Silicic acid reacts with insulin, albumin and amorphous nylon over a wide pH range but monolayers are tanned only near pH 6. Tanning requires (i) the combination of silicic acid with the protein or polyamide, and (ii) further polymerization of the adsorbed silicic acid. The roles of ionic links and hydrogen bonds are discussed.
One theory which seeks to explain the production of fibrous tissue in silicosis postulates the interaction of collagen precursors with polysilicic acid. This stage has been questioned because it is doubtful whether polymerized silicic acid is normally formed when quartz dissolves in aqueous media. A mechanism by which silicic acid may be polymerized in vivo has been demonstrated by studying the properties of polyamide monolayers, which behave like collagen monolayers when spread on silicic acid substrates. The silicic acid which is adsorbed beneath the monolayer polymerizes when the polyamide films are subjected to pressure.In fibrogenesis, the polysaccharides which are produced in large amounts may serve to prevent the premature agglomeration of the collagen units. Later, when the concentration of the polysaccharide is largely reduced, the polysaccharide may assist in aligning and cementing the units together to form fibres. The physiological process is probably reversible but, if silicic acid takes the role of the polysaccharide in this second stage, the building up of fibres is likely to be an irreversible process.
Polysilicic acid interacts with pepsin in the range pH 3.5 to 7. Unlike those of proteins previously studied, pepsin films are not tanned at any pH. Polysilicic acid interacts with laminarin near pW 6 to give rigid films but no interaction is apparent with cellulose acetate.
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