CHEMICAL MODELS OF ENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 173

4. Optimal disposition of primary and secondary catalytic groups within a single
super molecule or on a polymer or membrane template according to its sterical adjusting
for attacking substrates.
5. Including in the catalytic system are additional residues, which can form portions
capable of bounding and precisely orienting the substrate molecule.
6. A matrix, carrying the model catalysis active site, should provide unimpeded
entrance to reagents and exit to products, and free room for conversion of each
intermediate (the dynamic adaptation). In other words, the matrix should exhibit
optimum molecular dynamicssimilar to intramolecular dynamics of proteins.
7. Each stage of the catalytic process should obey the “principle of optimum motion”
(Sections 2. and 2.9). Eventually, constrained pretransition-state complex that activates
cleavage or formation of chemical bonds, have to be formed. The realization of this last
requirement is the most challenging and difficult problem of the mimicking enzymes
processes.
In the mimicking of an enzymatic process there is no need to copy the structure of
protein and coenzyme groups and all stages of this process. In the course of evolution,
Nature created enzymes in specific conditions in certain media and utilized certain
“building materials”. Besides chemical functions, enzymes bear many other obligations,
serving as units of complicated enzymatic and membrane ensembles. These conditions
have not always been the most favorable for catalytic properties and the stability of
enzymes.

6.2. Reduction of dinitrogen

The process of assimilation of atmospheric nitrogen by microorganisms was known
since 1838. Involvement of these molecules in a chemical reaction under mild conditions
seemed to desagree with theoretical considerations and experimental evidence of the
extraordinary chemical inertness of dinitrogen. The first break through in this problem
was made in the pioneering work of Volpin and Shur (1964). These authors
demonstrated the first reactions of dinitrogen reduction by such reducing agents as
in aprotic media in the presence of transition metals

In the 1970's, on the basis of the concept of the multi-electron mechanism of
dinitrogen reduction in polynuclear transition metal complexes (Likhtenshtein and
Shilov, 1970), dinirogen reduction of hydrazine and ammonia in protic media (methanol,
water) involving relative weak reducing agents was discovered (Denisov et al., 1970;
Shilov, 1984; Shilov and Likhtenshtein, 1971). The first systems discovered were
heterogeneous and included metal hydroxides or
which can be considered as giant clusters of transition metals. As a model of biological
dinitrogen fixation, was reduced by or in the presence of Mo
compounds in aqua and alcohol solutions, while CO strongly inhibited redaction. The
principal product was hydrazine, although was reduced to at higher
temperatures. One of this system, complexes of V(II) and catechol in the protic media,