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.

Much attention has been paid to the natural mechanism of silkworm spinning due to the impressive mechanical properties of the natural fibers. Our results in the present work show that the fractional changes of the conformational components in regenerated silk fibroin (SF) extracted from Bombyx mori fibers is remarkably pH- and Cu(II)-dependent as demonstrated by Cu(II) EPR, (13)C NMR, and Raman spectroscopy. Cu(II) coordination atoms in SF are changed from four nitrogens to two nitrogens and two oxygens as well as to one nitrogen and three oxygens when the pH is lowered from 8.0 to 4.0. The addition of a given amount of Cu(II) into a SF solution could induce efficiently the SF conformational fractional change from silk I, a soluble helical conformation, to silk II, an insoluble beta-sheet conformation. This behavior is strikingly similar to that seen in prion protein and amyloid beta-peptide. On the basis of the similarity in the relevant sequence in SF to the octapeptide PHGGGWGQ in PrP, we suggest that at basic and neutral pH polypeptide AHGGYSGY in SF may form a 1:1 complex with Cu(II) by coordination of imidazole N(pi) of His together with two deprotonated main-chain nitrogens from two glycine residues and one nitrogen or oxygen from serine. Such a type of coordination may make the interaction between two adjacent beta-form polypeptide chains more difficult, thereby leading to an amorphous structure. Under weakly acidic conditions, however, Cu(II)-amide linkages may be broken and Cu(II) may switch to bind two N(tau) from two histidines in adjacent peptide chains, forming an intermolecular His(N(tau))-Cu(II)-His(N(tau)) bridge. This type of coordination may induce beta-sheet formation and aggregation, leading to a crystalline structure.