Silaffins

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. 

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In contrast, such approaches have been much more developed with proteins, termed silicateins, that have been extracted from some silicified sponges [38]. The success of these approaches probably originates from the fact that the reactivity of these proteins towards silica precursors differs significantly from the processes occurring in diatoms. Whereas silaffins and poly-amines activate silica formation via electrostatic interactions due to the presence of positively-charged ammonium... 

Kroger, N., Lorenz, S., Brunner, E. and Sumper, M. (2002) Self-assembly of highly phosporylated silaffins and their function in biosilica morphogenesis. Science, 298, 584-586. 

Poulsen, N., Sumper, M. and Kroger, N. (2003) Silica formation in diatoms Characterization of native silaffin-2 and its role in silica morphogenesis. Proceedings of the National Academy of Sciences of the United States of America, 100, 12075-12080. 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

Both causes may be advocated for the widespread distribution of the silaffins. These polypeptides, as the name implies, show high affinity for silica, which is required to build up the skeleton of diatoms (Chart 8.2.P). Another example in this group is provided by certain tetrapyrroles that act as luciferins in marine dinoflagellates and other organisms (Chart 8.2.A Tables 9.1 and 13.5.II). 

In each of these formulas additional free OH groups are available on the silicon so that it is possible to crosslink more than two polysaccharide chains. Silicon may function as a biological crosslinking agent in connective tissue. Silaffins, small polypeptides containing polyamine side chains of modified lysine residues, apparently initiate silica formation from silicic acid in diatoms.0... 

Hypusine (Ne-(4-amino-2-hydroxybutyl)lysine)242 occurs in mammalian initiation factor 4D, which is utilized in protein synthesis (Chapter 29) and is formed by transfer of the 4-carbon butylamine group from spermidine to a lysine side chain followed by hydroxylation 280 2823 The lupine alkaloid lupinine283 is formed from two C5 units of cadaverine which arises by decarboxylation of lysine. Silaffins (pp. 178, 1381) also contain modified lysines. 

Three families of proteins have been identified in the organic matrix of the cellular wall of marine microorganisms [59], among which silaffins are responsible... 

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... 

Within seconds after their addition to dilute silicic acid, the silaffins induce co-precipita-tion of silica nanospheres35. Rates of formation of the nanoparticles are rapid and constant from pH 5 to pH 7 the precipitates form stoichiometrically with the silaffins, which become occluded within the silica. The nanoparticles produced in vitro are comparable in dimension to those that have been observed in the developing biosilica in diatoms36 37, as well as those revealed by differential etching of the biosilica produced by sponges38. 

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. 

Silaffins (proteins with silica affinity) are a family of phosphoproteins, which are loosely associated with diatom silica two other families of cell proteins, frustulins, and pleuralins are found bound to the cell walls of diatoms. 

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