Quantitative analysis of infrared absorption coefficient of spider silk fibers

Spider silk has a high degree of crystallinity comprising nanocrystals which are interlinked by amorphous regions containing pre-strained spidroin chains. Both, crystal volume fraction and crystal size were shown to be critical for the macroscopic mechanical properties of silk. In this study, for the first time, quantitative results were obtained from absorption IR spectra. Two main difficulties must be overcome: the cylindrical sample geometry and the absence of suitable reference spectra. We show that it is possible to get reproducible absorption coefficient spectra from dense parallel fiber grids immersed in a refractive index matching liquid. Comparison with model polypeptides yields the nanocrystal volume fraction that is close to the crystallinity values given by solid-state NMR or X-ray scattering.