Silicon is among the most abundant elements on the Earth. It occurs in many minerals and plays an important role in several biochemical processes. Some living organisms use silicon dioxide as a substrate for building elements of their bodies. Unicellular diatom algae build frustules from silicon dioxide. The skeleton of siliceous sponges is a silicaprotein composite. Similarly, rice hulls which protect seeds, contain silica as an important component. The living organisms assimilate silicon from the environment in the form of silicic acid. However, the biochemical mechanisms involved in the transformation of silicic acid to solid siliceous materials are still poorly understood.Evidently, condensation of silicic acid in the living organisms proceeds under control of biopolymers and it is important to know how various types of polymers influence the condensation. Bio-inspired chemistry involving the interaction between polymeric silicic acid and functional polymers results in interesting composite materials, including nanoparticles and bulk materials. This review contains a brief description of the mechanism of silicic acid condensation in aqueous medium and also includes a discussion on various precursors of silicic acid. The main focus of the review is on the influence of polymers bearing nitrogen and oxygen-containing functional groups on silicic acid condensation starting from monomer to three-dimensional polymer. Influence of molecular weight of the organic polymer on the condensation and structure of the resulting product is also elaborated. The biological importance of the obtained data and strategies for novel applications of the synthesized composite materials are described in the concluding section of the review. The biomimetic condensation processes open up new vistas for development of novel materials and applications in the biomedical and process industries.

The effect of three Group IV metals (titanium, zirconium and tin) on the growth, morphology and chemical composition of the freshwater diatom Synedra acus subsp. radians (Kützing) Skabichevsky was studied and compared with germanium. The elements in their highest oxidation states were introduced into the culture medium in the form of hydroxides. Germanium was found to be toxic at ≥5 mol. % of the total Ge-Si content in the culture medium. In the presence of other elements, a slight decrease in the cell division rate was observed independent of the element within 1-15% content interval. The analysis of the obtained biomass and silica valves revealed the presence of all the added elements within the cells. However, only germanium was incorporated into the valves in considerable amounts. S. acus cultivation with the addition of 5% Group IV elements resulted in cells having the following aberrations in the structure of the silica valves: changes in valve shape, thickening of valves, alterations of the areolae rows, irregularity or absence of the areolae and a decrease in the mechanical strength of valves. Moreover, the effect of Group IV elements on silica formation was simulated in vitro using a synthetic polymer bearing polyamine and phosphate groups found in silaffines (proteins from Electronic supplementary material The online version of this article (diatom frustules). The studied elements were observed to provoke the formation of unstable silica particles in solution. We propose that the observed effects of germanium, titanium, zirconium and tin on diatom growth and structure are due to uncontrollable silica condensation.

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Synthesis of methylated oligopropylamines by the stepwise repetition of reactions between methyl acrylate and amine, amidation and reduction of the resulting amide is reported. Longchain oligomers (average 15 nitrogen atoms) were obtained via condensation reactions utilising 1,3-dibromopropane. Three new acrylic monomers were prepared by reaction of oligopropylamines with acryloyl chloride. The synthesized amines and monomers are promising as model compounds for biosilicification reactions, as biologically active substances and as building blocks in organic and polymer chemistry.

Many organisms including unicellular (diatoms, radiolaria, and chrysophytes), higher plants (rice and horsetail) and animals (sponges) use silica as a main part of skeletons. The bioavailable form of silicon is silicic acid and the mechanism of silicic acid penetration into living cells is still an enigma. Macropinocytosis was assumed as a key stage of the silicon capture by diatoms but assimilation of monomeric silicic acid by this way requires enormous amounts of water to be passed through the cell. We hypothesized that silicon can be captured by diatoms via endocytosis in the form of partially condensed silicic acid (oligosilicates) whose formation on the diatom surface was supposed. Oligosilicates are negatively charged nanoparticles and similar to coils of poly(acrylic acid) (PAA). We have synthesized fluorescent tagged PAA as well as several neutral and positively charged polymers. Cultivation of the diatom Ulnaria ferefusiformis in the presence of these polymers showed that only PAA is able to penetrate into siliceous frustules. The presence of PAA in the frustules was confirmed with chromatography and PAA causes various aberrations of the valve morphology. Growth of U. ferefusiformis and two other diatoms in the presence of tri-and tetracarbonic fluorescent tagged acids points to the ability of diatoms to recognize substances that bear four acidic groups and to include them into siliceous frustules. Thus, partial condensation of silicic acid is a plausible first stage of silicon assimilation.

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