Manganese complexes outer sphere

At variance with the aqua ion, in most manganese(II) proteins and complexes the contact contribution to relaxation is found negligible. This is clearly the case for MnEDTA (Fig. 33), the relaxivity of which indicates the presence of the dipolar contribution only, and one water molecule bound to the complex 93). Actually the profile is very similar to that of GdDTPA (see Chapter 4), and is provided by the sum of inner-sphere and outer-sphere contributions of the same order. The relaxation rate of MnDTPA is accounted for by outer-sphere relaxation only (see Section II.A.7), no water molecules being coordinated to the complex 94). 

In all Mn(II) proteins and in most complexes the contact interaction is found negligible. In fact, the H NMRD profile of MnEDTA, for instance, indicates the presence of the dipolar contribution only, and one water bound to the complex. The relaxation rate of manganese(II) complexes with DTPA (see Fig. 5.56) is instead provided by outer-sphere relaxation only, since no water molecules are bound to the complex (see Section 4.5.2). 

Manganese(ii) forms both inner- and outer-sphere complexes in hydrochloric acid solutions. (279) For 1-5mF[C1, the species Mn(H20)e and Mn(H20)6 Cl are present in solution in relative amounts 3 2. An outer sphere complex has been suggested by the spectral data for the binding of Mn with the polymeric humic acid, fulvic acid. (280) This is in contrast to the corresponding Fe complex which is of the inner sphere type. 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

The chemical shift of the phosphorus resonance of various nucleotides has been studied as a function of pH in the presence and absence of RNase A. The signal shifts upheld on protonation of the phosphate and the apparent pATa of the phosphate group in 2 -CMP complex with RNase is the same as the pATa of histidine-119 in this enzyme as determined by n.m.r. From n.m.r. relaxation rates for the ternary complex manganese(n)-phosphate-E. coli alkaline phosphatase, it has been concluded that an outer-sphere complex is formed which has a shorter lifetime than the enzyme turnover rate. The latter conclusion is consistent with the participation of the complex in the enzymic reaction. 

The coupling constant is inconsistent with carboxyl coordination but consistent with carbonyl coordination 15). Similar data for -ketobu-tyrate 15) and oxalacetate (19) have been fit by exchange contributions (1/tm) and inner sphere contributions Tm and T2m)- The rates of formation of these metal bridge complexes from an outer sphere complex ( 3,4) are limited predominantly by the rate of dissociation of a water molecule from the coordination sphere of the enzyme-bound manganese (Figure 3, Table V) (15,19), as required by the Sj l-outer sphere mechanism of Eigen and Tamm (20),... 

The aziridination of olefins has also been studied, but fewer complexes catalyze this reaction as efficiently as iron and manganese complexes catalyze the epoxida-tion of olefins. Nevertheless, the aziridinations of olefins catalyzed by copper, ruthenium, and rhodium complexes have been reported. The source of nitrogen is usually [N-(p-toluenesulfonyl)imino]phenyliodinane (PhI=NTs) or a precursor to a related iodoarylimine. The aziridine is likely generated from these copper- and rhodium-catalyzed reactions by an outer-sphere process in which the olefin interacts with the LUMO of the complex, which is located at the nitrogen. This mechanism is more likely to be followed by these catalysts than a [2-t-2] process, followed by reductive elimination. 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

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