Inner-sphere binding metal complexes

In contrast to the relatively simple structures incorporating one of the first three binding motifs, the polymeric complex [Cu3(5 -GMP)3(H20)8] has three distinct coordination environments about different Cu + ions. While this complex is unusually sophisticated, the polymeric nature of the material is common for many metal complexes with GMP or IMP, in which inner-sphere binding occurs to both the N-7 atom and phosphate oxygens of a single nucleotide residue, but does not involve the same metal ion. As described in Section 5.5, there are also unusual structures ( open complexes ) of Cu + species and GMP, which involve only inner-sphere metal binding to the phosphate group. 

The anti conformation preferred by the adenine residue in 5 -AMP (Figure lb) disposes this nucleotide to simultaneous inner-sphere binding of metal ions both to the phosphate oxygen and N-7 atoms. As would be expected, neither 2 -nor 3 -AMP (Figure Ic) is able to participate in forming the same type of complex for steric reasons. As was shown in experiments involving Mg +, Mn +, Cd +, Co +, and Cu +,... 

The term Y-Z bond activation is traditionally understood as a reaction that cleaves the bond [1]. Often, the term is restricted to reactions involving organometallic complexes and proceeding by Y-Z coordination to the inner sphere of metal, either via an intermediate state or as a transition state. We are inclined to use the term activation for weaker (noncovalent) binding that results in the altered reactivity of a molecule through associated changes in the relative energies of its orbitals or in verified polarity. 

Copper also binds very strongly as an inner-sphere complex with organic matter while other divalent transition metals such as Ni2+ and Co2+... 

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

Sharma and Reed, 1976)]. In proteins the coordination number 4 is most common, where the zinc ion is typically coordinated in tetrahedral or distorted tetrahedral fashion. The coordination polyhedron of structural zinc is dominated by cysteine thiolates, and the metal ion is typically sequestered from solvent by its molecular environment the coordination polyhedron of catalytic zinc is dominated by histidine ligands, and the metal ion is exposed to bulk solvent and typically binds a solvent molecule (Vallee and Auld, 1990). The inner-sphere coordination number of catalytic zinc may increase to 5 during the course of enzymatic turnover, and several five-coordinate zinc enzyme—substrate, enzyme product, and enzyme-inhibitor complexes have been studied by high-resolution X-ray crystallographic methods (reviewed by Matthews, 1988 Christianson and Lipscomb, 1989). The coordination polyhedron of zinc in five coordinate examples may tend toward either trigonal bipyramid or octahedral-minus-one geometry. 

Reaction (63) is an example of 0 acting as an oxidant and it probably proceeds via an inner-sphere electron transfer mechanism in which incompletely coordinated Cu binds O2 prior to electron transfer [87]. HO2 and 0 also react readily with a number of other transition metal ions, either by electron transfer or through the formation of a complex [83], for example ... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

When metal cations are placed in aqueous solutions two kinds of spheres normally appear (a) a sphere of water molecules that binds directly to the metal, called inner coordination sphere (or simply, inner sphere), and (b) a more loosely bound group of water molecules (not directly bound to the metal), called outer coordination sphere (or simply, outer sphere). In this way, a cationic complex can have an outer sphere interaction with an ionic ligand or a solvent molecule without displacing the inner ligands directly bonded to the metal. At higher anion concentrations, the outer sphere complex [M(H20)6]n+An is more prevalent than its corresponding inner sphere complex, [M(H20)5A], Interestingly, the number of inner-... 

For many metal ions, the initial electrostatic interaction can be followed by stronger and more specific binding with nucleic acids via the formation of outer- and inner-sphere complexes. The formation of such complexes may be strongly accelerated (in comparison to binding to nucleosides and nucleotides) because of the polyelecfrolyte effect. 

In the stracture of the polymeric complex of Mn + and 5 -CMP, an unusual cytosine inner-sphere coordination complex has been demonstrated involving, rather than N-3, as well as four phosphate oxygen atoms of three different nucleotides and one water oxygen atom. Thus, in the case of CMP binding, it seems clear that can act as a significant metal-binding site alone, or together with N-3 in chelate structures. 

There is evidence for interligand interactions, without inner-sphere metal-nucleic acid binding, in three different types of complexes. In the first, the potential metal-binding sites are blocked by a proton, for example, in saltlike stmc-tures of the complexes of protonated bases 9-ethylguantne,... 

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