Bridging water molecule

Figure 9.26 The bisligand chelate structure of zinc eugenolate, showing bridging water molecules. After Wilson Mesley (1974).

Chevrier et al., 1983), solvent effects (Bensaude et al., 1979), and the effect of added salt on the rate of reaction (Bensaude et al., 1978) have been studied to provide information about this process. Molecular-orbital calculations confirm that a suitable transition state for the reaction is one involving bridging water molecules (Field et al., 1984). 

From the mechanistic viewpoint, a more informative structure is the complex of PLCBc with inorganic phosphate (Pi), which also inhibits the enzyme (50% inhibition at 50 mmol l1 Pi) [65]. In this complex, which was obtained at 2.1 A resolution, the three zinc ions were coordinated to the two non-bridging phosphate oxygens (Fig. 7 a). Comparison of this structure with the native one (Fig. 6) reveals that one of the non-bridging oxygens on the phosphate replaced the bridging water molecule between Znl and Zn3, while the other oxygen displaced one of the waters coordinated to Zn2. 

Bridging water molecules occur in several crystal hydrates,53 and in a very few ternary complexes (Table 2).53 55... 

Figure 3.87 X-ray structure of the disodium complex of 3.124 showing the two different Na + environments and the intra-cavity bridging water molecule.62...

Although mammalian liver arginases can be activated by Co2+, Ni2+, Fe2+, and Cd2+, only Mn2+ is considered [114] to be the physiological activator. Mn2+ is specifically required to activate the arginases from Neurospora crassa [115], Rhodobacter capsulatus [116], and the Agrobacterium TiC58 plasmid. This selectivity is attributed to the unique ability of Mn to orient the bridging water molecule properly for catalysis [81,117],... 

The CP-MD trajectories of the 18C6-H20 "supermolecule" were then calculated for the Ci and D3d crowns starting with one bridging water molecule. The latter dissociated from the C, crown in less than 1 ps, indicating that this complex is too weakly bound to compete with the kinetic energy at 300 K. This contrasts with the hydrate of the D3d crown, which remained bound when the dynamics was pushed up to 10 ps, indicating that it is kinetically stable. 

The 04-protonation mechanism was explored only briefly, due to the fact that the crystal structures do not show any acidic residues in the vicinity of 04 (Fig. 2). The authors did calculate an 04-protonation mechanism using a model in which methylammonium protonates 04 via a bridging water molecule as proposed previously by Houk et al. (16).23... 

Fig. 21.9. A channel-type anionic-water host lattice formed by edge-sharing (H20)4F- tetrahedra with bridging water molecules in the structure of 4 (C2Hj)4N+(F 11H20). In this structure, six waters are three-coordinated and five are only two-coordinated, i.e., are donors only [438]...

Mak TCW (1985) Crystal structure of tetraethylammonium fluoride-water (4/11), 4 (C2H5)4N+F 11H20, a clathrate hydrate containing linear chains of edge-sharing (H20)4F tetrahedra and bridging water molecules. J Inclusion Phenom 3 347-354... 

Table 24 contains data on approximately half the known structures, and a similar story emerges from the amino acid structures.253 Monomers are known and there is the first example of a transition metal conipound [M(RC00)(H20)s]RC00 but most structures involve carboxylate bridging of three basic types (82 (84), although one structure has cw-unidentate carboxylates and chooses to polymerize on a single bridging water molecule. The Mn polyhedron in the tetrahydrate for R = (89) is the only one to depart from the pattern of [MnOs] octahedra. It has trans-H20 molecules at relatively normal distances of 2.28 and 2.31(2) A, and in the plane normal to these are three... 

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