Silica compounds diatoms

Diatomaceous earth, a fairly pure silica, formed from the skeletons of minute organisms, known as diatoms. In rubber compounding it is used as an inert filler. 

NaBr/H20, LiBr on Amberlyst-15 resin, TiCU-LiCl, " SiCL, I2 with a Sml2 catalyst, and Lil on silica gel. Epoxides can be converted directly to 1,2-dichloro compounds by treatment with SOCI2 and pyridine, or with Ph3P and CCl4. These are two-step reactions a halohydrin is formed first and is then converted by the reagents to the dihalide (10-48). As expected, inversion is found at both carbons. Meso epoxides were cleaved enantioselectively with the chiral B-halodiisopinocampheylboranes (see 15-16), where the halogen was Cl, Br, or I. ° Diatomic iodine gives an iodohydrin with a 2,6-bis[2-(o-aminophenoxy) methyl]-4-bromo-l-methoxybenzene catalyst. ... 

Our English word silica has a very broad connotation it includes silicon dioxide in all its crystalline, amorphous, soluble, or chemically combined forms in which the silicon atom is surrounded by four or six oxygen atoms. This definitely excludes all the organosilicon compounds made by man in which carbon atoms have been linked directly to silicon atoms—commonly referred to as silicones , which do not occur in nature. Silica is soluble enough in water to play important roles in many forms of life. It forms the skeletons of diatoms, the earliest form of life that absorbed sunlight and began to release oxygen into the atmosphere. Many plants use silica to stiffen stems and form needles on the surface for protection. 

Metabolism involving silica has been studied in diatoms by measuring the effects of silica-deficient growth conditions. Lewin found that silica is not taken up by washed cells until supplied, with a sulfur compound. Cadmium inhibited uptake. 

The compound 2,3-cir,/rans-3,4-dihydroxyproline was identified in the cell walls of diatoms (57). The question whether this could be related to the mechanism o. silica metabolism and transport is a matter of speculation (58). ... 

Polyamines Nonpeptide biomimetic analogues to siUca-precipitating peptides are drawn from the isolated LCPAs found on the cell walls of diatoms. Some of these polyamines include poly(allylamine) [79], (PEI) [73], N-methylpropylamine (PA) [76] and amine-terminated dendrimers [62, 68-70]. Each of these classes of compound represents a different structural aspect to silica templating, including the number of amines, the number of spacer atoms between the amines, its relative degree of methylation, and the presence and number of branching elements. All of these factors affect the resultant physical properties and subsequent reactivity of the siUca nanoparticle produced. 

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