Quantitative Mapping of the Orientation of Fibroin β-Sheets in B. mori Cocoon Fibers by Scanning Transmission X-ray Microscopy

The spatial distribution of the linear dichroic signal associated with aligned β-sheets in a microtomed section of a Bombyx mori cocoon silk fiber was derived from scanning transmission X-ray microscopy (STXM). The intense C 1s → π*_amide peak at 288.25 eV was found to have negligible dichroic signal in transverse sections but a large dichroic signal in longitudinal sections. This is consistent with other measurements of the orientation of the aligned β-sheets in silk fibers, in particular with those obtained by polarized Raman microspectroscopy to which our results are compared. When the dichroic signal strength is mapped at better than 100 nm spatial resolution, microscopic variations are found. Although the magnitude of the dichroic signal changes over a fine spatial scale, the direction of the maximum signal at any position does not change. We interpret the spatial variation of the intensity of the dichroic signal as a map of the quality of local orientation of the β-sheets in the fiber. At sufficiently high magnification and resolution, this technique should image individual β-sheet crystallites, although the present implementation does not achieve that. A map of the orientation parameter 〈P₂〉 is derived. The average value of 〈P₂〉 (−0.20 ± 0.04) from STXM is smaller than that derived from the analysis of the amide I band in polarized Raman spectra (−0.41 ± 0.03). This deviation is attributed to the fact that the STXM results also include the signal from unaligned regions of the protein.