Transition metal ions, ligated

Independently, Ruan etal. (1990) demonstrated that unnatural metal-ligating residues may likewise be utilized toward the stabilization of short a helices by transition metal ions (including Zn " ")—these investigators reported that an 11-mer is converted from the random coil conformation to about 80% a helix by the addition of Cd at 4°C. These results suggest that the engineering of zinc-binding sites in small peptides or large proteins may be a powerful approach toward the stabilization of protein secondary structure. 

It has been shown that many redox reactions in anions and similar species may be induced by ligation to transition metal ions. One may consider metal ions as bridging between the oxidant and the oxidized ligand somewhat in an analogous manner to a ligand bridge in a metal-metal redox reaction. 

The formation of ligated transition metal ions at unstable high states of oxidation, its implications in the mechanisms of metal-catalyzed autoxidation, and the effect of configuration of a metal-ligand system on its redox stability have been pointed out. These considerations may be helpful in interpreting more complex metal-ligand systems including metal-enzyme reactions. 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

In a study on the effect of divalent cations on the mode of action of DNAase I, Campbell and Jackson (1980) found that in the presence of the Mn++ ions DNAase I is able to cut both DNA strands within a duplex at or near the same point. This ability to cut both strands is inhibited at lower temperatures and by the addition of a monovalent ion or another divalent cation which is not a transition metal ion. Transition metal ions thus appear to promote the localized unwinding of duplex DNA into a form where DNAase I can introduce breaks into both strands. These observations therefore suggest another route for producing a more or less random set of fragments which, after limited polishing of the ends with DNA polymerase, could be ligated to the RI linker and subsequently cloned into M13mp2. 

Finally, some ion mobility data is available for a number of clusters composed of an ion and small neutral molecules. Such clusters include H30+(H20)3, NHJ(NH3)n, n=l-3, NO+(CH3COCH3)n, n=2, 3 [139-141], complexes between protonated amines and polyethers [21], and ligated transition metal ions... 

One possible way to prevent transition metal ions from precipitating, and thus allow them to coordinate to peptides, is to have a primary ligating site or anchor, which would allow the amide oxygen to chelate to the metal. This will allow for subsequent substitution of the amide hydrogen by the metal ion. This primary binding site reduces the importance of metal ion hydrolysis and permits attainment of pH values where substitution of a metal ion for an amide hydrogen may occur (equation 3). 

Mn(IV) ion functions as a Lewis acid, in the same manner as the early transition metal ions, to activate the 0-0 bond in hydrogen peroxide through ligation but with cleavage of the 0-0 bond only during the transfer of an oxygen atom to the olefin double bond [25,94 96]. 

The real power of electron spin resonance spectroscopy for structural studies is based on the interaction of the impaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons, and of the ligation scheme around paramagnetic transition-metal ions. The more complete Hamiltonian is given in equation 2 for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

In a similar way collisional spin exchange between unlike species can be studied, as has been demonstrated for Cu and Ni complexes of crown ethers and the nitroxide radical TEMPO [40]. Comparison of the rate constant of spin exchange with the rate constant of diffusion collisions gave information on the steric factor, which characterizes the accessibility of the ligated transition metal ion to the nitroxide radical. 

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