Ligands biologically relevant

Polylactones (for an example, see Fig. 14.4) are synthetic analogues of naturally occurring ionophores such as enniatin (species that transport ions across biological membranes). Molecular mechanics calculations have been used to predict the stability and selectivity with respect to Li+, Na+, and K+ of a series of new polylactones12661. Metal-ligand interactions were again modeled using a combination of van der Waals and electrostatic terms. 

Similar studies were carried out on other enniatin analogues12671, enniatin itself12681, and valinomycin12691. The Ca2+ complexes of a series of C3 symmetric trispep-tides were also modeled, and it was shown that some of these have the ability to form chiral Ca2+ complexes12701. 

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Table 1. EPR parameters for tetragonal copper(II) complexes with biologically relevant ligands...

Enrichment of Co(II) by early polymers thus would occur at Ej (L) > -0.47 V, that is, for almost all biologically relevant ligands other than thiolates. In considerable contrast with this, amino acids have E (L) -0.05 V while the corresponding value for peptides is about zero, oligocarboxylates ranging from about -0.13 V (malate) down to some -0.26 V (citrate). The difference in Ej (L) caused by chemical evolution thus will suffice to promote step 1 if the absolute value of E (L) of the educts is not higher than some h-0.2 V - provided... 

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