Eike Brunner

Application of a chiral metal–organic framework in enantioselective separation

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.

Silica Biomineralisation in Diatoms: The Model Organism Thalassiosira pseudonana

After complete genome sequencing, the diatom Thalassiosira pseudonana has become an attractive model organism for silica biomineralisation studies. Recent progress, especially with respect to intracellular silicic acid processing, as well as to the natures of the biomolecules involved in diatom cell wall formation, is described. On the one hand, considerable progress has been made with respect to silicon uptake by special proteins (SITs) from the surrounding water, as well as to the storage and processing of silicon before cell division. On the other hand, the discovery and characterisation of remarkable biomolecules such as silaffins, polyamines and—quite recently—of silacidins in the siliceous cell walls of diatoms strongly impacts the growing field of biomimetic materials synthesis.

Biomimetic synthesis of silica nanospheres depends on the aggregation and phase separation of polyamines in aqueous solution

Long-chain polyamines extracted from the highly siliceous cell walls of diatoms are known to precipitate silica nanospheres from aqueous, silicic-acid containing solutions at near-neutral pH in vitro. The same is true for synthetic polyamines such as polyallylamine. In the present contribution we show that the microscopic phase separation of polyallylamine in aqueous solution is strictly correlated with the silica precipitation activity of polyallylamine/silicic acid solutions. Multivalent anions such as phosphate or sulfate efficiently induce this microscopic phase separation. At higher anion concentrations, macroscopic phase separation occurs. In contrast to the multivalent phosphate and sulfate ions, the monovalent chloride ions are much less efficient in polyallylamine aggregate formation.

Chitin-based scaffolds are an integral part of the skeleton of the marine demosponge Ianthella basta

The skeletons of demosponges, such as Ianthella basta, are known to be a composite material containing organic constituents. Here, we show that a filigree chitin-based scaffold is an integral component of the I. basta skeleton. These chitin-based scaffolds can be isolated from the sponge skeletons using an isolation and purification technique based on treatment with alkaline solutions. Solid-state (13)C NMR, Raman, and FT-IR spectroscopies, as well as chitinase digestion, reveal that the isolated material indeed consists of chitin. The morphology of the scaffolds has been determined by light and electron microscopy. It consists of cross-linked chitin fibers approximately 40-100 nm in diameter forming a micro-structured network. The overall shape of this network closely resembles the shape of the integer sponge skeleton. Solid-state (13)C NMR spectroscopy was used to characterize the sponge skeleton on a molecular level. The (13)C NMR signals of the chitin-based scaffolds are relatively broad, indicating a high amount of disordered chitin, possibly in the form of surface-exposed molecules. X-ray diffraction confirms that the scaffolds isolated from I. basta consist of partially disordered and loosely packed chitin with large surfaces. The spectroscopic signature of these chitin-based scaffolds is closer to that of alpha-chitin than beta-chitin.

Three-dimensional chitin-based scaffolds from Verongida sponges (Demospongiae: Porifera). Part I. Isolation and identification of chitin.

Marine invertebrate organisms including sponges (Porifera) not only provide an abundant source of biologically active secondary metabolites but also inspire investigations to develop biomimetic composites, scaffolds and templates for practical use in materials science, biomedicine and tissue engineering. Here, we presented a detailed study of the structural and physico-chemical properties of three-dimensional skeletal scaffolds of the marine sponges Aiolochroia crassa, Aplysina aerophoba, A. cauliformis, A. cavernicola, and A. fulva (Verongida: Demospongiae). We show that these fibrous scaffolds have a multilayered design and are made of chitin. (13)C solid-state NMR spectroscopy, NEXAFS, and IR spectroscopy as well as chitinase digestion and test were applied in order to unequivocally prove the existence of alpha-chitin in all investigated species.

