Conformations, copper complexe

Key words Acidities, amino acids, binding mechanisms, calixarenes, choline, conformations, copper complexes, NMR, polyphenolates, resorcinarenes, supramolecular complexes. 

The complex trans-[Cun(hfac)2(TTF—CH=CH—py)2](BF4)2-2CH2Cl2 was obtained after 1 week of galvanostatic oxidation of Cun(hfac)2(TTF CH=CH py)2 [61]. The molecular structure of the copper complex is identical to its neutral form. There is one TTF CH=CH py molecule per BF4 and one dichloromethane solvent molecule. The copper is located at the center of a centrosymetric-distorted octahedron two TTF CH=CH py ligands in trans- conformation are bonded to Cun by the nitrogen atoms of the pyridyl rings. From the stoichiometry, the charge distribution corresponds to fully oxidized TTF CH=CH—py+" radical units. 

Calixarenes, when in their cone-conformation (54), represent versatile host systems for metalated container molecules and many examples have been reported in the literature (55-61). Reinaud and coworkers have carried out extensive work concerned with calix[6]arenes that are functionalized at the small rim by nitrogen arms (62), aiming to reproduce the hydro-phobic binding site of mononuclear zinc and copper metalloen-zymes. A recent example is the calix[6]arene ligand L1 (Fig. 3), in which a tris(2-methylpyridyl)amine unit covalently caps the calixarene small rim (63). The ligand forms copper complexes of... 

More advanced semiempirical molecular orbital methods have also been used in this respect in modeling, e.g., the structure of a diphosphonium extractant in the gas phase, and then the percentage extraction of zinc ion-pair complexes was correlated with the calculated energy of association of the ion pairs [29]. Semiempirical SCF calculations, used to study structure, conformational changes and hydration of hydroxyoximes as extractants of copper, appeared helpful in interpreting their interfacial activity and the rate of extraction [30]. Similar (PM3, ZINDO) methods were also used to model the structure of some commercial extractants (pyridine dicarboxylates, pyridyloctanoates, jS-diketones, hydroxyoximes), as well as the effects of their hydration and association with modifiers (alcohols, )S-diketones) on their thermodynamic and interfacial activity [31 33]. In addition, the structure of copper complexes with these extractants was calculated [32]. 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

When n = 2 or 3, such structurally characterized ICC, in the majority of cases, have slightly tetrahedrizated c/.v-planar conformations [100]. The effect of the factor examined is so high that even zinc chelates 882 (M = Zn, R = Me, X = S, n — 2) show a considerably planar-distorted structure [209]. An analogous structural situation is observed for cobalt, nickel, and copper complexes containing R — cyclo-(C 12)3, annelated aromatic, and pyrazol fragments [100]. When n increases, for example n = 4 in the complexes capable of tetrahedrizating a structure, the influence of the examined factor decreases. This is observed in the ICC not only with X = S [100,210 212], but also for X = NH 222 [187]. 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

ORD spectra of copper complexes with a variety of proteins and peptides give conformational as well as quantitative information on the metal binding (9, 10, 35). This method also has been successfully applied to study metal complexes with nucleotides (53, 70). 

However, when calculating the binding constant for patellamide C with copper the change in conformation, from the figure of eight ( >) to the square ( ), that takes place must also be taken into consideration, as shown below, as this causes changes to the CD signal as well as the formation of the copper complex. 

Patellamide A, adopting mainly the square form in solution, was also shown to bind copper, but in doing so only small conformational changes occurred. This can be related to the XRD structure of ascidiacyclamide which distorts only slightly, from the square form to what has been termed the saddle form, to acconunodate two copper atoms. No bridged 2Cu complexes of patellamide A were detected, indeed only small amounts of 2 1 copper complexes were detected by MS. The 2Cu bridged complex of... 

A number of investigations have been based on the conclusions of Strominger and Tipper [183,215]. It was estimated that by structural analogy between the terminal D-alanyl-D-alanine moiety of N-acetylmuramylpentapeptide and 6a-methylpenicillin the latter should possess an enhanced bioactivity. The first experiments aimed at obtaining this compound were unsuccessful [216,217], as the copper complex (112) could not be split after alkylation. Starting from 6-APA, Reiner and Zeller [216] were able to introduce the hydroxymethyl function into C-6, but the low activity of the end-product suggested the presence of (113) in the epi-conformation. The use 4>f formaldehyde yielded the spiro derivative (114). 

Fig. 10. Principle of the electrochemically induced molecular motions in a copper complex rotaxane. The stable 4-coordinate monovalent complex [top left, the black circle represents Cu(I)] is oxidized to an intermediate tetrahedral divalent species [top right, the white circle represents Cu(II)]. This compound undergoes a complete reorganization process to afford the stable 5-coordinate Cu(II) complex [bottom right]. Upon reduction, the 5-coordinate monovalent state is formed as a transient [bottom left]. Finally, the latter undergoes the conformational change which regenerates the starting complex...

The different oxidation states of copper ions not only favor distinct ligands but distinct stereochemistries of the copper-complexes as well. The conformational geometry of a complex depends on the orbital geometry of the central ion [17]. Sterical hindrances excluded, Cu+-ligands favor a tetrahedral conformation, while those of Cu2+ favor a square planar conformation (Table 4) [17]. In the copper-binding sites of proteins, ligands and conformations may differ greatly from those normally preferred. 

Figure 6.20. (a) Acryloyloxazolidinone in bidentate coordination. strain favors the s-cis conformation, (b) Cycloaddition of Ci-symmetric bisoxazoline-magnesium complex [206]. (c) Cycloaddition of C2-symmetric bisoxazoline-copper complex [205]. (d) Rationale for the different topicities of the bisoxazoline complexes, even though both ligands have the same absolute configuration. The dienophile is awn in the plane of the paper, and the favored approach is from the direction of the viewer. 

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