Tobacco mosaic virus gels from

Urea affects the gel as well as the state of aggregation of solutes. Stepa-now et al. (1961) have shown that formation of peptide-peptide complexes may be avoided in phenolate or alkaline urea solutions. Two peptides derived from tobacco mosaic virus protein could be separated with Sephadex only in the presence of 8 M urea. Urea need only be included in the sample solution, not in the eluting solvent (0.01 M sodium hydroxide). Upon filtration on a Sephadex G-50 column, urea and the two peptides moved as well-separated zones. The authors did not comment on the choice of G-.50. It was probably found to be superior to G-25 since urea seems to close the pores and meshes of a Sephadex gel. The reduction in effective pore and mesh size is perhaps caused by urea being bound to the carbohydrate network since the swelling is in fact increased in strong urea solutions. 

As early as 1938, Langmuir observed the phase separation of clay suspensions into an isotropic phase and a birefringent gel at the macroscopic level in test-tubes [9]. However, in the same report, he noted that this property of phase separation was gradually lost with time, which he tentatively explained by the incorporation of impurities diffusing from the glass tubes. He also compared this system to normal liquid crystals. Later, in 1956, Emerson observed a banded texture similar to that displayed by the Tobacco Mosaic Virus [48]. The investigation of clay suspensions from the structural point of view has been recently resumed. However, the study of the nematic order of suspensions of montmorillonite clays is in fact complicated by their gel properties. In spite of sustained efforts to understand its nature, the gelation mechanism has not yet been fully elucidated [49]. 

It is true that recent investigations have shown that possibly there may exist systems of a gel-like character not corresponding to (c). The coherence of the dispersed component is here due to long range forces. Such systems seem to occur in the tobacco mosaic virus and perhaps also in certain soap jellies. We shall however abstract from these particular cases. 

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