Effects of pH and calcium ions on the conformational transitions in silk fibroin using 2D Raman correlation spectroscopy and 13C solid-state NMR.
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.
Zhou
Ping
P
Department of Macromolecular Science, Fudan University, Shanghai 200433, China. pingzhou@fudan.edu.cn
Xie
Xun
X
Knight
David P
DP
Zong
Xiao-Hong
XH
Deng
Feng
F
Yao
Wen-Hua
WH
D000818:Animals; D032701:Biomimetics / Q000379:methods; D012831:Bombyx / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D002247:Carbon Isotopes; D002413:Cations, Divalent / Q000737:chemistry; D005349:Fibroins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013059:Spectrum Analysis, Raman / Q000379:methods
D002247:Carbon Isotopes; D002413:Cations, Divalent; D005349:Fibroins; D002118:Calcium