Biomimetic activity

A study of electro-assisted biomimetic activation of molecular oxygen by a chiral Mn(salen) complex in [BMIMJPFs showed that a highly reactive oxomanganese(V) intermediate could transfer its oxygen to an alkene (229). 

Drenth, Nolte and their coworkers developed polyisonitriles as channel mimics in the 1980s (Beijnen et al., 1982). These compounds were characterized more fully later (Roks and Nolte, 1992), but this pioneering never provided true biomimetic activity. 

Larger capsules prepared by the same group have been used extensively as nanoreactors in which exotic chemical species can be formed. The group has also used a molecular panel approach as shown in Fig. 3.8. These compounds are, however, too small for any biomimetic activity. 

The first structurally characterized mononuclear Cu nitrite complex with biomimetic activity was [(L )Cu(N02)] (L = l,4,7-triisopropyl-l,4,7-triacyclononane) in which nitrite binds in an y -N mode (Figure 7)- On treatment with acetic acid or the Lewis acid MesSiOsSCFs, this complex yielded NO and L Cu(02CCH3) 2 or L " Cu(03SCF3)2, respectively. 

Figure 7 Stable mononuclear 7j -A-nitrite Cu complex with biomimetic activity.

Considerable attention has been given to the synthesis of compounds in which a single mono- or multinuclear coordination unit is at the center of a dendrimer array. These systems have been prepared by the construction of ligands with pendant dendritic wedges with subsequent convergent assembly upon coordination of a metal ion at the center or by covalent synthesis of an appropriate dendrimer-functionalized macrocyclic core. The primary interest for the study of such compounds lies in the very unusual, usually hydrophobic, environment in which the metal(s) at the center find themselves. This environment has been likened to that at the active site of a metalloenzyme and numerous studies have dealt with the catalytic and biomimetic activity of such conjugates. A general introduction to the area and the concept of chemistry within dendrimers is available. " The term dendrizyme has been used to describe these systems. 

We recently reported for the first time on the electroassisted-biomimetic activation of molecular oxygen by a newly prepared electropolymerized polypyrrole-manganese phthalocyanine film " The prepared films and their intervention in the electroassisted-catalytic reduction of molecular oxygen were analyzed by cyclic voltammetry and UV-visible spectrophotometry on optically transparent electrodes. The obtained results demonstrate the probable existence of the... 

Gaillon, L. and F. Bedioui (2001). First example of electroassisted biomimetic activation of molecular oxygen by a (salen)Mn epoxidation catalyst in a room-temperature ionic liquid. Chem. Commun. (16), 1458-1459. 

The Mannich reactions plays an important role in pharmaceutical chemistry. Many /3-aminoalcohols show pharmacological activity. The Mannich reaction can take place under physiological conditions (with respect to pH, temperature, aqueous solution), and therefore can be used in a biomimetic synthesis e.g. in the synthesis of alkaloids. 

In summary, these recently obtained results demonstrate that certain amphi-pathic peptoid sequences designed to mimic both the helical structure and approximate length of magainin helices are also capable of selective and biomimetic antibacterial activity. These antibacterial peptoids are helical in both aqueous buffer and in the presence of lipid vesicles. Ineffective (non-antibacterial) peptoids exhibit weak, random coil-like CD, with no spectral intensification in the presence of lipid vesicles. Selective peptoids exhibit stronger CD signals in bacterial-mimetic vesicles than in mammalian-mimetic vesicles. Non-selective peptoids exhibit intensely helical CD in both types of vesicles. 

Fig. 1.9 S urface pressure ( r)-area (A) isotherms obtained for a lipid mixture (DPPC POPG PA, 68 22 9 (by weight)), alone and with 10% (w/w) of either SP-C peptide or SP-C peptoid added. Results indicate that the addition of the SP-C mimics engenders biomimetic surface activity, as indicated by lift-off at a higher molecular area and the introduction of a plateau...

Within the past 10 years, various biomimetic Fe model complexes were prepared and their catalytic activities in the electrochemical reduction of protons to H2 were investigated (Scheme 57). 

A biomimetic oxidation with perfluorinated porphyrin complexes [(F20TPP) FeCl] showed high catalytic activity with secondary alcohols with over 97% yield in all cases [144]. Furthermore, this catalyst is able to oxidize a broad range of alcohols under mild conditions with wCPBA as terminal oxidant. Here, an a-hydroxyalkyl radical species is proposed as central intermediate. 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

When discussing seco alkaloids the question of their genesis should not be disregarded. Are they true alkaloids or artifacts of isolation It is difficult to answer this question with certainty. Some of them, e.g., secophthalide-isoquinoline ene lactams, are postulated to be formed during the extraction process however, most of them are believed to be metabolites produced naturally. This may be evidenced by the fact that some of these alkaloids retain optical activity, and in addition many of them can be synthesized in biomimetic syntheses in the laboratory. Thus, one can generalize the opinion of Shamma (10), whose significant contribution to the field of secoisoquinoline alkaloids should be acknowledged, that [a process]. .. could presumably occur in vivo at least as readily as it could in vitro. ... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

---