Evidence from ¹³C solid‐state NMR spectroscopy for a lamella structure in an alanine–glycine copolypeptide: A model for the crystalline domain of Bombyx mori silk fiber

Abstract

13 C high-resolution solid-state NMR coupled with selective 13 C isotope-labeling of different Ala one methyl carbons was used to clarify the structure of (AG) 15 peptide in the silk II structure as a model for the crystalline domain of Bombyx mori silk fiber. At the inner part of the peptide, the fraction of the peak at 16.6 ppm of the Ala Cb resonance assigned to b-turn structure increased at 11th and 19th positions. These data indicate the appearance of the most probable lamellar structure having a turn structure at these two positions, although the position of turn was distributed along the chain.

The Influence of Ser and Tyr Residues on the Structure of Bombyx Mori Silk Fibroin Studied Using High-resolution Solid-state 13C NMR Spectroscopy and 13C Selectively Labeled Model Peptides

“…This is clearly due to presence of Ser residue. The (AG) 15 with silk I form changed to silk II form after formic acid treatment 4 and therefore formic acid treatment was also applied to (AGSGAG) 5 . The fraction of 16.7 ppm peak was 37% and 44% for [3-13 C]A 11 and [3-13 C]A 19 labeled positions, respectively, after 9MLiBr/ dialysis treatment, and 31% and 36% for the same labeled positions after formic acid treatment.…”

“…However, multiple existence of AGSGAG sequence appears to modulate the pattern of the hydrogen bonding interactions involving O H group of the Ser side-chains and the backbone C=O oxygen. Our 2 H NMR measurements of uniaxially aligned [3,3-2 H 2 ] Ser-labeled B. mori silk fiber with silk II form, indicated that the dominant conformer of the Ser side-chain is gauche þ and this orientation could be a good candidate for facilitating intermolecular hydrogen bonds with the carbonyl groups on adjacent peptide chains, strongly favoring the -sheet structure as shown in Figure 2. 9 Figure 3 shows Ala C regions in 13 C CP/MAS NMR spectra of (a) IV: (AGSGAG) 4 …”

“…Recently, 13 C solidstate NMR on selectively 13 C labeled 15 peptides labeled singly at different Ala methyl carbons was used to examine the silk II structure of (AG) 15 which was considered to be a model for the crystalline domain, with emphasis on a possible lamellar structure containing -turns. 4,5 However, there are also Ser and Tyr residues and the relative content is reasonably high; 12.2% and 4.8%, respectively. They stand third and fourth behind the Gly and Ala residues.…”

“…The 13 C chemical shifts were calibrated indirectly through the methine peak of adamantane observed at 28.8 ppm relative to TMS at 0 ppm. 4,5 The peak fitting was performed as follows: After fixing the chemical shifts of three peaks of Ala C carbon, the fraction and width of each peak was changed to reproduce the observed line shape by assuming Gaussian for each peak. The uncertainty of the percentages is about AE1%.…”

“…The change in the peak intensity of 16.7 ppm depending on the 13 C labeled position of (AG) 15 has been used for study of the lamellar structure. 4,5 In this study, two Ala positions in I, Ala 11th and Ala 19th , were selected for the 13 C labeling position because of relatively high distorted -turn probabilities in the proposed lamellar structure of (AG) 15 . The most remarkable difference between (AG) 15 and (AGSGAG) 5 after 9M LiBr/dialysis treatment was that (AG) 15 took silk I form, 4 but (AGSGAG) 5 took silk II form as shown in Figure 1 (left).…”