Biomimetic chemistry

The ability of the ubiquitous Co(salen) complex and its tetradentate Schiff base analog complexes to bind 02 reversibly has been central to most investigations of its coordination chemistry. A density functional computational investigation has been carried out on the 02 carriers 

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222 

Well-dispersed silica and polymer/Co(salen) segments at a molecular level were obtained. Nondestructive immobilization of Co(salen) complexes within silica aero- and xerogels was also achieved with the sol-gel method using silylether-appended salen.1227 

DNA (phosphodiester) hydrolysis has also been examined with [Co(cyclen)(OH2)(OH)]2+ as the active species.1239,1246 Significant rate enhancement of linear1247 and supercoiled1248 double-stranded polydeoxyribonucleotide hydrolysis is observed by immobilizing the Co complex on a polystyrene support. Wider exploration of reactions with DNA follow. 

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B. S. Green, Y. Ashani, and D. Chipman, eds.. Chemical Approaches to Understanding En me Catalysis Biomimetic Chemistry and Transition State Analogs, Elsevier, Amsterdam, 1982. 

Dolphin, D., McKenna, C., Murakami, Y. and Tabushi, I. (1980) Biomimetic Chemistry, Advances in Chemistry Series 191 (American Chemical Society, Washington, DC). 

Mazumdar S, Mitra S (1993) Biomimetic Chemistry of Hemes Inside Aqueous Micelles. 81 115-145... 

These were relatively low-resolution structures, and with refinement some errors in the initial structural assignments have been detected (4-7). Since the structures were first reported the subject has been extensively reviewed in this series (8) and elsewhere 9-15). This review will focus on the structure, biosynthesis, and function of the met-allosulfur clusters found in nitrogenases. This will require a broader overview of some functional aspects, particularly the involvement of MgATP in the enzymic reaction, and also some reference will be made to the extensive literature (9, 15) on biomimetic chemistry that has helped to illuminate possible modes of nitrogenase function, although a detailed review of this chemistry will not be attempted here. This review cannot be fully comprehensive in the space available, but concentrates on recent advances and attempts to describe the current level of our understanding. 

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

Bakke et al. (1982) have shown how montmorillonite catalyses chlorination and nitration of toluene nitration leads to 56 % para and 41 % ortho derivative compared to approximately 40 % para and 60 % ortho derivatives in the absence of the catalyst. Montmorillonite clays have an acidity comparable to nitric acid / sulphuric acid mixtures and the use of iron-exchanged material (Clayfen) gives a remarkable improvement in the para, ortho ratio in the nitration of phenols. The nitration of estrones, which is relevant in making various estrogenic drugs, can be improved in a remarkable way by using molecular engineered layer structures (MELS), while a reduction in the cost by a factor of six has been indicated. With a Clayfen type catalyst, it seems possible to manipulate the para, ortho ratio drastically for a variety of substrates and this should be useful in the manufacture of fine chemicals. In principle, such catalysts may approach biomimetic chemistry our ability to predict selectivity is very limited. 

Biomimetic chemistry of nickel was extensively reviewed.1847,1848 Elaborate complexes have been developed in order to model structural and spectroscopic properties as well as the catalytic function of the biological sites. Biomimetic systems for urease are described in Section 6.3.4.12.7, and model systems for [Ni,Fe]-hydrogenases are collected in Section 6.3.4.12.5. 

Macrocyclic complexes of zinc have inspired interest in varied areas such as supramolecular and biomimetic chemistry including hydrolysis enzymes, such as phosphatases and esterases, and also for the fluorescent detection of zinc. The polyaza macrocycles and their A--functionalized derivatives are particularly well represented. An important aspect of macrocycle synthesis is the use of metal templates to form the ligand. Examples of zinc as a template ion will be discussed where relevant. 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

Much of the impetus for the study of reactions in micelles is that they model, to a limited extent, reactions in biological assemblies. Synthetic vesicles and cyclodextrins are other model reaction media and the term Biomimetic Chemistry has been coined to describe this general area of study. Work in this area is reviewed in recent publications (Kunitake and Shinkai, 1980 Fendler, 1982). 

Popovitz-Biro, R. Chang, H. C. Tang, C. P. Shochet, N. Lahav, M. Leiserowitz, L. In Chemical Approaches to Understanding Enzyme Catalysis Biomimetic Chemistry and Transition-State Analogs Green, B. S. Ashani, Y. Chipman, D., Eds. Elsevier Amsterdam, 1982, pp. 88-105. 

In the context of this chapter, biomimetic is defined as the nse of simple synthetic media to mimic a complex biological process . Earlier Fendler (1984) defined membrane biomimetic chemistry as processes in simple media that mimic aspects of biomembranes . Thus, classical biomimetic approaches target specific... 

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic LC have been of much interest in such fields as biomimetic chemistry. In fact, biological membranes and cell membranes are a form of LC. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface yet, the membrane is fluid and elastic. The constituent molecules can flow in plane quite easily but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These LC membrane phases can also host important proteins such as receptors freely floating inside, or partly outside, the membrane. 

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