Biomimetic experiments

In fact, such biomimetic molecules demonstrate the ability to tailor the growth of silica nanoparticles in a way that is very similar to diatom-extracted species. However, they demonstrate the same limitations in terms of morphological control of nanoparticle assembly. This is because the diatom shell architecture results not only from interactions of silica precursors with templating molecules but also benefits from a cell-driven molding of the vesicular compartment where silicification occurs [29]. Thus, it is very likely that diatom-like synthetic silica will only be achieved when such confinement/molding effects are taken into account in the design of biomimetic experiments [30]. 

However, very recently in a series of biomimetic experiments it has been established that under virtually physiological conditions vincoside is transformed into akuammigine (140b) and strictosidine into tetrahydroalstonine (139b) and ajmalicine i.e. the precursor for the 3a Corynanthe alkaloids was the 3a glycoalkaloid and hence no inversion at C-3 was required (73). Concurrent biosynthetic work on V. rosea it was found that strictosidine was... 

Reminder of the weaknesses inherent in, and the reassurance possible from, biomimetic experiments. 

Evidence for the intermediacy of the enamine (138) and the immonium ion (140) derives from the condensation reaction in which potassium cyanide replaced sodium cyanoborohydride the immonium ion (140) was thus trapped by reaction with nucleophilic cyanide ion, with formation of 21-cyanotetrahydroalstonine (141a) and 21-cyanoakuammigine (141b). Both cyano-compounds suffered slow reduction to the parent alkaloid by sodium borohydride, and were re-converted (by silver acetate) into the precursor immonium ions (140), which could be rapidly reduced (NaBH4) to the alkaloids. Scheme 17 illustrates some, but not by any means all, of the numerous interconversions involved in these biomimetic experiments. 

The assumption was made that this addition takes place after p-lactam formation (105). For biomimetic experiments see ref. (24, 105, 274, 275). 

U. Schmidt and coworkers 287, 352) condensed dehydrovaline ester with aromatic aldehydes and pyridoxal to yield Schifif bases directly, which were employed in biomimetic experiments (VI. A. 2.1). 

Reduction with sodium borohydride and tritiated sodium boro-hydride (28, 61,144,164, 434) has been used in the structural elucidation of natural products with dehydroamino acid units, as has the addition of mercaptans or nitromethane. Addition of S-nucleophiles is the method of choice for the preparation of sulfur-containing amino acids such as cysteine, penicillamine, lanthionine and cystathionine. Such additions have also been often carried out in biomimetic experiments in the study of penicillin biosynthesis (see Table 3). 

Structural analogs of the tripeptide were converted to analogs of isopenicillin N in C. aaremonium. No authentic intermediates between the LLD-ACV-tripeptide and isopenicillin N have been obtained . It has been proposed that free radicals are involved in this conversion, and biomimetic experiments support this suggestion . The subsequent conversion of isopenicillin N to penicillin N, and thence to desacetoxycephalosporin C, desacet-ylcephalosporin C, and cephalosporin C, has been acieved in this species and Streptomyaes alavuligema. ... 

Both mechanisms can also rationalize an increase in due to the production of superoxide/HO2 (18.16), which appears to dominate the flux of partially reduced oxygen species generated by certain biomimetic catalysts [Boulatov et al., 2002 Boulatov, 2004]. It remains to be estabhshed if either of these two mechanisms does indeed operate in simple Fe porph5Tins, for example by carrying out single-turnover experiments similarly to the approach used to study ORR by C5hochrome c oxidase. 

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Bianco, A., Chiacchio, U., Rescifina, A., Romeo, G., and Uccella, N. (1997). Biomimetic supramolecular biophenol-carbohydrate and biophenol-protein models by NMR experiments. ]. Agric. Food Chem. 45, 4281-4285. 

As already mentioned, due to progress on the experimental side the last decade has seen tremendous research efforts in the area of ion-molecule reactions, and especially, the reactions of metal cations with neutral organic molecules have received a great deal of interest53. In particular, the possibilities of studying biomimetic effects and obtaining data about the intrinsic properties of unligated ( bare ) metal ions in mass spectrometric experiments have prompted intense research activities. 

The self-catalyzed model is a simplification of the actin polymerization model of Oosawa and Kasai (1962) [and more recent elaborations of it (Niranjan et al., 2003)], and both equilibrium constants K and Kn, and hence the free energies g and ga can in principle be obtained by fitting the theory to assembly experiments. Typical values of g for, for example, actin vary between —10 and —20 kBT and ga between +2 and +3 kBT (Oosawa and Asakura, 1975 Oosawa and Kasai, 1962). For the biomimetic compound oligo(phenylene vinylene) similar to compound 2 shown in Figure 6, dissolved in alkane solvents, similar values were found for g but much larger ones for ga of +8 to +10 kBT, making the supramolecular polymerization of this compound an extremely highly co-operative process (Jonkheijm et al., 2006). 

The transport of molecules across biological cell membranes and biomimetic membranes, including planar bilayer lipid membranes (BLMs) and giant liposomes, has been studied by SECM. The approaches used in those studies are conceptually similar to generation-collection and feedback SECM experiments. In the former mode, an amperometric tip is used to measure concentration profiles and monitor fluxes of molecules crossing the membrane. In a feedback-type experiment, the tip process depletes the concentration of the transferred species on one side of the membrane and in this way induces its transfer across the membrane. 

This chemical experiment aims to imitate the biosynthesis of terpenes. A mixture of products results. Draw a mechanism for the reaction. To what extent is it biomimetic, and what can the natural system do better ... 

Based on the above activation mechanism we wondered whether it would be possible to develop a biomimetic, organocatalytic reductive amination or transfer hydrogenation of ketimines. We reasoned that the activation of the imine by catalytic protonation through the Brpnsted acid should enable the hydrogen transfer from a suitable NADH mimic to yield the corresponding amine (Fig. 2). Hence, initial experiments focused on the examination of various Brpnsted acids in combination with different hydride sources (Rueping et al. 2005a). 

Maackolin, C25H24O8, isolated from Maackia amurensis, was identified as 8-epi-8 -epi-3-de-0-methyl-5 -methoxygnetifolin F (143), based on the spectroscopic comparison between maackolin pentacacetate and gnetifolin F tetraacetate, and NOE difference NMR experiments on maackolin (755). Lehmbachol D (142), isolated from G. cleistostachyum, is structurally similar to 143 and it possesses a methoxy group in place of the hydroxy group at the C-3 position. The NOE correlation of H-8 and H-8 indicated their mutual cis orientation. The biomimetic synthesis of 142 has also been described (752) (Scheme 11). 

---