Study of Protein Conformation and Orientation in Silkworm and Spider Silk Fibers Using Raman Microspectroscopy

Abstract

Raman microspectroscopy has been used for the first time to determine quantitatively the orientation of the β-sheets in silk monofilaments from Bombyx mori and Samia cynthia ricini silkworms, and from the spider Nephila edulis. It is shown that, for systems with uniaxial symmetry such as silk, it is possible to determine the order parameters 〈P₂〉 and 〈P₄〉 of the orientation distribution function from intensity ratios of polarized Raman spectra. The equations allowing the calculation of 〈P₂〉 and 〈P₄〉 using polarized Raman microspectroscopy for a vibration with a cylindrical Raman tensor were first derived and then applied to the amide I band that is mostly due to the CO stretching vibration of the peptide groups. The shape of the Raman tensor for the amide I vibration of the β-sheets was determined from an isotropic film of Bombyx mori silk treated with methanol. For both the Bombyx mori and Samia cynthia ricini fibroin fibers, the values of 〈P₂〉 and 〈P₄〉 obtained are equal to −0.36 ± 0.03 and 0.19 ± 0.02, respectively, even though the two types of silkworm fibroins strongly differ in their primary sequences. For the Nephila edulis dragline silk, values of 〈P₂〉 and 〈P₄〉 of −0.32 ± 0.02 and 0.13 ± 0.02 were obtained, respectively. These results clearly indicate that the carbonyl groups are highly oriented perpendicular to the fiber axis and that the β-sheets are oriented parallel to the fiber axis, in agreement with previous X-ray and NMR results. The most probable distribution of orientation was also calculated from the values of 〈P₂〉 and 〈P₄〉 using the information entropy theory. For the three types of silk, the β-sheets are highly oriented parallel to the fiber axis. The orientation distributions of the β-sheets are nearly Gaussian functions with a width of 32° and 40° for the silkworm fibroins and the spider dragline silk, respectively. In addition to these results, the comparison of the Raman spectra recorded for the different silk samples and the polarization dependence of several bands has allowed to clarify some important band assignments.