Metalloproteins metal coupling

Table II reports the contact coupling constant for different aqua ion systems at room temperature. The contact coupling constant is a measure of the unpaired spin density delocalized at the coordinated protons. The values were calculated from the analysis of the contact contribution to the paramagnetic enhancements of relaxation rates in all cases where the correlation time for dipolar relaxation is dominated by x and Tig > x. In fact, in such cases the dispersion due to contact relaxation occurs earlier in frequency than the dispersion due to dipolar relaxation. In metalloproteins the contact contribution is usually negligible, even for metal ions characterized by a large contact contribution in aqua ion systems. This is due to the fact that the dipolar contribution is much larger because the correlation time increases by orders of magnitude, and x becomes longer than Tig. Under...

In situ STM of metalloproteins with localized low-lying redox levels can be expected to follow ET patterns similar to metalloprotein ET in homogeneous solution and at electrochemical surfaces. The redox level is thus strongly coupled to the protein and solvent environment. A key notion is that the vacant local level (oxidized form) at equilibrium with the environmental nuclear motion is located well above the Fermi levels of both the substrate and tip, whereas, the occupied level (reduced form) at equilibrium is located well below the Fermi levels. Another central notion is that the local redox level at the transition metal centre is still much lower than environmental protein or solvent electronic levels. The redox level therefore constitutes a pronounced indentation in the tunnel barrier. This alone would strongly enhance tunnelling. Configurational fluctuations in the environment can, secondly take the redox level to such low values that temporary physical population occurs. This requires nuclear activation but can still be favourable due to the much shorter electron tunnel distances... 

The artificial intelligence-superexchange method in which the details of the electronic structure of the protein medium are taken into account was used for estimating the electronic coupling in the metalloproteins (Siddarth and Marcus, 1993a,b,c). Fig.2.11 demonstrates a correlation of experimental and calculated ET rate constants for cytochrome c derivatives, modified by Ru complexes. The influence of the special mutual orientation of the donor and acceptor orbitals in Ru(bpy)2im HisX-cytochrome c on the rate of electron transfer was analyzed by the transition amplitude methods (Stuchebrukhov and Marcus, 1995). In this reaction the transferring electron in the initial and the final states occupies the 3d shell of the Fe atom and the 4d shell of Ru, respectively. It was shown that the electron is localized on t2g subshells of the metal ions. Due to the near-... 

Paramagnetism in a metalloprotein does not have to be associated with only a single metal site but can arise from metal clusters. In these clusters the spins of the different metal sites couple either ferromagnetically or antiferromagnetically to produce a net spin state of the cluster. 

Table 3. Jahn-Teller coupling constants for transition metal ions commonly involved in metalloproteins ... 

Metal ions in metalloproteins tend to be rather feebly bound (Williams 1986), except for Cu(II) and few others, using just two to three coordination sites which link them to amino acid side chains of the protein. This fact has various ramifications for conditions of essentiality in a reproducing system. Figures 3.1 and 3.2 show some correlation between certain values of c and x (which are coupled to each other as long as the oxidation state is kept constant)... 

ICP-MS was found to be compatible with LC for the trace metal speciation. The role of ICP-MS in trace element speciation studies at the FSL was described (Crews et al., 1987). The characteristics of LC-ICP-MS for the study of metalloprotein species were assessed and the chromatographic efficiency of ICP-MS was found to be similar to that obtained with a UV detector (Dean et al., 1987). Information about the chemical nature of trace elements from food can be obtained by first treating the foods in vitro with enzymes to broadly simulate the action of enzymes in the gastrointestinal tract (Crews et al., 1988). The soluble components can be separated by size exclusion chromatography (SEC) and an estimate of their molecular size obtained. By coupling SEC... 

Two-dimensional NMR methods that yield a two-dimensional frequency spectrum have not yet been attempted successfully to study metal interactions with HS, although instances of successful applications of these approaches can be found in the study of metalloproteins (Kingery et al., 2001). Mononuclear ( H) two-dimensional NMR experiments, such as total correlation spectroscopy (TOCSY), can show H- H coupling throughout the complete spin system, and exchange protons can provide information on sites to which metal attach. For example, N-containing units in HS that bind the metal can be identified since the amido protons from these structures will exchange and disappear from the spectrum. 

The chapters in this volume offer overviews of electronic properties, electron transfer and electron-proton coupled charge transfer of biological molecules and macromolecules both in the natural aqueous solution environment and on metallic electrode surfaces, where the electrochemical potential controls biomolecular function. Redox metalloproteins and DNA-based molecules are primary targets, but amino acid and nucleobase building blocks are also addressed. Novel enviromnents where proteins and DNA-based molecules are inserted in metallic nanoparticle hybrids or in situ STM configurations are other focus areas. 

Within the context of toxicological and clinical importance, speciation studies have been focused on relatively few elements, mainly aluminum, antimony, arsenic, chromium, iodine, lead, mercury, platinum, selenium and tin. However, coupled HPLC-ICP-MS has most often been used for speciation of arsenic, selenium, iodine and, to a lesser extent, mercury. The primary species of these elements include different oxidation states, alkylated metal and/or metalloid compounds, selenoamino acids and selenopeptides.In addition, applications in smdies on the pharmacokinetics of metal-based drugs (mainly platinum complexes) and metalloproteins should be included. " In the following sections, the advances in speciation smdies of individual elements are reviewed. 

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