Evidence by infrared spectroscopy of the presence of two types of β-sheets in major ampullate spider silk and silkworm silk

The major ampullate (MA) silk of spider is known to be composed of oriented beta-sheet nanocrystals dispersed within an amorphous matrix. The presence of an interphase has also been proposed, but it has not been reported for the fibroin of the silkworm Bombyx mori (B. mori). To obtain quantitative information regarding this third phase, the deuteration of B. mori silk and Nephila clavipes MA silk has been probed by attenuated total reflection infrared spectroscopy. The spectral decomposition of the amide II region has allowed determination of the level of orientation and content of the different secondary structures. The data reveal that, in addition to the amorphous domains, part of the beta-sheets is deuterated upon immersion in D2O for both silks. The D2O-inaccessible beta-sheets are associated with crystallites, while the interphase is composed of D2O-accessible ones. It is found that the former beta-sheets are slightly more oriented along the fiber axis than the latter ones, which suggests that the interphase beta-sheets are located at both ends of the crystals. The total beta-sheet content is similar for B. mori silk (50 + 4%) and MA silk (46 + 4%). However, 27 + 3% of the beta-sheets of MA silk are D2O-accessible compared to 8 +/- 3% for B. mori silk. These data suggest that around 5 amino acids for B. mori silk and 9 amino acids for MA silk would be involved in the interphase beta-sheets. The higher amount of interphase beta-sheets for MA silk is believed to contribute to its higher toughness.