vitro production of [15N]Gly and [15N]Ala labeled B. mori silk fibroin was accomplished by culturing the posterior silk glands isolated from five-day-old silkworm larvae in the fifth instar stage. Orientation-dependent 15N and 13C solid-state NMR spectra of these isotope-labeled silk fibroin fibers were observed when the fiber axis was arranged at various angles between 0° and 90° to the magnetic field direction. Such data from the silk II structure was simulated on the basis of the chemical shift anisotropy to determine the Euler angles that relate the principal axis system (PAS) to the fiber axis coordinate system. The dipolar modulated 15N and 13C chemical shift powder pattern spectra of 13C−15N double-labeled silk fibroin model peptides were observed and simulated to determine the Eu...