Elena N. Danilovtseva

From biosilicification to tailored materials: Optimizing hydrophobic domains and resistance to protonation of polyamines

Considerable research has been directed toward identifying the mechanisms involved in biosilicification to understand and possibly mimic the process for the production of superior silica-based materials while simultaneously minimizing pollution and energy costs. Molecules isolated from diatoms and, most recently sponges, thought to be key to this process contain polyamines with a propylamine backbone and variable levels of methylation. In a chemical approach to understanding the role of amine (especially propylamine) structures in silicification we have explored three key structural features: (i) the degree of polymerization, (ii) the level of amine methylation, and (iii) the size of the amine chain spacers. In this article, we show that there are two factors critical to their function: the ability of the amines to produce microemulsions and the presence of charged and uncharged amine groups within a molecule, with the latter feature helping to catalyze silicic acid condensation by a proton donor/acceptor mechanism. The understanding of amine–silicate interactions obtained from this study has enabled the controlled preparation of hollow and nonporous siliceous materials under mild conditions (circumneutral pH, room temperature, and in all aqueous systems) possibly compatible with the conditions used by biosystems. The “rules” identified from our study were further used predictively to modulate the activity of a given amine. We believe that the outcomes of the present contribution will form the basis for an approach to controlling the growth of inorganic materials by using tailor-made organic molecules.

Novel fluorescent dyes based on oligopropylamines for the in vivo staining of eukaryotic unicellular algae

Weakly basic fluorescent dyes are used to visualize organelles within live cells due to their affinity to acidic subcellular organelles. In particular, they are used to stain the silica deposited in the silica deposition vesicles (SDVs) of diatoms during the course of their frustule synthesis. This study involved the synthesis of fluorescent dyes derived from oligopropylamines, compounds similar to those found in diatoms. The dyes were obtained by reacting oligopropylamines with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole. The reaction was realized using methylated oligopropylamines with two or three nitrogen atoms and yielded two novel fluorescent dyes: NBD-N2 and NBD-N3. The dyes appeared to be highly efficient in the in vivo staining of growing siliceous frustules of diatoms at concentrations at least 10 times lower than those required for staining with HCK-123. NBD-N3 also efficiently stained other subcellular vesicles of eukaryotic unicellular algae. NBD-N2 stained only growing diatom frustules, whereas NBD-N3 also stained various subcellular organelles of different eukaryotic unicellular algae. NBD-N2 and NBD-N3 were not removed from stained diatom frustules by drastic treatments with H(2)SO(4) and H(2)O(2). Fluorescent silica can also be obtained by its chemical precipitation in the presence of NBD-N2 and NBD-N3.

Controlled stabilisation of silicic acid below pH 9 using poly(1-vinylimidazole)

We show, for the first time, inhibition of silicic acid condensation over a wide range of pH, especially below 9 using certain molecular mass fractions of poly(1-vinylimidazole) (PVI). This is achieved by stabilisation of molybdate-active Si species, which are crucial to condensation and growth to form silica. The structure of the resulting composites depends on the molecular mass of the PVI chains. Long-chain macromolecules can “encapsulate” Si species giving rise to stable soluble complexes. Short PVI chains stimulate association of silica particles and at neutral pH precipitation occurs. Protonation of imidazole units in acidic pH results in dissolution of the precipitates. We believe that the results presented herein using PVI as a model system will help elucidate the mechanisms underpinning the molecular interactions between (bio)macromolecules and inorganic materials.

Imidazole catalyzed silica synthesis: Progress toward understanding the role of histidine in (bio)silicification

Histidine is an amino acid present in proteins involved in biosilica formation and often found in peptides identified during phage display studies but its role(s) and the extent of its involvement in the silica precipitation process is not fully understood. In this contribution we describe results from an in vitro silicification study conducted using poly-histidine (P-His) and a series of different molecular weight synthetic polymers containing the imidazole functionality (polyvinylimidazole, PVI) for comparison. We show that the presence of imidazole from PVI or P-His is able to catalyze silicic acid condensation; the effect being greater for P-His. The catalytic mechanism is proposed to involve the dual features of the imidazole group—its ability to form hydrogen bonds with silicic acid and electrostatic attraction toward oligomeric silicic acid species.

Bioinspired thermo- and pH-responsive polymeric amines: multimolecular aggregates in aqueous media and matrices for silica/polymer nanocomposites.

Polymeric amines have been intensively studied for application in smart systems and as matrices for the design of composite materials, including bioinspired substances. A new thermo- and pH-responsive polymer was obtained by radical polymerization of N-(3-(diethylamino)propyl)-N-methylacrylamide. Upon heating, the polymer precipitated from aqueous solutions above pH 9; the observed cloud point was dependent on the polymer concentration and decreased from 95°C at pH 9 to 40°C at pH 11. The basicity of the polymer decreased at elevated temperatures owing to an increase in the hydrophobicity-driven compaction of the macromolecules. Dynamic light scattering analysis demonstrated that the formation of large multimolecular associates with radius 1000-2000 nm was initiated from 1 to 2°C below the cloud point. The new polymer is demonstrated to be an effective matrix for various siliceous composite structures, including 200-300 nm solid spherical raspberry-like particles and hollow hemispherical particles of more than 1000 nm diameter. Condensation of silicic acid in the presence of polymeric amines is a model reaction in biosilicification studies, and the obtained data are also discussed from the perspective of the matrix hypothesis for biosilica formation.

