Organization of microfibrils in keratin fibers studied by X-ray scattering modelling using the paracrystal concept.

ABSTRACT

Low-angle X-ray scattering patterns of hard alpha-keratin fibers have been studied for more than 50 years but a completely convincing modelling has never been presented. The models which have been proposed so far are specific to the sample and cannot be adapted to others, mainly because they do not use a parametric analytical expression of the distribution function describing the relative positions of the microfibrils. Our new approach is based on a paracrystal distribution function. In addition, a huge background originating from a non-ordered matrix is taken into account. Various hard alpha-keratins from different origins have been studied using our approach. From the rather good modellings obtained, it appears that the diameter of the microfibril is not origin dependent (7.4 nm) whereas the distances between microfibrils and their electron density profiles are. Hair microfibrils can be reasonably approximated by a solid cylinder but a core and an outer ring are necessary for porcupine. Our method is of course not limited to keratin microfibrils; it can be used for modelling equatorial X-ray scattering profiles of all types of hexagonal fibrillar assemblies, which are in fact widely found in biological tissues.