Vibrational ¹³C‐cross‐polarization/magic angle spinning NMR spectroscopic and thermal characterization of poly(alanine‐glycine) as model for silk I Bombyx mori fibroin

Abstract

This study focuses on the conformational characterization of poly(alanine-glycine) II (pAG II) as a model for a Bombyx mori fibroin silk I structure. Raman, IR, and 13C-cross-polarization/magic angle spinning NMR spectra of pAG II are discussed in comparison with those of the crystalline fraction of B. mori silk fibroin (chymotryptic precipitate, Cp) with a silk I (silk I-Cp) structure. The spectral data give evidence that silk I-Cp and the synthetic copolypeptide pAG II have similar conformations. Moreover, the spectral findings reveal that silk I-Cp is more crystalline than pAG II; consequently, the latter contains a larger amount of the random coil conformation. Differential scanning calorimetry measurements confirm this result. N-Deuteration experiments on pAG II allow us to attribute the Raman component at 1320 cm(-1) to the amide III mode of a beta-turn type II conformation, thus confirming the results of those who propose a repeated beta-turn type II structure for silk I. The analysis of the Raman spectra in the nuNH region confirms that the silk I structure is characterized by the presence of different types of H-bonding arrangements, in agreement with the above model.

Vibrational infrared conformational studies of model peptides representing the semicrystalline domains of Bombyx mori silk fibroin

“…In fact, silk I shows amide I, II, and III bands at about 1654, 1540, and 1240 cm À1 , 27 and silk II at about 1700-1626, 1534, and 1264-1234 cm…”

“…[24][25][26][27][28] Recently, the Raman and IR spectra of the Cp fraction (chymotryptic precipitate of fibroin) and of the synthetic polypeptide (AG) 130 , both with silk I structure, have been reported. 27 N-deuteration experiments confirmed the structural results reported by Asakura et al 16 The aim of the present study is to contribute deeper insights into the structural characteristics of the semicrystalline domains of B. mori silk fibroin. To this purpose, model peptides of varying chain length, incorporating Y, V, and S residues into the basic (AG) n sequence, were synthesized by the solid phase method.…”

“…been reported 27 that other bands characteristic of silk I fall at about 1411 cm À1 ( C a H 2 ), 44 1387 cm À1 ( CH 3 ), 44 and 1335 cm À1 ( s CH 3 ), 45 which can be used to discriminate between silk I and unordered structures.…”

Silk: Optical Properties over 12.6 Octaves THz-IR-Visible-UV Range

“…The Amide III band ranging from 1200 cm${}^{-1}$ to 1300 cm${}^{-1}$ is a complex spectral region with overlapping contributions from various amide side-chains in differing conformations [44]. The components are typically assigned for $\beta$-sheets at 1219 cm${}^{-1}$, random coil at 1240 cm${}^{-1}$ and $\alpha$-coils at 1268 cm${}^{-1}$ [41], with B. mori exhibiting a peak at 1232 cm${}^{-1}$ consistent with the Silk I form.…”

“…The absence of an equivalent peak in A. pernyi fibers, where alanine and glycine rich regions are segregated, further reinforces this assignment. The low energy bands are also found at the silk-I polypeptide recognisably by signatures at about 1415, 1105, 950, 930, 865, 260, and 230 cm${}^{-1}$ [44]. The spectral features of A. pernyi fibers at 20 cm${}^{-1}$ and 67 cm${}^{-1}$ (0.6 THz and 2 THz) are, most probably, related to the hydrogen bonding O-H$···$O where interaction between the neighboring segments define the acoustic response of material in a similar way as in water, where a 70 cm${}^{-1}$ band has been ascribed to the third or fourth neighbor network response [56].…”