Cyclotrimerization and linear oligomerization of phenylacetylene on the nickel(I) monocyclopentadienyl complex CpNi(PPh3)2

The reactions of the CpNi(PPh3)2 monocyclopentadienyl complex with phenylacetylene and diphenylacetylene in toluene have been studied by ESR. When an alkyne is in twofold molar excess over nickel, it substitutes rapidly for PPh3 ligands to form the bisalkyne π complex CpNi(η2-C2PhR)2, where R = H or Ph. In the case of phenylacetylene, two structural isomers of the Ni(I) π complex have been identified. Irreversible clustering occurs in the system as time passes. When phenylacetylene is in excess, it oligomerizes actively at ambient temperature. The composition of the oligomerization products depends substantially on the reaction temperature: at T = 20°C, the main product is 1,2,4-triphenylbenzene (97% of the conversion products); at T = 40°C, the main products are linear oligomers with an average molecular weight of 1050. The formation and stabilization of active complexes in the system take place when the substrate is in excess. Phenylacetylene trimerization and linear oligomerization schemes in which the Ni(I) monocyclopentadienyl complex stabilized by substrate molecules is the active species are suggested.

Silicic acid condensation under the influence of water-soluble polymers: from biology to new materials

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 silica–protein 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.

Fluorescent Amines as a New Tool for Study of Siliceous Sponges

Siliceous sponges (Hexactinellida and Demospongiae classes) are aquatic invertebrates which are important both for marine and freshwater ecology and also as the source of biologically active compounds. The sponge skeleton consists of spicules - needle-like or branched composite structures based on silicon dioxide. Mechanisms of silicon assimilation and synthesis of high-ordered glass-like structures at ambient temperatures by sponges are intriguing for biologists, chemists and nanotechnologists. Fluorescent amines are in-vivo dyes that stain growing siliceous frustules of diatom algae so the use of these agents for the sponge study was attempted. We found that cultivation of the Lubomirskia baicalensis (Pallas, 1773) sponge in the presence of fluorescent tracers of biosilica - N1,N3,N3-trimethyl-N1-(7-nitro-2,1,3- benzoxadiazol-4-yl)propane-1,3-diamine and N1,N3-dime thyl-N1-[3-(dimethylamino)propyl]-N3-(7-nitro-2,1,3-benz oxadiazo-4yl)propane-1,3-diamine results in the staining of growing siliceous spicules. This finding shows that amine dyes accompany silicon from the environment to sponges spicules which opens a new way to study of silicon assimilation by sponges. Fluorescent staining of the growing spicules following with the confocal microscopy can be a powerful tool for morphological studies, revealing information about the dynamics of spiculogenesis and for bio-fabrication of new fluorescent materials.

Spiculogenesis in the siliceous sponge Lubomirskia baicalensis studied with fluorescent staining

Siliceous sponges are the most primitive multicellular animals whose skeleton consists of spicules - needle-like constructions from silicon dioxide surrounding organic axial filaments. Mechanisms of spicule formation have been intensively studied due to the high ecological importance of sponges and their interest to materials science. Light and electron microscopy are not appropriate enough to display the process from silicon-enriched cells to mature spicules because of composite structure of the sponge tissues. In this article, spiculogenesis in the siliceous sponge has been studied for the first time with the use of fluorescent microscopy. Fluorescent vital dye NBD-N2 was applied to stain growing siliceous structures in the sponge and primmorph cell system. The main stages of spicule growth in the fresh-water sponge Lubomirskia baicalensis (Pallas, 1773) were visualized: silicon accumulation in sclerocytes; formation of an organic filament protruding from the cell; further elongation of the filament and growth of the spicule in a spindle-like form with enlargement in the center; merger with new sclerocytes and formation of the mature spicule. Fluorescent microscopy combined with SEM allows us to overcome the virtual differentiation between intra- and extracellular mechanisms of spicule growth. The growing spicule can capture silicic acid from the extracellular space and merge with new silicon-enriched cells. Visualization of the growing spicules with the fluorescent dye allows us to monitor sponge viability in ecological or toxicological experiments and to apply genomic, proteomic and biochemical techniques.

Aluminum Complexes with a Donor Polymer: New Route to Organic/Inorganic Polymer Hybrids

The formation of alumina particles from aluminum salts in the presence of poly(1-vinylimidazole) was investigated by quantitative 27Al NMR, potentiometry and FTIR spectra. The interaction of poly(1-vinylimidazole) with aluminum chloride and nitrate in an aqueous medium stabilizes [AlO4Al12(OH)24]7+ and less ordered polymeric particles. The addition of NaOH (NaOH: Al ≥ 2) results in water-insoluble organic/inorganic hydrogen-bonded composites. Stabilization of the complexes is realized by cooperative hydrogen N···H—O—Al bonds, without N → Al donor-acceptor interactions.