New aspects of the α-helix to β-sheet transition in stretched hard α-keratin fibers

The putative transformation of α-helices into β-sheets has been studied for more than 50 years in the case of hard α-keratin. In a previous study of stretched keratin fibers, we specified the conditions for β-sheet appearance within horsehair: the formation of β-sheets requires at least 30% relative humidity. However, this phenomenon was observed in the whole tissue. Then there was no clear chemical identification of the α-sheets (keratin or matrix proteins) and the exact location of the β-sheets across the fiber could not be specified. In this study, using wide-angle x-ray scattering and high spatial resolution infrared microspectroscopy, we could determine and charactedze the structural elements across hair sections stretched in water, which provides new information about the aforementioned transition. Our results show that the process can be split into three steps: 1), unraveling of the α-helical coiled-coil domains, which starts at roughly 5% macroscopic strain; 2), further transformation of the unraveled coiled-coils into β-sheet structures, which occurs above roughly 20% macroscopic strain; and 3), spatial expanding of the β-structured zones from the sample center to its periphery.