Structural and biochemical properties of bidentate tetraaquarhodium(III) complexes of inorganic pyrophosphate and adenosine 5'-diphosphate.

The structural and biochemical properties of the alpha,beta-bidentate tetraaquarhodium(III) complexes of inorganic pyrophosphate [Rh(H2O)4PP] and adenosine diphosphate [Rh(H2O)4ADP] are examined. These Rh(III) complexes are exchange-inert analogues of the corresponding physiologically important MgIIPP and MgIIADP complexes. The crystal structure of [Rh(H2O)4H2P2O7]+Cl- shows that the six-membered chelate ring adopts a twist-boat conformation with an unusually high puckering amplitude of 0.756 (3) A. The Rh coordination distances average 2.02 (1) A, while the bridge P-O bonds are virtually equal in length. All 10 protons of the complex participate in hydrogen bonding. There are two intramolecular hydrogen bonds between the phosphate oxygen atoms and the axially coordinated water molecules. The Rh(H2O)4PP complex was found to be a substrate for yeast inorganic pyrophosphatase, with Ki = 0.063 (7) mM and Vm = 500 (100) min-1. The two screw sense isomers of Rh(H2O)4ADP were prepared from (Rp)-[alpha-16O,18O]ADP and assigned configuration on the basis of the magnitude of their 31P NMR isotopic chemical shifts. The Rh(H2O)4ADP complex binds a number of kinases as tightly as MgADP. Arginine kinase and creatine kinase were shown to bind the delta Rh(H2O)4ADP isomer 7 and 45 times tighter, respectively, than the lambda isomer. The reactivity of Rh(H2O)4PP with pyrophosphatase is comparable to that of Cr(H2O)4PP, and the binding affinities of the Rh(H2O)4ADP screw sense isomers for kinases are also comparable to those observed for the corresponding Cr(H2O)4ADP screw sense isomers.