Effect of pH and copper(II) on the conformation transitions of silk fibroin based on EPR, NMR, and Raman spectroscopy

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, ¹³C 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 β-sheet conformation. This behavior is strikingly similar to that seen in prion protein and amyloid β-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_π 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 β-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_τ from two histidines in adjacent peptide chains, forming an intermolecular His(N_τ)-Cu(II)-His(N_τ) bridge. This type of coordination may induce β-sheet formation and aggregation, leading to a crystalline structure.