Abstract
Biosilicification sets the standard for the localized in vitro precipitation of silica at low orthosilicate concentrations in aqueous environment under ambient conditions. Numerous parameters must be controlled for the development of new technologies in designing inventive nanosilica structures, which are able to challenge the biological templates. A long neglected requirement that came into focus in the recent years are the cellular techniques of preventing unintentional lithification of cellular structures since numerous cellular components such as membranes, DNA, and proteins are known to precipitate nanosilica. The diatom metabolism makes use of techniques that restrict silicification to an armor of silica around the cell wall while avoiding the petrifying gaze of Medusa, which turns the whole cell into stone. Step by step, biochemistry unveils the hierarchical interplay of an arsenal of low-molecular weight molecules, proteins, and the cytoskeletal architecture and it becomes clearer why the organisms invest much metabolic effort for an obviously simple chemical reaction like the precipitation of amorphous silica. The discrimination between different soluble components in the silicification process (chemoselective silicification) is not only vitally important for the diatom but poses an interesting challenge for in vitro experiments. Until now, silica precipitation studies were mainly focused on the amount, the morphology, and composition of the precipitate while disregarding a quantitative analysis of the remaining soluble components. Here, we turn the tables and quantify the soluble components by 1H NMR in the progress of precipitation and present experiments which quantify the additivity, and potential cooperativity of long chain polyamines (LCPAs) and cationic peptides in the silicification process.
Highlights
Modifications of the Stöber method [1] are in use today for the synthesis of largely monodisperse silica particles with entrapped enzymes for NMR studies [2] or numerous other applications [3]
We investigate the simplest scenario of chemoselective precipitation, which is the differentiation between two dissolved components, a cationic peptide and an oligoamine, that are both capable of precipitating silica on their own (Figure 1)
We and others analyzed to what extent the mineralisation process and the morphology of the precipitate depends on the number of propyleneimino repeating units [11], the type of KXXK-boxes in silaffin proteins (K = Lys, X = other amino acid) [12], or the number of POG tripeptide repeating units in collagens [13]
Summary
Modifications of the Stöber method [1] are in use today for the synthesis of largely monodisperse silica particles with entrapped enzymes for NMR studies [2] or numerous other applications [3]. The unmitigated silicification entraps the dissolved molecules as far as possible in the silica precipitate. Biosilicification relies on the sharp differentiation between soluble and entrapped molecules, a sophisticated form of chemoselective silicification. Accepted models are the LCPA–phosphate model [4] and the silaffin-matrix hypothesis [5]. Both formulate varying concentrations of soluble components at the surface of the forming silica beads. Poulsen et al investigated the mutual influence of peptides and LCPAs. Here we investigate the simplest scenario of chemoselective precipitation, which is the differentiation between two dissolved components, a cationic peptide and an oligoamine, that are both capable of precipitating silica on their own (Figure 1). Observing dissolved molecules next to the precipitate gives answers to questions such as these: Is the amine completely consumed by the precipitate formed or is there a fixed N/Si ratio leaving the surplus amine untouched? What happens with the less capable Si precipitator in the presence of the better precipitator? Is there a measurable cooperativity between peptides and amines?
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