33—RECENT OBSERVATIONS ON THE STRUCTURE OF α-KERATIN

Lead-revealed lipid organization in human hair

Human hair lipids form a complex mixture of composition close to that of sebum. Part of these lipids appears in an organized state that has been studied by diffraction techniques in the literature. Nevertheless, information on the structure of these lipids remains very scarce due to their low contribution to global hair diffraction pattern. Here we show that appropriate lead treatment considerably enhances organized lipid features observed by microbeam synchrotron radiation diffraction experiments. We attribute these features to the formation of lead soaps of free fatty acids. Specific orientation of hair "lipid crystals" in planes parallel to the hair axis is clearly demonstrated. Inclusion of these lipids in the bulk of the hair shaft is shown by diffraction experiments following removal of the cuticular outermost layer of hair. Moreover, microfluorescence and diffraction experiments are consistent with part of the lipids being present as calcium soaps in native hair. We therefore consider lead fixation as a powerful tool to evaluate the lipid organization in human hair for medical, environmental and archaeological purposes, including lead poisoning.

The structure of the alpha-keratin microfibril

Quantitative measurements of the intensity of the meridional reflections in the X-ray-diffraction pattern of alpha-keratin are shown to be consistent with a microfibril structure in which a surface lattice with an axially projected period around 200 A is subject to a periodic interruption with an axially projected period of 470 A. Taken in conjunction with recent evidence on the chemical structure of alpha-keratin and other intermediate filaments this finding enables an elaboration to be made of a model proposed earlier by RDB Fraser, TP MacRae, & E Suzuki (J. Mol. Biol. 108, 435-452, 1976) for the alpha-helical framework of the microfibril. The disposition and connectivity of the helical segments suggested here provides a straightforward explanation of a number of recent physicochemical and electron-microscopical observations on intermediate filaments and provides a starting point for the development of models for the framework of other intermediate filaments.

An x-ray diffraction study of thermally-induced structural changes in alpha-keratin

A series of events takes place as wool is heated under vacuum from room temperature to 250 degrees C: Loosely and strongly bound water molecules are removed at temperature at and below 150 degrees C; a glass transition of the amorphous keratin occurs at 160-175 degrees C; helices melt at 215 and 235 degrees C. The structural changes take place between the glass transition and the helix melting temperature are observed as reflected in the low-angle X-ray diffraction patterns: The 39 A meridional reflection is intensified; a 4-point diagram at azimuthal angle of 45 degree with spacing around 46 A appears; the intensity of the 33 A meridional reflection decreases, and the 66 A meridional reflection is the most heat-resistant.