Introduction. Mucus plays an exceptionally wide range of important biological roles. It operates as a protective, exchange, and transport medium in the digestive, respiratory, and reproductive systems of humans and other vertebrates (for review see ref 1). It protects fish skin and decreases its hydrodynamic drag,2 and it also functions as a tegumental dehydration barrier, a surface defense, a navigation aid, and a snare for prey in several invertebrate species.3 Its rheological properties are of special interest in biomedical contexts, particularly respiratory diseases like cystic fibrosis and asthma in which defective mucus plays a critical pathophysiological role.

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Boundaries on the phase diagram for aqueous solutions of levan (a branched polymer of fructose) were located quantitatively by transmitted light measurements performed with a U V-visible spectrophotometer.Data were collected in the range 10-70 °C; the minimum concentrations required for separation of a liquid crystalline phase and the minimum concentration required for a fully liquid crystalline solution were identified within this range. The liquid crystalline nature of the anisotropic phase was confirmed by transmitted polarized light microscopy. The boundaries of the biphasic region (separated isotropic and anisotropic phases) are parallel, and they have a positive slope, suggesting that phase separation is dictated by hard rod interactions and that conformational disorder decreases the rod axial ratio with increasing temperature.

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The liquid-crystalline phase of cell-free and nominally pure bacterial levan in water is shown to depend on the presence of DNA. The necessary concentrations of DNA are 2 orders of magnitude lower than those needed to stabilize a mesophase in aqueous solutions of DNA on its own and are similarly small in comparison to the number concentration of the globular levan molecules. A partial phase diagram for liquid crystallinity in the ternary system water/levan/DNA is mapped out. A model is proposed in which the levan and DNA associate noncovalently, to produce supramolecular, rodlike aggregates. The term “chimeric liquid crystal” is coined to denote a mesophase based on such rods assembled from otherwise unrelated molecular species. The model can account for several characteristics of the liquid-crystalline phase of bacterial levan, including the unexpected observation that liquid crystal phase formation is accompanied by a continuous increase in viscosity.