Molecular Structure of Human Hair


Hair is one of the most distinctive features of mammals. It is a filamentous biomaterial, mainly consisting of proteins, primarily keratin, and structural lipids. It is composed of several layers, including the cortex and the medulla, and it forms in follicles in the dermis (the layer of skin).

The coiled-coil phase of keratin molecules plays an important role in the organisation of the hair fibre mass. In the cortex, the coiled-coil phase organizes keratin intermediate filaments which are then further shaped into larger fibres. These are surrounded by the cuticle, a dead cell layer. These layers are bound together by a membrane complex.

We investigated the molecular structure of human scalp hair from 12 healthy individuals using X-ray diffraction. Two-dimensional data were collected in the q and qz ranges, covering a range of length scales from about 3-90 A. We observed a number of signals related to the coiled-coil keratin phase, formation of intermediate filaments in the cortex, and lipid molecules in the cell membrane complex. These general features were observable in all specimens, regardless of gender and optical appearance, within the resolution of this experiment.

This X-ray experiment was conducted on an ultra-fast X-ray system in the laboratory at RWTH Aachen University, Germany. The X-rays were oriented with the long axis of the hair strands parallel to the vertical z-axis, in order to study the coiled-coil keratin signal and the corresponding intermediate filaments in a very fast time frame. The X-ray data are shown in Figure 1.

In this experiment, the scattering intensity of the coiled-coil keratin phases and the corresponding lipid molecules in the cortex and the cell membrane complex was determined with the help of an X-ray powder detector, which is operated with a beam voltage of about 50 kV. The diffraction signals were analyzed with the software PICCAD, and the scattering features were assigned to the corresponding signal assignment in the figure.

The X-ray data are characterized by the ring-like scattering at q values of about 0.1 A-1 and broad ring-like scattering at qz values of about 1.5 A-1, which reflect the order in the lipid mono- and bilayers in the cell membrane complex. This feature is a direct result of the structure of lipids, which have a lamellar periodicity of about 45 A and are arranged in rings at spacings of about 4.3 A.