Models second coordination sphere

Fig. 1. Construction of a computational model for TauD. (A) the solvated TauD enzyme (PDB code 1GY9, solvating water molecules in red) (B) the desolvated enzyme (C) the active site with the substrate and a-ketoglutarate bound to the iron centre, and the most important amino acids in the first and second coordination sphere (D) a minimal model for TauD including only the first coordination sphere and the substrate.

Measurements of Tj have been made (269,270) to probe the structure of the second coordination sphere around Cr(acac)3. Acetone, chloroform, and methylene chloride were chosen as second sphere ligands. The shortening of their Tj values is found to be independent of solute concentration, and the value measured is determined by the diffusional correlation time of the solute molecule. No detectable second coordination sphere therefore exists in these solvents. However, methanol forms a discrete second sphere, and above a certain concentration the measured Ty values vary linearly with solute concentration. The observed Tj values are compatible with a model having a coordination number of 8, a Cr-CH3 separation of 700 pm, and an equilibrium constant for displacing solvent (CHCI3) of ca. 10. This equilibrium constant value is consistent with values previously... 

Although there are aqua ions still to be identified, many have been characterized as already described. On the other hand, information about the number of water molecules in the second-coordination sphere of metal ions and their residence times is scarce, and the only experimentally determined lifetime of a water molecule exchanging between the 12 H20 of the second-coordination sphere and bulk water is 1.28 x 10-los (Ih o = 7.8x 109s 1) at 25°C for [Cr(H20)6]3+whieh compares with 1.44 x 10-los from molecular dynamics calculations.24 63 Similar calculations show Nd3+, Sm3+, and Yb3+ to have 17.61, 17.13, and 16.74 water molecules in the second-coordination sphere, with residence times of 1.3 x 10-11 s, 1.2 x 10—11 s, and 1.8 x 10 11 s, respectively.211 These studies are consistent with the exchange of water between the second-coordination sphere and the bulk solvent being close to diffusion controlled, as has generally been assumed in mechanistic models for the substitution of water in the first-coordination sphere. 

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