Orientation of silk III at the air-water interface

A threefold helical crystal structure of Bombyx mori silk fibroin has been observed in films prepared from aqueous silk fibroin solutions using the Langmuir Blodgett (LB) technique. The films were studied using a combination of transmission electron microscopy and electron diffraction techniques. Films prepared at a surface pressure of 16.7 mN/m have a uniaxially oriented crystalline texture, with the helical axis oriented perpendicular to the plane of the LB film. Films obtained from the air-water interface without compression have a different orientation, with the helical axes lying roughly in the plane of the film. In both cases the d-spacings observed in electron diffraction are the same and match a threefold helical model crystal structure, silk III, described in previous publications. Differences in the relative intensities of the observed reflections in both types of oriented samples, as compared to unoriented samples, allows estimations of orientation distributions and the calculations of orientation parameters. The orientation of the fibroin chain axis in the plane of the interfacial film for uncompressed samples is consistent with the amphiphilic behavior previously postulated to drive the formation of the threefold helical silk III conformation.