Significance of research
Biomimetic strategies have paved the way for synthesis of complex organic molecules from simple precursors. Though much progress has been seen in this field, developing synthetic equivalents from naturally occurring substances is still a challenge (Enders, 2008, p. 46). Biomineralisation is a process which produces synthetic minerals or their precursors under moderate conditions of supersaturation in contrast to colloidal or solution processing techniques normally used. The ‘engineered substrate’ surface would influence the ‘morphology, location, orientation and crystallographic phase’ (Leonor et al, 2004, p. 123). Most of the biological model systems are complex. The microenvironment and the relationship between the organic matrix and the precursors of mineral formations decide the kind of product obtained. Complex architectural products are formed measuring from nanometres to millimeters in the form of minerals, structural biological polymers of proteins and polysaccharides, lipids and cells and at minimum energy costs. The biomineralisation pathway has the added advantage of being environmental friendly. (Liu and Lim, 2003). The resultant tissue has multifunctions many times more than its precursor and minimizing the amount of material used. Biological materials have the ‘self-repairing ability’.
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Technologies like ‘electronics, biotechnology, biosensors, material’s fabrication such as ceramics, nanocomposites, coatings use enzymes, proteins, peptides, functional groups, anchoring units and growth modifiers’ for inorganic compound synthesis (Leonor et al, 2004, p. 143). These methods are also useful in developing material for biomedical applications. Multifunctional biomolecules could be manufactured through recombinant proteins, genetic engineering site-directed mutagenesis to obtain peptides that bind specifically to inorganic surfaces (Shiba et al, 2003). The enormous number of enzymes in Nature enhances the possibility of designing new multifunctional synthetic products using natural techniques. The experience of the natural world can be effectively used for making great advances in materials research and a huge contribution to society (Leonor et al, 2004, p.144).
New polypropionate metabolites have been isolated from terrestrial and marine
ecological systems from phylogenetically diverse structures as sponge and bacteria (Moses, 2003, p. 3670). Many of the isolated compounds like erythromycin, monensin, methymycin, tylosin and nonactic acid are biologically important. Their functions range from immunosuppression to cytotoxicity to antibiotic properties.
Review of background research
Many unusual natural products which have potential uses in mankind have been synthesized in a biomimetic manner. A short total synthesis of Aureothin 1 and N acetyl aureothamine 2 has been reported (Jacobsen, 2005, p. 641). N-acetyl aureothamine 2 has been isolated from Streptomyces netropsis and found to be highly potent against the Helicobacter pylori organism which causes a kind of chronic gastritis (Taniguchi, 2000). Aureothin 1 is found in Actinomycetes mycelia and has anti tumour, antifungal and pesticidal actions (Hirata, 1961). AurF is the N oxygenase which oxidizes the p-aminobenzoate to the corresponding nitro conmpound which is the first unit for the formation of Aureothin 1 (He, 2004). AurH catalyses the formation of the exomethylene tetrahydrofuran ring of Aureothin1 (He, 2004).
A restricted group of marine molluscs belonging to the order Sacoglossa regularly produced pyrone containing compounds (Moses, 2005, p. 1687). These compounds, derived by a polyketide pathway, account for the specific ecophysiological pathways of the molluscs, acting as mediators in tissue regeneration and chemical defence (Cimino, 2001). John Faulkner isolated the pyrone from compounds of tridachione and 9, 10- deoxytridachione from the ether soluble oil of the ascoglossan mollusc Tridachiella diomedea or Mexican dancer (Moses, 2005, p. 1687). In vitro studies were done by Ireland and Faulkner (1981) while in vivo studies were done by Ireland and Scheuer (1979). The chemical reactions are believed to have proceeded through a pericyclic pathway as there was optical activity during photolysis. The extraordinary chemical behavior and molecular structures associated with the marine molluscs have made researchers investigate further into the synthesis of the metabolites (Moses, 2005, p. 1687). The origins of the highly unsaturated natural products may be explained by cascade isomerisation and electrocyclisation processes of polyene precursors.
