Introduction

Throughout this book there has been reference to work carried out on the synthetic polypeptide PBLG <25> (poly(γ-benzyl-L-glutamate)), a homopeptide with a single repeat unit along the chain. This molecule forms lyotropic phases readily in a variety of organic solvents (see e.g. Sections 3.6.2, 4.4.5 and 5.3.2) by virtue of the fact that it is a rather stiff chain, due to the molecule adopting a helical conformation. Macroscopically this molecule exhibits cholesteric structures. Many naturally occurring biopolymers (and, as we shall see, their aggregates) are similarly stiff molecules with a large persistence length, and it is becoming increasingly apparent that they too can form liquid crystalline phases. This recognition dates back to the early work on tobacco mosaic virus (TMV) which showed the existence of lyotropic phases (Bawden and Pirie, 1937) (see also Sections 1.1 and 3.6.7). More recently, a wide range of biopolymers have been shown to be capable of demonstrating liquid crystallinity in solution, ranging from the nucleic acid DNA (Livolant, 1986), to proteins such as silk (Kerkam et al., 1991), to polysaccharides such as cellulose microfibrils (Orts et al., 1998) and to the glycoprotein mucin (Viney et al., 1993). In other words, liquid crystalline phases have been identified in all the main classes of biological macromolecules. However, it is less clear whether the liquid crystalline ordering is always relevant to the biological function associated with each biopolymer, and this will be further discussed later.