Phosphates metal-binding properties

Mn has often been employed as a substitute for Mg to investigate potential metal binding sites in nucleic acids. High-spin Mn is similar but not identical to Mg in properties such as ionic radius, enthalpy of hydration, and coordination preferences." Since the earliest crystal structures of tRNA it has been observed that some sites may be occupied by either divalent Mg or Mn, but that coordination details may differ between the two ions (Figure 6b). The most common difference is that the indirect interactions of hydrated Mg ions with RNA that are mediated by aqua ligands are sometimes replaced by direct coordination between Mn and a phosphate or nucleic acid base position. 

The importance of phosphate in DNA and RNA macromolecules has been described. It serves as an integral part of the molecular backbone as well as the means by which the monomeric units are connected. It has important binding properties, being able to participate in strong (electrostatic) bonding interactions with metal and amine cations. 

Overall, these results confirm the above conclusion that a purine-coordinated Pt " unit affects metal ion binding at the phosphate group of the same nucleotide only relatively little. This is of relevance for multiple metal ion binding to nucleotides and nucleic acids as is the already discussed result (Section 4.1) regarding (Dien)Pt(9EtG-A7) and its metal ion-binding properties, which are, despite the Pt " at N7 of the guanine residue, still quite remarkable at the (Nl) site [82] (see also ref. [69]). 

Chin and coworkers have recently evaluated the anion binding properties of mononuclear Co(III) complexes as a function of the presence of an internal H-bond donor [51]. Using nuclear magnetic resonance (NMR), equilibrium binding constants for phosphate diester monoanion coordination to the Co(III) aqua complexes 5 and 6 shown in Fig. 8.16 were found to be 210 and 6.2 respectively, in water at 80 °C. As monodentate coordination of phosphate diester monoanions to metal ions is typically weak in aqueous solution (K< 10 M ) [108], the incorporation of a H-bond donor clearly influences the anion coordination properties. In sum, the combination of metal coordination and H-bonding produces an equilibrium binding constant that is still well below that found for coordination of both phosphoryl oxygens of the diester to a dinuclear metal structure (around 400 M ) [109]. 

Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin. We have shown that the tightly bound divalent metal ion (Ca or Mg ) interacts with the P- and y-phosphates of ATP bound to actin, and that the Me-ATP bidentate chelate is bound to G-actin in the A configuration. The nature of the bound metal ion affects the conformation of actin, the binding kinetics of ATP and ADP, and the rate of ATP hydrolysis. 

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