Water primary coordination environment

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... 

Only nuclei with a net nuclear spin will give a signal. However, this is no problem for hydration studies since it is the chemical shift of the proton, H, signal of H2O which is being measured. In certain cases the signal from in Hj O is used. In a solution the O atom is the atom closest to the cation and the H atom is the one closest to an anion. Essentially the experiment looks at the environment of the atom whose resonance is being studied. In an electrolyte solution there are three species of interest the solvated cation, the solvated anion and bulk water. Coordinated water will exchange with secondary solvation of the ion under study, with the primary and secondary solvation of the counter ion and with the bulk water. 

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