Metal Ions and Their Relationship Towards Biocatalysis

Reactivities of other ligands such as N2, NO or CN- which are or get bound to metal complexes or metalloproteins are likewise influenced by the El(L) of the coligands (Chatt et al. 1980a; Rehder 1991). Accordingly, there is also a relationship between binding properties of a central ion as defined by Eq. 2.4 and its catalytic properties which extends to metalloproteins. Thus essentiality patterns -including biocatalytic activities - can be directly linked with chemical properties of biorelevant metal ions. Another issue that arises here is whether or inhowfar features of biological uses of metal ions (biocatalysis) match the "optima" for promoting the same reaction which are known from technical or bench-scale catalytic chemistry or else differ somehow. Of course, a meaningful comparison does imply the non-biological reactions to occur in similar to physiological conditions, also. We already mentioned one conspicuous example before: transport (including reversible attachment to metal ions) of molecular oxygen in biology is effected by either Fe (haemoglobin, haemerythrin) or Cu (haemocyanin) rather than Co; many more such "discrepancies" are listed in tab. 1.1. Activation of CO2 by coordination towards electron-rich metal centers (Ni(I), Co(I)) and/or reduction to carbonyl-besides carbonatoligands is known for long (Floriani 1983), likewise reductive terminal addition to alkene or alkyne ligands causing chain extension and eventually direct carboxylation of phenolates or carbanionoids (Li or Mg organyls). On the other hand, there is not yet a model (whether using Mg or any other metal ion) complex which mimics the function of rubisco, that is, can add CO2 to partially oxidized organic molecules splitting their C-C backbones. The coordination chemistry of formaldehyde at V(II) [vanadocene] or Zr sites (Floriani 1983) interaction can be considered to mimic the interactions between (aldo-)sugars and metal centers even though neither V nor Zr are used for related purposes in biochemistry.

In Table 1.1, reactions or transport modes of some 30, usually small, biorelevant molecules are listed together with the metal ions which effect these reactions in (a) biochemistry and (b) catalytic inorganic chemistry, giving an impression of how abundant differences are between "procedures" in biochemistry and chemical technology even after several biollion. years of evolution. Among the substrates in this list small molecules and "simple" functional groups do prevail over larger ones or even macromolecules for the simple reason that, because the behaviour of the former is better understood also in terms of quantum chemistry, "optimum" catalysts (last column in Table 1.1) can be pinpointed there more easily. This table forms some semi-theorical background for a theoretical analysis of limiting conditions set by both evolution and geochemistry.

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