Liquid Crystallinity in Living Systems
One thing we all share as organisms is our physical situation in a state between the solidness of a crystal and the fluidity of a liquid, a fact responsible for other, larger-scale commonalities.
Liquid crystals, or ordered fluids, are organic compounds that exist in stable states intermediate between the more common solid and fluid. They are intermediate in characteristics such as molecular regularity, confinement, and ability to flow and mix. About five percent of known organic compounds exhibit liquid crystalline characteristics (Makow 1982). An apparently necessary property of the types of molecules that exist in these intermediate states is that they be rod-like or disc-like. Many liquid crystal forming molecules have both rigid and flexible regions:
“First of all, the molecule must be elongated in shape; that is, it must be significantly longer than it is wide. Second, the molecule must have some rigidity in its central region. A molecule that flops around like a piece of cooked spaghetti is unlikely to have a liquid crystal phase. Finally, it seems to be advantageous if the ends of the molecule are somewhat flexible. A good model of a typical liquid crystal molecule is therefore a short pencil with a short piece of cooked spaghetti attached to each end” (Collings 2002).
According to King (1969), liquid crystals were first described by the Austrian botanist Friedrich Reinitzer in 1888. They were named by Otto Lehmann in 1889. The state is also sometimes referred to as “mesomorphic” or “ordered fluid.” Makow (1982) gives a human analogy for what things are like on the inside of matter in a liquid crystalline condition:
“To visualize the intermediate liquid crystalline state one may use the analogy of a column of soldiers standing in neat rows (or planes for a 3-dimensional case) with the soldiers equally spaced and facing the same direction. This column of soldiers could represent a model of molecules in a crystal which is an anisotropic solid and be described as having positional and directional order. If the commander of the columns calls out to break ranks, the soldiers run in a disorderly way in all directions at different speeds. This would be a model of an isotropic liquid, since the molecules have lost their positional and directional order. Should the soldiers run and face the same direction, they would represent a model of a liquid crystal since the molecules would have lost their positional but would have retained their directional order. The molecules can slide against each other, and thus liquid crystals appear as a viscous liquid.”
The old idea that organisms must incorporate both fluidity and solidness, or disorder and order, or motion and stillness in their bodies to live is confirmed by the modern discovery of liquid crystals in living systems. Some of the biomolecules that have liquid crystalline properties include myosin, hemoglobin and trypsin, DNA, RNA (Stewart 1967), chitin (Clark 1928, Brown and Wolken 1979) collagen, cellulose, elastin, spongin, fibrin, muscle protiens (Clark 1928), phosphatidlylethanolamine, phosphatidylcholine (lecitin), phosphatidylserine and amyloid (Corrigan et al. 2006). Lyotropic mesophases (liquid crystal phases that depend on a solvent, in contrast to thermotropic liquid crystals which depend on temperature) are likely essential to, at least, the organization of certain components of protoplasm such as chromatin (Leforestier et al. 2001), muscle, collagen, reticulin, adrenal cells, ovaries, other tissues, and plant viruses (Stewart 1967).
Lehmann (1909) takes distinctions between crystalline hardness and shape and the relative softness and curvature of organisms to suggest that life is somewhat more liquid than a solid:
“The treatises on crystallography define a crystal as a solid, homogeneous, anisotropic body. Living organisms have curved forms, while crystals are polyhedra bounded by plane faces. Organisms are soft, and the simplest organisms, such as amobae, are liquid, while crystals are rigid. Curved and liquid crystals would contradict the fundamental definition of a crystal, and also the theory of molecular arrangement adopted by all crystallographers.”
Goodby (1998) opens his article on life and liquid crystals by saying molecules in living systems “invariably exhibit both thermotropic and/or lyotropic liquid crystalline properties.” He quotes Dervichian (1977):
“Liquid crystals stand between the isotropic liquid and the strongly organized solid state, life stands between complete disorder, which is death, and complete rigidity, which is death again.”
Different substances exist as liquid crystals across different temperature ranges, many of which contain typical animal body temperatures, and when different liquid crystals are mixed wide ranges of stability can be achieved (Makow 1982). Membranes of the cell are in a liquid crystalline state (Vance 2002). Their fluidity is adjustable in a variety of ways and serves a range of essential functions (Alberts 2004). Within the membrane there is further variation; some sections (e.g., lipid rafts) are less fluid than others (Vance 2002). Stewart (1967) suggests that solvent concentration, polar solubility, intermolecular forces, ionic versus nonionic nature of amphiphiles, length of hydrocarbon chains and temperature are some of the parameters determining phase in such systems. Lyotropic liquid crystals can incorporate a wide variety of related or unrelated molecules while remaining in the liquid crystalline phase, although they may be altered, for instance in the degree of order or fluidity (Stewart 1967). King (1969) quotes Joseph Needham’s Biochemistry and Morphogenesis:
“…the aspect of molecular pattern which seems to have been most underestimated in the consideration of biological phenomena is that found in liquid crystals…. Living systems actually are liquid crystals, or, it would be more correct to say, the paracrystalline state undoubtedly exists in living cells…. most of the protein, fat and myelinic substance of the cell probably exists in these states, but this is only directly visible when all the molecules are oriented in enormous swarms in one direction, as in muscle fibrils…. This state seems the most suited to biological functions, as it combines the fluidity and diffusability of liquids while preserving the possibilities of internal structure characteristic of crystalline solids.”
