Hydroxides, crystal structure

H. Bode, K. Behmelt, and]. Witte, Crystal Structures of Nickel Hydroxides, ClTCE Meeting, Strashourg, France, 1965. 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

Figure 1. The crystal structure of Zr2(0H)2(SO4)3(H2O)4> reprinted with permission from Ref. 5, copyright 1966, American Chemical Society. Zirconium atoms are shown as solid circles, oxygen atoms as open circles. The Pu compound is isomorphous, Zr being replaced by Pu. la shows the manner in which the bridging sulfates link Pu atoms to form layers, lb shows the manner in which layers are linked through the double hydroxide bridges.

Fig. 26. (a) A stereoview of the hydroxide-bridged complex and (b) the crystal structure (93) of the complex cation CudIXOHXO-BISTREN), showing the hydrogen bonding that appears to promote formation of the bridged hydroxy complex. 

Before our work [39], only one catalytic mechanism for zinc dependent HDACs has been proposed in the literature, which was originated from the crystallographic study of HDLP [47], a histone-deacetylase-like protein that is widely used as a model for class-I HDACs. In the enzyme active site, the catalytic metal zinc is penta-coordinated by two asp residues, one histidine residues as well as the inhibitor [47], Based on their crystal structures, Finnin et al. [47] postulated a catalytic mechanism for HDACs in which the first reaction step is analogous to the hydroxide mechanism for zinc proteases zinc-bound water is a nucleophile and Zn2+ is five-fold coordinated during the reaction process. However, recent experimental studies by Kapustin et al. suggested that the transition state of HDACs may not be analogous to zinc-proteases [48], which cast some doubts on this mechanism. 

Trivalent Co complexes of the mixed donor amino arsine emda (2-aminoethyl)dimethylarsine) have been synthesized.936 Examples include trans(X,X), cA(As,As)-[CoX2(edma)2]+ (X = C1, Br, and I), trans( As,As)- and trara(As,N)-[Co(acac)(edma)2]2+, trara(As,N)-[Co(C03)(edma)2]+, [Co(acac)2(edma)]+, and /ao[Co(edma)3]3+. The /ao[Co(edma)3]3+ complex was resolved and the absolute configuration of the (+)49o CD isomer was assigned as A on the basis of the CD spectrum. Racemization of this complex was first order in both complex and hydroxide ions. The crystal structure of [Co(acac)2(emda)]C104 has been reported.937... 

Hydrothermal synthesis using hard and soft acids and bases gave the metal hydroxide-based phases M3Pd2(0H)2[NC5H3(C02)2-2,4]4(H20)4 (M = Co, Ni, Zn).391 A crystal structure is shown in Figure 29. 

Phosphate ester crystal structures have been determined of zinc 1,5,9-triazacyclononane including an interesting structure containing an oligophosphate bridged zinc unit.450 The zinc complex of 1,5,9-triazacyclododecane was studied as a hydrolysis catalyst for substituted phenyl acetates.451 Kinetic analysis suggested that hydrolysis occurs by a mechanism involving hydroxide attack of a metal-bound carbonyl. 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

The carbonic anhydrase (Cam) in M. thermophila cells is elevated several fold when the energy source is shifted to acetate, suggesting a role for this enzyme in the acetate-fermentation pathway. It is proposed that Cam functions outside the cell membrane to convert CO2 to a charged species (reaction A4) thereby facilitating removal of product from the cytoplasm. Cam is the prototype of a new class (y) of carbonic anhydrases, independently evolved from the other two classes (a and P). The crystal structure of Cam reveals a novel left-handed parallel P-helix fold (Kisker et al. 1996). Apart from the histidines ligating zinc, the activesite residues of Cam have no recognizable analogs in the active sites of the a- and P-classes. Kinetic analyses establish that the enzyme has a zinc-hydroxide mechanism similar to that of Cab (Alber et al. 1999). 

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