As a conclusion, for metal-forming chemical elements pathways of transport through and strength/stability of binding to biomass, including possible bioaccumulation, are closely connected to their coordination properties. This defines the current endeavour: we are to express these coordination properties in a way (quantitatively) which allows to account for such abundance correlations (summed up in the BSE), biochemical functions and catalytic uses likewise. This requires a method of description which does not just address one "biochemical" ligand or another but encompasses the entire range of materials seen in biological materials and their interconversions.
Corresponding numbers thus must describe properties of some metal ion - be it essential, slightly or highly toxic - with respect to almost all kinds of donors. Some representation which borrows from data directly related to (semi-empirical) quantum chemistry (perturbation theory and ligand-field approaches, for example, measured by electrochemistry) will be most versatile provided it can link some easily measurable (or already measured) term to complex formation constants in a quantitative way referring to electronic properties of the complex thus formed -most versatile for understanding reasons of essentiality (which is neither caused nor implied by environmental abundances (cp. Al, Ti - very abundant yet completely non-essential so far - vs V, Mo, I which are rare but essential for many different living beings)).
This approach thus relies upon the following matters of fact:
- Complexes of one identical central ion with different ligands use to differ with respect to stability, however, stabilities of complexes containing ligands bearing similar or identical functional groups are very much similar, however, these implications of relative hydrolytic stabilities among, e.g. halido ligands F-...I- may vary with metal center. Water solubilities (net hydration enthalpies) of phenyl alanin(-ate) and glycinate or of ammonia and benzyl amine differ considerably, and so do those of their complexes. Hence the expected different lability towards hydrolytic cleavage of the complexes is almost cancelled, producing closely similar complex formation constants in these pairs of complexes for a multitude of metal ions each (e.g. Kiss et al. 1991; Sovago et al. 1993; Irving and Williams 1953; Irving and Rossotti 1956). As a result, binding stabilities just depend on the particular coordinating group, that is, carboxylate or an amino group while hydrophilic or hydrophobic behaviour (e.g. log kOW) of the entire ligand molecule or anion is insignificant. Thus, the same "focussing" on the very binding (ligation) site may apply to very large ligands - that is, apoproteins or other macromolecules - also, allowing for a kausal interpretation of both the BSE and the observed patterns of (metal) essentialities.
- Upon exchange of a ligand by another there will be a change of both binding (hydrolytic) stability and of metal-centered orbital energy levels, causing the conspicuous changes of colour which are so typical for reactions in coordination compounds, classically accounted for by ligand field theory (Figgis and Hitchman 2000). Such energetic changes of highest occupied or lowest unoccupied orbitals influence the tendency of complexes to give away or take up electrons which in turn can be measured as a change of redox potentials which occurs due to exchange of some ligand.
Originally appropriate scales - that is, ligand-based electrochemical series describing redox potential changes - were developed in the Brighton-based workgroup of Chatt (starting in 1974, see especially Chatt et al. 1980a,b) and almost simultaneously by others (Sarapu and Fenske 1975; Bursten 1982). In 1990, Lever defined a kind of electrochemical series of ligands which refers to potential changes in the Ru(II/ III) couple (solvent: acetonitrile). Accordingly the principal term was called the ligand electrochemical parameter El(L). It (more precisely, its difference) gives the amount by which the redox potential in the Ru(II/III) couple changes upon some ligand replacement (when a ligand X gets substituted by Y). For multi- or oligodentate ligands this value has to be multiplied by denticity (hapticity) to obtain the change of potential which is actually seen (Lever 1990). The potential-changing effect is additive: replacing one carboxylate moiety by an aminomethyl donor site in a ligand system consisting of two carboxylates close to each other (oxalate, malonate turned into glycinate) produces an effect half as large as with replacing both sites (i.e. replacing oxalato ligands with ethylene diamine). The "absolute" scale also is derived from an assumption/observation (Chatt et al. 1980a; Bursten 1982; Lever 1990b; Rocha et al. 2002) of ligand additivity: for the famous redox couple [Ru(bipy)3]2+/3+ (bipy = 2,2'-bipyridyl), a complex which is used for photochemical water splitting, with six pyridine donor sites the standard potential is +1.56 V, for the respective Ru aquaion it is +0.24 V. Thus EL(L) = 1.56/6 = +0.26 V for (half of) a 2,2'-bipyridyl ligand and 0.24/6 = +0.04 V for water ([Ru(H2O)6]2+/3+) as standard compound.
Like Lever did in 1990, aqueous redox potentials of other homoleptic ruthenium complexes - excluding ligand protonation like in [Ru(CN)6]4-/3- - were used to determine solvent effects (see below). Let us give some example for calculation purposes only: EL(L) for a fluoroligand is +0.42 V, for chloride -0.24 V and for (bidentate) glycinate it is -0.05 V (Lever 1990). Thus, when [RuF6]3- is treated by hydrochloric acid, replaving one fluoroligand to produce [RuF5Cl]3- will increase the Ru(II/III) redox potential by 0.18 V whereas a reaction of [RuF6]3- with glycinate to afford [RuF4(glyc)]2- by replacing two fluorides will even enhance it by 0.74 V (Fig. 2.1).
Change of electrochemical ligand parameters (Lever 1990) and of redox potentials
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