Cadmium II Complexes of Nucleotides

For purine-nucleoside 5 -phosphates the formation of macrochelates was proposed nearly 60 years ago [131] and more than 50 years ago it was concluded that they actually exist [132-135], i.e., a metal ion coordinated to the phosphate residue of a purine nucleotide may also interact in the dominating anti conformation with N7 of the purine moiety. Nowadays formation of macrochelates in complexes formed by purine nucleotides with various metal ions including Cd is well established [117,120,136-138]. Clearly, the formation of such a macrochelate must give rise to the intramolecular equilibrium (22)  

As already discussed in Section 4.2, any kind of equilibrium between an open isomer, M(L) p, and a chelated or closed isomer, M(L)d, must be reflected in an increased complex stability compared to the situation where rally the open complex can form [51]. This stability enhancement, log zIm/l (eq. 14), can be evaluated by equations (16) to (20) (Section 4.2), which furnish values for ATj (eqs 16, 17) and thus for the formation degree of the closed species, % M(L)d (eq. 20). 

One important aspect needs to be emphasized and kept in mind when dealing with purine derivatives All of them show a pronounced tendency for selfassociation, which occurs via stacking of the purine rings [117,137,139,140]. Hence, experiments aimed at determining the properties of mOTiomeiic metal ion complexes of purine nucleosides or their phosphates should not be carried out in concentrations higher than 10 M. To be on the safe side, actually a maximum concentration of only 5 x 10 M is recommended (cf., e.g., [122,136]). The equilibrium constants discussed below were mostly obtained by potentiometric pH titrations and the corresponding experimental conditions adhere to the above request. 

Correspondingly, one has to consider the complexes M(H NMP) and M(NMP), the stabilities of which are defined in equilibria (24) and (2), respectively (charges neglected)  

At high pH values also Cd(NMP - H) species (eq. 8) might form but at present no information is available about these. 

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Lomozik, L. and Bregier-Jarzebowska, R. (1999) Complexes of cadmium(II) and mercury(II) with polyamines, nucleosides and nucleotides. Pol. J. Chem., 73, 927-940. 

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