Collagen Fibrils as Examples of Smectic A Biological Fibres

Collagen fibrils provide a biological example of smectic A liquid crystals. They demonstrate spiralling of their constituent chiral molecules, about the normal to their layers, when they undergo a transition to smectic C. Under tension, perpendicular to the planes of the layers, their molecules are tilted and some of them rearrange so as to describe a lattice whose unit cell has a square base. Novel features of the collagen fibril are that the layer thickness is dictated by the amino acid sequence of its collagen molecules, and not by their length, and that extra stability is conferred on the structure by covalent cross-links.