Researchers have also found a new thermally induced tandem pericyclic process producing a new complex tricyclic core structure. This new process opens up the possibilities of high yielding and highly selective synthetic products originating from the class of highly strained polyene esters (Moses, 2003, p. 6626).
Two new polypropionate metabolites have been isolated from Streptomyces spectabilis: SNF4435C and SNF4435D (Kurosawa, 2001). Both have been formed from complex natural products through cycoladditions and/or electrocyclisations (Tchabanenko, 2005 in Jacobsen, 2005, p.2473 ). In vitro these two compounds selectively suppress induced B-cell proliferation against induced T-cell proliferation (Jacobsen, 2005, p. 2473). This mode of immunosuppression is different from the actions of cyclosporine A (CsA) and FK-506 which are also immunosuppressants. The possibility of producing newer immunosuppressants is raised here. The two compounds are compact molecules with five stereogenic centres, four residing on the rare cyclobutane ring. Spectinabilin is a third metabolite from Streptomyces spectabilis. The significance of this compound is the complex polypropionate-acetate structure having a highly substituted tetraene moiety. It is worth pursuing investigation of spectinabilin. The total synthesis of spectinabilin and subsequent biomimetic conversion to SNF4435C and then SNF4435D through cascading isomerisations and electrocyclisations have been done (Jacobsen, 2005, p.2474).
The tissue regeneration of an ascoglossan species of mollusc, cyerce crystallina, which has a peculiar manner of fighting off predators and then regenerating itself, has inspired researchers to study its activities and production of cyercene (Moses, 2004, p. 6447). When attacked, this mollusc secretes a thick toxic mucus and if further assaulted, it lets go of its cerata or appendages which form the bulk of its body. The appendages however go on contracting and secreting the mucus. Within 10 days it regenerates its cerata. Analysis of the mollusc’s cerata led to the isolation and characterization of 7 new pyrone containing compounds called cyercenes. The signaling for the regeneration was caused by cyercene A1 (Moses, 2004, p. 6447).
Specific aims for research
The aims of most researches of biomimetic chemistry are the pursuit of novel materials. Mimicking the processes for synthesis of compounds of technological interest has become the resort of many researchers. Yeasts like Candida Glabratu and Schizosac-charoyces have given rise to crystallites of Cadmium sulphide (Nedoluzhko, 2001, p.37). Similar studies have led to the synthesis of use immunosuppressants, cytotoxic drugs and antibiotics. Aims of the researches are concentrating on the various molecular interactions between inorganic and organic material and subsequent production of synthetic materials which possess more functions than the precursor. A biomimetic approach to materials synthesis influences the morphology, size and other features of the substance. Living organisms synthesise inorganic materials to suit a particular function. The interactions are mediated by the organisms based on the ‘localization of the organic template, availability of the inorganic precursors, control of pH and ionic strength and lead to complex structures (Nedoluzhko, 2001, p.10).
Two broad types of approaches have been adopted by researchers in biomimetic synthesis. In one approach, molecular interactions are the main features. They lead to nucleation and crystal growth (Nedoluzhko, 2001, p.10). In the other approach, mineral components are assembled into complex shapes and structures (tectonics). The tectonics provide the new dimension to the properties of the material. Crystal nucleation and growth are influenced by organic molecular assemblies and growth modifiers in solution. Langmuir monolayers, Langmuir-Blodgett films, phospholipids vesicles, water-in-oil microemulsions, proteins, protein–nucleic acid assemblies, nucleic acids, gels, and growth additives are all growth modifiers. The interactions between the inorganic and organic molecules which are essential for crystal formation could be stereochemical, electrostatic, geometric and spatial in nature (Nedoluzhko, 2001, p.10). Two length scales are used for measurement in biomimetic chemistry. Crystal fidelity is ensured by the molecular length scale while the organismal length scale is used for the other approach.
Crystalline materials form from supersaturated solutions and undergo the stages of nucleation and growth. Molecular interactions influence subsequent growth by inhibiting certain crystal faces and lead to a change in shape. Nucleation could be homogenous or heterogenous. Slow growing crystals maintain their shape (Nedoluzhko, 2001, p.10).
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