Water coordination environment, carbonic

Zinc aqua (Zn-OH2) species are prevalent in biological systems. When coordinated to a Zn(II) center, a water molecule can have a pATa value that varies from to 11, with [Zn(OH2)6]2+ having a pKa — 9.0.5 The position of a specific Zn-OH2 unit in this range depends on the primary and secondary ligand coordination environment of the zinc center. A more Lewis acid Zn(II) center, and hence a lower Zn-OH2 pK value, is produced when the total number of primary ligands is low (e.g. 4) and these ligands are neutral donors. For example, the Zn-OH2 moiety in [(THB)Zn(OH2)]2+ (Fig. la) exhibits a pATa value of 6.2.16 This indicates that a tetrahedral (NHis)3Zn(II)-OH2 moiety, as is found in active site of carbonic anhy-drase (CA), could have a pATa at or below physiological pH for the zinc-bound aqua... 

Mononuclear biological zinc centers generally exhibit an overall coordination number of 4 and a tetrahedral or distorted tetrahedral geometry [4]. This structural motif enhances the Lewis acidity of the zinc center relative to an octahedral coordination environment. Thus, whereas [Zn(H20)g] has a pfCa of 9.0 [26], the pfCa value for the zinc-coordinated water in carbonic anhydrase 11 is 6.8 (Fig. 8.1) [22]. During catalysis, the coordination number of the active site zinc center in a metaUohydrolase can increase to 5 [13]. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

The study of zinc-aqua complexes as synthetic carbonic anhydrase models has shown a low coordination number and a hydrophobic environment to be prerequisites for a low pK value of the aqua ligand, which is essential for efficient enzyme function. The pK value in the case of 13 is expected to be greater than 10.7, which is the value determined for pentacoordinate [Zn(tren)(H20)](C104)2, in which the water ligand is more strongly bound and thus expected to be more acidic (cf. the discussion of bond lengths, above). Thus, 13 is not expected to show carbonic anhydrase-like reactivity. However, in related tetraazamacrocyclic systems it has been shown that upon... 

The bioavailability of a given metal is influenced by its chemical speciation in the ambient environment. Although some metals occur predominantly in their free or aquo form (that is, the inner coordination sphere of the metal ion is occupied solely by water molecules), most bioactive metals occur as complexes in the natural environment. Hydroxide, carbonate, and chloride anions can all bind transition metals to a significant extent. For example, Fe + forms hydroxide... 

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