Occurrence, Fluctuations and Significance of Liquid Crystallinity in Living Systems

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Abstract
Many molecules and macromolecules present in the living systems are known to exhibit the mesomorphic or the so-called liquid crystalline state under appropriate conditions. This state is characterised by relative mobility when compared with true solids and by association and alignment in one crystallographic axis or in one plane when compared with true liquids. It may be brought about by the variation of temperature yielding thermotropic liquid crystals or by the variation of amount and properties of the solvent giving rise to the lyotropic ones. Of these the former mechanism is obviously unimportant for the living systems, particularly for the homeothermic organisms, since there are but few compounds which undergo liquid crystalline transitions at the body temperature within the range of probable local temperature fluctuations in the tissues and body surfaces. However, many molecular species of biological relevance, including some which form thermotropic mesophases, are capable of associating into a lyotropic mesophase under the influence of the water which is the common solvent in the living systems. This association has been hitherto recognised in the case of relatively small molecules like phospholipids, or large natural macromolecules and discrete globular units like sickel cell hemoglobin, or virus particles. Although the existence of mesophases is accepted it has not been critically investigated if the state is essential for the characteristic features of the living state. It is contended here that the association of segments of biological macromolecules in the case of quaternary structures and the association of whole molecules with others follows the same essential rules as do lyotropic liquid crystals in general. The conformation of molecules resulting from intramolecular attractions and repulsions seem also to be governed similarly to a considerable extent. Thus the “liquid crystallinity,” in this sensc, is perhaps of general occurrence. In essence, the associations are a consequence of the balance between interatomic or intermolecular affinity within the molecule or molecules and the affinity toward the solvent. Correlation amongst electrons in associated molecules establishes basic continuity. By this is implied that the electrons in one molecule induce appropriate states in the electrons in the neighbouring molecules across distances commonly found in the living systems and in the liquid crystals. Since the lyotropic systems undergo fluctuations of internal energy and structure as a consequence of the input of quantum chemically defined species or of mass or momentum, or electromagnetic fields it is suggested that this state, in the manner defined, is essential for the following features of the living state: specificity, asymmetry, dynamic transformations, rhythmicity, control and communication in immediate molecular domains and evolution. The advantage of this statement, is that it permits the application of the concept of mesophase and electron correlation to biologic systems. Also, in the reverse, the large body of data pertaining to the molecules in the living systems can help us to understand the liquid crystalline state where data on numerous physical parameters of the mesomorphogenic molecules have been accumulated but there is as yet no direct link between them and the dominant concept in the liquid crystal field: the behaviour as a continuum of a system composed of discrete molecules.