Effects of pH and Calcium Ions on the Conformational Transitions in Silk Fibroin Using 2D Raman Correlation Spectroscopy and ¹³C Solid-State NMR

Abstract

Silk fibroin exists in a number of different states, such as silk I and silk II, with different properties largely defined by differences in secondary structure composition. Numerous attempts have been made to control the transitions from silk I to silk II in vitro to produce high-performance materials. Of all the factors influencing the structural compositions, pH and some metal ions play important roles. This paper focuses on the influence of pH and Ca(2+) ions on the conformational transition from silk I to silk II in regenerated (redissolved) Bombyx mori fibroin. One- and two-dimensional correlation Raman spectroscopy was used to describe qualitatively the transitions in secondary structure in silk I, silk II, and their intermediates as pH and Ca(2+) ion concentration were changed, while (13)C cross polarization magic angle spinning (CP/MAS) solid-state NMR was used to quantify these changes. We showed that conditions (low pH, pH 5.2; a defined range of Ca(2+) ion concentrations; gradual water removal) that mimic natural silk spinning promote the formations of beta-sheet and distorted beta-sheet characteristic of silk II or silk II-related intermediate. In contrast, higher pH (pH 6.9-8.0) and higher Ca(2+) ion concentrations maintain "random coil" conformations typical of silk I or silk I-related intermediate. These results help to explain why the natural silk spinning process is attended by a reduction in pH from 6.9 to 4.8 and a change in the Ca(2+) ion concentration in the gland lumen as fibroin passes from the posterior division through the secretory pathway to the anterior division.

Conformational transition and liquid crystalline state of regenerated silk fibroin in water

“…This result is basically consistent with that determined by simulation of C b peak (d14.5-22) in [15][16][17][18] C CP-MAS solid state NMR spectrum. 28 The conformational transition of SF macromolecules with pH would be ascribed to their structural amphiphilicity just like an amphiphilic block copolymer. 26 Lower pH may suppress the negative charges in two regions of the fibroin molecules, allowing a closer approach of neighboring molecules and thus the formation of hydrophobic interactions and later hydrogen bonds between molecules.…”

Deposition of bone‐like apatite on modified silk fibroin films from simulated body fluid

“…Thus, the effect of casting conditions, such as initial concentration, drying rate, pH, and temperature, on conformational change has been investigated. [22][23][24] The proportion of conformations can be correlated with absorption rates, determined from changes in tensile strength. 11,17,25,26 The metal ions, such as calcium, copper, zinc, and potassium ions, influence the conformational transition from ␣-helix to ␤-sheet and/or ␤-sheet intermediates in cast silk fibroin films.…”

“…11,17,25,26 The metal ions, such as calcium, copper, zinc, and potassium ions, influence the conformational transition from ␣-helix to ␤-sheet and/or ␤-sheet intermediates in cast silk fibroin films. 22, 26 Zhou et al studied the conformations of silk fibroin films prepared from the regenerated silk fibroin solutions with various calcium chloride contents and pHs (from pH 5.2 to 8.0). The addition of calcium ions at contents Ͻ10 wt % relative to the fibroin and high pH promoted the formation of "random coil" conformation.…”

“…22 The calcium ions affect the conformational transition from the ␣-helical or the intermediate with random coil to the ␤-sheet structure. 21 The effect of calcium ion concentration and pH value on the conformational changes have been clearly described from NMR and Raman analyses by Freddi et al, 21 over the pH range from 6.9 to 8.0.…”

Chromium(VI) Adsorption Behavior of Silk Sericin Beads

“…After the adsorption experiment, the SS beads were recovered from the can induce structural deformation of proteins (Zhou et al, 2004;Ahern and Garrell, 1991). However, there was no change in the amide I (1600-1700 cm -1 ) peak after Cr(VI) adsorption, indicating that there were no structural changes.…”