Mineralization of the metre-long biosilica structures of glass sponges is templated on hydroxylated collagen

The minerals involved in the formation of metazoan skeletons principally comprise glassy silica, calcium phosphate or carbonate. Because of their ancient heritage, glass sponges (Hexactinellida) may shed light on fundamental questions such as molecular evolution, the unique chemistry and formation of the first skeletal silica-based structures, and the origin of multicellular animals. We have studied anchoring spicules from the metre-long stalk of the glass rope sponge (Hyalonema sieboldi; Porifera, Class Hexactinellida), which are remarkable for their size, durability, flexibility and optical properties. Using slow-alkali etching of biosilica, we isolated the organic fraction, which was revealed to be dominated by a hydroxylated fibrillar collagen that contains an unusual [Gly-3Hyp-4Hyp] motif. We speculate that this motif is predisposed for silica precipitation, and provides a novel template for biosilicification in nature.

Biomineralization in diatoms: Characterization of novel polyamines associated with silica

Pattern formation during silica biomineralization in diatoms appears to depend on long‐chain polyamines as well as proteins covalently modified with polyamines (silaffins). Recently, the complete genome of the diatom Thalassiosira pseudonana has been sequenced making this species an attractive model organism for future studies on biomineralization. Mass‐ and NMR‐spectroscopic analysis of the long‐chain polyamines from this diatom species reveals the existence of a complex population with as yet unknown structural features. These include complex methylation patterns, different attachment moieties as well as the existence of quaternary ammonium functionalities.

Magic-angle-spinning NMR studies of acid sites in zeolite H-ZSM-5

1H, 11C, 17Al, and 29Si magic-angle-spinning (MAS) NMR was used to elucidate the nature of the catalytic activity of zeolite H-ZSM-5. 1H MAS NMR of sealed samples after mild hydrothermal dealumination shows that the enhanced activity for n-hexane cracking is not due to an enhanced Bronsted acidity. The concentrations of the various OH groups and aluminous species suggest that the reason for the enhanced catalytic activity is the interaction of the n-hexane molecule with a bridging hydroxyl group and with extra-framework aluminium species. Loading the samples with HCOOH or HCl shows that those extra-framework aluminium species, which give rise to the enhanced activity, cannot be easily removed from their positions, and are therefore immobilized by the zeolitic framework.

Biomimetic silica formation: analysis of the phosphate-induced self-assembly of polyamines.

The highly siliceous cell walls of diatoms are probably the most outstanding examples of nanostructured materials in nature. Previous in vitro experiments have shown that the biomolecules found in the cell walls of diatoms, namely polyamines and silaffins, are capable of catalysing the formation of silica nanospheres from silicic/oligosilicic acid solutions. In a previous publication, silica precipitation was found to be strictly correlated with a phosphate-induced microscopic phase separation of the polyamines. The present contribution further characterises the phase separation behaviour of polyamines in aqueous solutions. In particular, a pronounced pH-dependence of the average particle diameter is found. It is, furthermore, shown that the ability of phosphate ions to form polyamine aggregates in aqueous solutions cannot be a purely electrostatic effect. Instead, a defined hydrogen-bonded network stabilised by properly balanced electrostatic interactions should be considered. Finally, solid-state 31P NMR studies on phase-separated polyamines, synthetic silica precipitates, and diatom cell walls from the species Coscinodicus granii support the assumption of a phosphate-induced phase separation process taking place during cell wall formation.

Three-dimensional chitin-based scaffolds from Verongida sponges (Demospongiae: Porifera). Part II: Biomimetic potential and applications

In order to evaluate the biomedical potential of three-dimensional chitinous scaffolds of poriferan origin, chondrocyte culturing experiments were performed. It was shown for the first time that freshly isolated chondrocytes attached well to the chitin scaffold and synthesized an extracellular matrix similar to that found in other cartilage tissue engineering constructs. Chitin scaffolds also supported deposition of a proteoglycan-rich extracellular matrix of chondrocytes seeded bioconstructs in an in vivo environment. We suggest that chitin sponge scaffolds, apart from the demonstrated biomedical applications, are highly optimized structures for use as filtering systems, templates for biomineralization as well as metallization in order to produce catalysts.