The Role of Mn(II) in Silk Fibroin Based on EPR and NMR Spectroscopy

The formation of the natural silk fiber depends on the conformational transition of silk fibroin (SF) from a soluble random coil and/or helix state (Silk I) to an insoluble -sheet state (Silk II). Previous investigations have shown that many factors including H+, K+, Na+, Ca2+, Cu2+, and Fe3+ have an effect on the transition. In the present investigation, we report that 13C nuclear magnetic resonance (NMR) spectroscopy indicates that Mn2+ ion does not promote the conformational transition of silk fibroin. Using electron paramagnetic resonance (EPR), we show that Mn(II) ions are likely to exist in the silk fibroin molecules in two states: a six-coordination Mn(II)/SF complex and a [image omitted] complex. The binding of Mn(II) to the amino acid residues located in the hydrophilic spacers of the silk fibroin heavy chain might favor the Silk II state, while the [image omitted] complex might counter this effect by stabilizing the Silk I state. pH change has no considerable influences on the conformation transition of the silk fibroin because of the [image omitted] complex existence.