A wide variety of biological structures such as biomembranes and layered tissues and the processes associated with them likely rely on the “limited flow” characteristic of lyotropic liquid crystals (Stewart 1967) suggesting that that liquid crystallinity, in part, distinguishes living and nonliving matter:
“The presence of lipid, protein, and other substances in various forms of lyotropic mesophases explains many of the properties which distinguish protoplasm from inanimate colloid or even, in more general terms, some of the essential differences between the physical structure of living and nonliving matter.”
The Origin of Life
Based on the importance of lyotropic liquid crystalline phases of matter in contemporary organismal structure and function, Stewart (1967) says the origin of life likely coincided with the development of complex liquid crystalline mesophases. Or, more specifically, that aggregations of amphiphiles, water solvent, and ions could have spontaneously developed into stable, but mobile, complex liquid crystalline structures, given the appropriate conditions of concentration and temperature which likely existed in many places in the waters of the early earth.
As might have been expected, biological systems are capable of transforming substances that exist primarily in a crystalline state into those which exist primarily in a liquid crystalline state. For example, microcrystalline cholesterol injected into the peritoneum of a mouse is transformed within about two hours into a liquid crystalline form found within peritoneal leukocytes and regional lymph glands (Stewart 1967). Crystalline cholesterol fed to a rabbit is transformed into a liquid crystalline component of the blood plasma and later deposited in a similar liquid crystalline form on the walls of arteries (Stewart 1967).
Liquid crystallization of spider silk is likely to be at least partially responsible for its great tensile strength (the amount of force it withstands before breaking), and its toughness (the amount of energy needed to break it). Material in a cholesteric liquid crystalline state has been found in silk secreting glands. Knight and Volrath (1998), using polarizing microscopy, found in the spider silk secretory pathway a “large scale texture” consisting of “a chequered pattern of rather regularly alternating, approximately rectangular, blue and yellow areas”, resembling the texture of a synthetic, lyotropic nematic discotic liquid crystal in a cylindrical tube.
Liquid crystalline cholesterol and cholesterol esters are likely important in the opossum tapetum lucidum (a mirror-like structure at the back of the retina). Such substances are sensitive to small changes in strain, presence of vapors, and temperature and are therefore probably functional in many other animal sensory organs (Stewart 1967).
Oriented molecules in a liquid crystal can reflect light to give brilliant colors, some more so than is possible with pigments (Makow 1982). Cholesteric liquid crystal molecules are usually elongate, with axes pointing along a plane. Many such planes sit on top of each other, with molecular axes in each neighboring plane pointing at a slightly different angle so that the axes rotate periodically throughout the substance. The color of light reflecting from planes arranged in this way varies depending on the angle at which they are viewed. Iridescent, angle dependent coloration in animals is often produced by the same mechanism. Cholesteric liquid crystals are becoming popular in human art:
“Paintings with free liquid crystals sparkle with rainbow colors and appear like nothing ever seen before in a painting. The best comparisons of color effects are with some beetles, butterflies and wings of some birds” (Makow 1982).
Large Scale Liquid Crystals
Ho et al. (1996) consider “organisms as polyphasic liquid crystals,” suggesting the importance of this state of matter, or rather many intermediate states, collected together at the whole organism level. They say:
“Liquid crystals in organisms include the amphiphilic lipids of cellular membranes, the DNA in chromosomes, all proteins, especially cytoskeletal proteins, muscle proteins, collagens and proteoglycans of connective tissues. These adopt a multiplicity of mesophases that may be crucial for biological structure and function at all levels of organization from processing metabolites in the cell to pattern determination in development, and the coordinated locomotion of whole organisms.”
Luzatti and Husson (1962) studied a phospholipid in tissue samples from the human brain and found that it exhibited two liquid crystalline phases, one lamellar and one hexagonal. They say that lipoprotein conditions in the brain itself are probably not far from a phase transition between liquid crystal and coagel (hydrated crystal), determined by temperature, concentration, or electric potential parameters, and that this may prove important in regulation of substances in brain cells. They point out it may not be coincidental that the phase transition temperature is close to body temperature.
R. K. Mishra published a paper in the Indian Journal of Psychiatry (1965) proposing that the liquid crystallinity of the brain is involved in perception, memory, thinking, original thought and dreams. Mishra writes of solid structures made up of plates and fibers creating long distance repetitive order throughout a liquid matrix within the brain, of freedom versus confinement and restraint of molecules, phases with variable amounts of order, and lattices being perturbed (“lattice alterations”) from one to another thermodynamic equilibrium, coming to “life” in response to certain types of stimuli, thus creating new memories. Mishra suggests that analogies between the structure of liquid crystals and the mind will explain consciousness itself:
“In this scheme of things consciousness is an emergent property of the liquid crystal, which need be no more mysterious than the “wateryness” of water or the “colour” associated with a wavelength.”
The liquid crystal/mind analogy applies to all conscious animals. If the characteristics of liquid crystals do prove to be essential to such things as memory, thinking, originality, and consciousness then it would be reasonable to implicate liquid crystal characteristics (e.g., solidness, fluidity, order, disorder, regularity, randomness) in aesthetic preferences.
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