Protein secondary structure and orientation in silk as revealed by Raman spectromicroscopy

Taking advantage of recent advances in polarized Raman microspectroscopy, and based on a rational decomposition of the amide I band, the conformation and orientation of proteins have been determined for cocoon silks of the silkworms Bombyx mori and Samia cynthia ricini and dragline silks of the spiders Nephila clavipes and Nephila edulis. This study distinguished between band components due to beta-sheets, beta-turns, 3(1)-helices, and unordered structure for the four fibers. For B. mori, the beta-sheet content is 50%, which matches the proportion of residues that form the GAGAGS fibroin motifs. For the Nephila dragline and S. c. ricini cocoon, the beta-sheet content (36-37% and 45%, respectively) is higher than the proportion of residues that belong to polyalanine blocks (18% and 42%, respectively), showing that adjacent GGA motifs are incorporated into the beta-sheets. Nephila spidroins contain fewer beta-sheets and more flexible secondary structures than silkworm fibroins. The amorphous polypeptide chains are preferentially aligned parallel to the fiber direction, although their level of orientation is much lower than that of beta-sheets. Overall, the results show that the four silks exhibit a common molecular organization, with mixtures of different amounts of beta-sheets and flexible structures, which are organized with specific orientation levels.