Zinc dimer

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

The X-ray structure of the bis(l,4,7-trithiacyclononane) zinc shows an unusual S6 thioether-coordinated zinc center.540 A rare monodentate thioether donor is observed in a phenyl-bridged zinc dimer that has been structurally characterized with a tetrahydrothiophene coordinated to each zinc center.541... 

In 1999, Nozaki and co-workers were the first to report an asymmetric copolymerization, catalysed by a chiral amino-alkoxide zinc complex 15 (Table 6) and producing optically active PCHC with 70% ee (measured by hydrolysing the copolymer and analysing the resulting diol using chiral GC) [138,148], The crystal structure of the catalyst, reported subsequently, showed a dimeric structure it was unclear whether the dimer was maintained during the copolymerization [148], In the solid state, the zinc-zinc distance in the catalyst was determined to be 3.00 A (vs. ca. 4 A, for the loosely bound BDI zinc dimers). 

UV-visible titrations confirmed that in the experimental conditions of the photophysical experiments, for which concentrations of free base are in the low micromolar range, 92% of the zinc dimers were chelated around pyrazines in the assembled structures of 86, 87, and 88. Upon excitation at 564 nm, fluorescence occurred primarily from the central free base energy acceptor, due to energy transfer according to the Forster mechanism. Array 86 shows a 77-fold increase of the free base emission compared to the individual free base component (where R = PY2) with the same number of energy collecting zinc dimers located at a longer distance, array 87 shows only a 20-fold increase of the free base emission. 

This is comparable with array 88 in which eight zinc dimers are also arranged around the free base, for a 12% fluorescence increase. In this latter... 

Zhao Y, Truhlar DG (2006) Comparative DFT study of van der Waals complexes rare-gas dimers, alkaline-earth dimers, zinc dimer, and zinc-rare-gas dimers. J Phys Chem A 110 5121-5129... 

This type of cement has been further improved by the substitution of -hexyl van ill ate [84375-71-3] and similar esters of vanillic acid [121 -34-6] and/or syringic acid [530-57 ] for eugenol (93—95). These substituted cements are strong, resistant to dissolution, and, unlike ZOE and EBA cements, do not inhibit the polymerization of resin-base materials. Noneugenol cements based on the acid—base reaction of zinc and similar oxides with carboxyhc acids have been investigated, and several promising types have been developed based on dimer and trimer acids (82). 

Calcium Chelates (Salicylates). Several successhil dental cements which use the formation of a calcium chelate system (96) were developed based on the reaction of calcium hydroxide [1305-62-0] and various phenohc esters of sahcyhc acid [69-72-7]. The calcium sahcylate [824-35-1] system offers certain advantages over the more widely used zinc oxide—eugenol system. These products are completely bland, antibacterial (97), facihtate the formation of reparative dentin, and do not retard the free-radical polymerization reaction of acryhc monomer systems. The principal deficiencies of this type of cement are its relatively high solubihty, relatively low strength, and low modulus. Less soluble and higher strength calcium-based cements based on dimer and trimer acid have been reported (82). 

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

The two zinc ions fulfill important but different functions in the DNA-binding domains. The first zinc ion is important for DNA-bindlng because it properly positions the recognition helix the last two cysteine zinc ligands are part of this helix. The second zinc ion is important for dimerization since the five-residue loop between the first two cysteine zinc ligands is the main component of the dimer interaction area. 

Subsequently Stephen Harrison s group determined the x-ray structure of a PPRl-DNA complex and showed that the zinc cluster domain of PPRl and its mode of binding to DNA was very similar to that of GAL4, and that PPRl also dimerized through a coiled-coil region. However, the linker region... 

Dimerization of the Ce-zinc cluster transcription factors involves an a-helical coiled coil in the dimerization region. Coiled coils, often called leucine zippers, are also found in a large group of transcription factors that do not contain zinc. The leucine zipper is made up of two a helices in a coiled coil with every seventh residue leucine or some other large hydrophobic residue, such as isoleucine or valine. Leucine zipper transcription factors (b/zip) include factors characterized by heterodimerization, for example Fos and Jun. The a-helical DNA-binding motifs of the heterodimers recognize quite different base sequences and are continous with the a helices of the zipper. 

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