Silaffins structure

FIGURE 1. The lysine family of cationic amino acids. Structures include the typical a-amino acid found in proteins, and the cationic side-chains of lysine, arginine and the two substituted lysine derivatives recently found in high abundance in the silaffins occluded within the biosilica of a diatom35. The derivatives with multiple methylaminopropyl units were previously unknown in biological systems... 

Other than accelerating the formation of spherical nanoparticles of silica from metastable silicic acid solutions, the silaffins appear to have no structure-directing activity. If they are responsible for silica formation in the living diatom, as seems quite likely, control of the higher order architecture of the resulting silica apparently must be determined by the pre-formed shape of the silica deposition vesicle (the envelope within which the silica grows) serving as a complex three-dimensional mold. 

Many other polyamines are known. Among these are a 4-aminobutylcadaverine isolated from root nodules of the adzuki bean and very long partially aromatic hydroxylamine derivatives from venom of common funnel-web spiders (structures at top of page). Cationic polypeptides called silaffins, with masses of 3 kDa, apparently initiate the growth of the silica cell walls of diatoms (Box 4-B). These peptides contain polyamines consisting of 6 to 11 repeated N-methylpropylamine imits covalently attached to lysine residues and also many phosphoserines. ... 

It is well known that a variety of organisms are able to produce intricately patterned species-specific siliceous materials and that they are able to do so with great fidelity. In order to understand the role(s) that biomolecules play in the formation of nano- and micro-structured biosilica, organic material associated with biosilica has been isolated, identified and studied. To date, the main focus has been a variety of investigations of R5 (in both its modified and unmodified forms) and its ability to form silica in vitro. The R5 peptide is a nineteen amino acid sequence which corresponds with silaffin-lAi of the diatom Cylindrotheca fusiformis. In order to study the role of biomolecules in (bio)silicification, we have chosen to systematically study the silica forming ability of three peptides derived fi om the diatom C. fusiformis - namely, Rl, R2, and R5. The R1 peptide is a thirty three amino acid... 

In order to understand the roles that biomolecules play in the fabrication of nano- and micro-structured biosilica, organic material associated with biosilica was identified, isolated and studied. In addition to some earlier studies, the following recent findings are of particular importance. Several proteins isolated from plants, diatom species (silafFins) and sponges (silicateins) have previously been shown to facilitate the in vitro polymeri tion of silica from a silicon catecholate complex, tetramethoxysilane (TMOS) and tetra-ethoxysilane (TEOS), respectively. Bioextracts from plants Equisetum talmateia and Equisetum arvense have been isolated from biosilica. These... 

Kroger, N. Deutzmann, R. Sumper, M., Silica-precipitating peptides from diatoms - The chemical structure of silaffin-lA from Cylindrotheca fiisiformis. J. Biol. Chem. 2001,276, (28), 26066-26070. 

The zwitterionic structure of the native silaffins leads to the self assembly of these molecules, which explains their extremely efficient induction of silica precipitation. Following up on this model, Sumperl l proposed, on the basis of simulations, a model that consists of multiple steps in which phase separation processes occur. At the initial stage phase separation of protein phase and silica permits the formation of the large, honeycomb structures, followed by several intermittent steps of silica formation - each mediated by phase separation processes to create smaller structures (see Fig. 9.18). 

The acid-catalyzed polycondensation of tetraethoxysilane, a silicic acid precursor, results in the formation of an aggregated silica gel (Figure 1). Con aratively, the three-dimensional structure of die aggregated silica gel was similar to the natural morphologies of the silica particles catalyzed by proteins (9) and polypeptides (12J3J7). Furthermore, e use of external force was demonstrated (21) to control the morphology (e.g. arched, fibrillar) of a silica material catalyzed by a biomimetic silaffin peptide (i.e. a non-modified Sil Ip R5... 

When another cationically charged polymer (poly-L-lysine discussed below) was added to the reaction mixture along with PAH, disk-like silica particles were observed rather than spherical silica particles as seen with just PAH (Fig. 8). It is postulated that the two polymers affect the chain conformations of each other, thus altering the product morphology. Experiments in vitro using a mixture of polyamines and silaffins extracted from diatoms resulted in similar disk-like structures. ... 

Figure 1.14 Chemical structure of silaffin-1 Ai from C. fusiformis. Reproduced with permission from Ref. [55] 2006, Wiley-VCH Verlag GmbH Co. KGaA.

The major differences between biological and biomimetic silica formation mainly lies in precursor concentrations (undersaturated in biosilicification), time required for biosilica deposition, involvement of other molecules (ions, organic molecules, and membranes), and the presence of a confined environment in diatoms. Nonetheless, proteins that direct biomineralization in nature can be used to control the production of nanostructured materials and fecilitate the febrica-tion of new structures in vitro under ambient conditions. Indeed, polycationic silaffins and silicateins isolated from diatoms and sponges, respectively, were shown to generate networlcs of silica nanospheres within seconds when added to a solution of silicic acid. 

Figure 19.7 Chemical background, (a) Schematic chemical structure of native silaffin-IA (pH 5). (Adapted from Ref. [56].) (b) Silicatein topology and the key role of the hydroxy group of serine-26 and the imidazole side chain of histidine-165 in the proposed mechanism for silicatein-mediated catalysis. (Adapted from Ref. [1001.)...

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