Oxyanions structure

Only a brief mention will be made of the role of polynuclear oxyanions as ligands [X,OJ" as these tend to involve infinite rigid lattices (one-, two- or three-dimensional) and thus lose their ability to act as independent ligands. In these rigid oxyanion structures the oxygens still act as ligands, as... 

Table 3 The Number, p, of Oxyanions (XO,) per Metal Atom M as a Function of the Oxyanion Structural Type... 

Remember that the oxyanion and carbanion structures are just two different ways to represent the same thing. We shall usually prefer the oxyanion structure as it is more realistic. You can say the same thing in orbitals. 

Notice that in drawing this mechanism It is not necessary to locate the negative charge on the carbon atom. You should always draw enolate mechanisms using the better oxyanion structure. 

Serine proteinases such as chymotrypsin and subtilisin catalyze the cleavage of peptide bonds. Four features essential for catalysis are present in the three-dimensional structures of all serine proteinases a catalytic triad, an oxyanion binding site, a substrate specificity pocket, and a nonspecific binding site for polypeptide substrates. These four features, in a very similar arrangement, are present in both chymotrypsin and subtilisin even though they are achieved in the two enzymes in completely different ways by quite different three-dimensional structures. Chymotrypsin is built up from two p-barrel domains, whereas the subtilisin structure is of the a/p type. These two enzymes provide an example of convergent evolution where completely different loop regions, attached to different framework structures, form similar active sites. 

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups. 

Some metals are amphoteric. That is, they form simple cations (in acid solutions) and soluble oxyanions (in alkaline solution) only in the mid-pH range is a protective film stable. Since cathodic protection produces alkali at the structure s surface, it is important to restrict the polarisation, and thereby the amount of hydroxyl ion produced, in these cases. Thus both lead and aluminium will suffer cathodic corrosion under cathodic protection if the potential is made excessively electro negative. 

The lipase (PAL) used in these studies is a hydrolase having the usual catalytic triad composed of aspartate, histidine, and serine [42] (Figure 2.6). Stereoselectivity is determined in the first step, which involves the formation of the oxyanion. Unfortunately, X-ray structural characterization of the (S)- and (J )-selective mutants are not available. However, consideration of the crystal structure of the WT lipase [42] is in itself illuminating. Surprisingly, it turned out that many of the mutants have amino acid exchanges remote from the active site [8,22,40]. 

Structure 3 is the intermediate oxyanion adduct. TS2 is the structure leading to cyclization of the oxyanion to the oxaphosphetane. Structure 4a is the oxaphosphetane, and the computation shows only a small barrier for its conversion to product. 

Another type of hypervalency is encountered in textbook descriptions of the oxyanions of common laboratory acids. Generations of chemistry students have been taught that the correct representations of these species are in terms of resonance-delocalized hypervalent Lewis-structure diagrams, such as sulfate (S042-),... 

Structure (3.226c), for example, depicts a central heptavalent Cl atom (Fa = 7), exceeding the normal valence octet by six electrons (These excess electrons are assumed to be accommodated in chlorine 3d orbitals, whereas d-orbital participation is prevented in first-row compounds.) Hypervalent structures such as (3.226a)-(3.226c) are claimed to be justified by the electroneutrality principle, which stipulates that second-row central atoms have zero formal charge (whereas first-row oxyanion Lewis structures commonly violate this principle).148... 

The vanadate (1, 2), molybdate (1-5), and tungstate (1-3) systems have been described in previous reviews. Although the focus in this chapter is on more recent developments, earlier well-established knowledge is included where needed for perspective and also to present a coherent picture of the hydrolysis behavior of these oxyanions. Equilibria of mono- and polynuclear species are described and information about known structures are given. Some recent work about mixed polyoxoanions is briefly reviewed. 

Clearly, the oxyanion hole is now as significant a feature of the binding site of such acyl transfer abzymes as it is already for esterases and peptidases — and not without good reason. Knossow has analysed the structures of three esterase-like catalytic antibodies, each elicited in response to the same phosphonate TSA hapten (Charbonnier et al., 1997). Catalysis for all three is accounted for by transition state stabilization and in each case there is an... 

Lawson, D. M., Williams, C. E., Mitchenall, L. A., and Pau, R. N. (1998). Ligand size is a major determinant of specificity in periplasmic oxyanion-binding proteins the 1.2 A resolution crystal structure of Azotobacter vinelandii ModA. Structure 6, 1529-1539. 

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