Binding of Platinum Complexes in Solution

Very useful reviews of relevant aqueous platinum chemistry are available 

One approach has been to use the kinetically more labile palladium complexes as analogues of the platinum systems, although it has been emphasized that even if the reaction is 10 times faster than platinum, the palladium complexes still react slowly [21]. Using NMR and poten-tiometric studies on the tridentate diethylenetriamine chelate, [Pd(dien) (H20)], which allows substitution at only one site, an order of thermodynamic binding at pH 7 was obtained for nucleosides [22]  

The relative orders for the two guanine possibilities, and Ny, confirm that in solution there is a competition between these two sites. Since metallation at Ny in general renders the hydrogen at the Nj site more acidic (see 4.2.1), the potential for binding at the second site increases upon initial Ny binding. In the case of adenosine both sites are deprotonated at normal pH, and Ni is favored. The tendency for binding at Nj or Ny has been considered in detail in the [Pd(dien)(H20)] system, and the 

An interesting feature of the adenine system is that the presence of a 5 -phosphate group (i.e. nucleotide compared to nucleoside) favors metallation of by approximately 0.7 log units, and thus the relative order in AMP is Ny N. The extent of Ny binding for both guanosine and 5 -GMP is much greater than could be expected from purely basicity considerations. In competition studies with mixtures of nucleotides, binding of 5 -GMP Ny is favored over the pH range 5—9, but for other nucleotides is pH-dependent [21]. 

The results quoted above are claimed to be extendable to platinum [21] but may also be considered to represent the simplest and most favorable case for study attainment of thermodynamic equilibrium at one substitution site. In the case of the [Pd(en)2] cation, both Ny and binding are observed for IMP, but the system is quite complicated [23]. 

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