Dinuclear site measurements

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

NH3)5]"+ (n = 4, 5, or 6) and [(bipy)2ClRu(pyz)RuCl(bipy)2]"4 (n = 2, 3, or 4).50 These studies reveal equivalent metal sites even in the mixed-valency Run-Ru,u species, despite the speed with which ESCA monitors electron distribution (10 17 s). Another investigation shows that the complex [Ru(NH3)5(pyz)]2+ can associate with several aqueous first-row transition-metal ions to form pyrazine-bridged dinuclear complexes.51 Equilibrium formation constants with Ni", Cu , and Zn11 were measured spectroscopically. 

Figure 22 Metals centers of pMMO. The distances are measured between metal centers. Anomalous difference Fourier maps, yellow for Cu absorption edge and red for Zn absorption edge, are superimposed on the finai 2Fo-Fc eiectron density map. (a) The mononuclear copper site, (b) The dinuclear copper site, (c) The zinc site. Reproduced from R. L. Lieberman ...

Magnetic susceptibility measurements carried out on (15) in the temperature range 5 to 300 K are consistent with a system composed of an antiferromagnetically coupled copper(II) pair and a magnetically independent third copper(II) ion. The macrocyclic complex therefore reproduces qualitatively features of the oxidase active site and the observed 2J value of -192 cm indicates the presence of a moderate antiferromagnetic interaction within the dinuclear moiety, albeit weaker than that of the protein type-3 site which is diamagnetic even at room temperature. 

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