THE STRUCTURE ANALYSIS OF Philosamia cynthia ricini SILK FIBROIN AND ITS MODEL COMPOUNDS, L-ALANINE/β-ALANINE COPOLYMERS, BY MEANS OF ¹³C NMR SPECTROSCOPY

Abstract

The structures of Philosamia cynthia ricini silk fibroin and its model compounds, L-alanine/β-alanine copolymers, were studied in organic solvents, dichloroacetic acid (DCA) or trifluoroacetic acid (TFA) by means of ¹³C NMR spectroscopy. In DCA, the carbonyl region of a series of L-alanine/β-alanine copolymers is divided into three peaks, corresponding to β-alanine (random coil), L-alanine (α-helix and random coil). On the other hand, the carbonyl region is more complex in TFA, indicating further splitting due to the primary structure in the coil state. Using the chemical shift data for the α-helix and random coil forms of the L-alanine residues of these copolymers in both solvents and statistical thermodynamic analysis for helix-coil transition, the spectra of the alanine carbonyl region of P. c. ricini silk fibroin in these solvents could be theoretically interpreted. Especially, the Lifson-Roig parameters in the thermodynamic treatment and the local helicity of each alanine residue of the -(Ala)₂₂-sequence were determined.