Adeninium cation

The effect of metal coordination to any of the endocyclic N atoms can be also expressed in terms of a loss in basicity of the remaining endocyclic N atoms, hence in a reduced tendency to accept a proton and to become a metalated adeninium cation. As the pXa values of these species are frequently in the range 2-0, their determination usually is facile. In the case of Ptn binding to 9-MeA, relevant pXa values are 1.93 0.1 for N7 coordination and 0.45 0.11 for N1 coordination in the mixed nucleobase complex di-[(NH3)2Pt(9-MeA)(l-MeC-A5)]2+ (103). This characteristic difference permits a ready assignment of the metal binding site in Pt(II) adenine complexes. The apparent increase in basicity of N7 upon N1 Pt(II) binding disappears if microacidity constants are applied (103). 

A certain number of N6-substituted adenines display kinetin activity. In this context, cadmium derivatives have been reported with the N6-benzyl-adeninium-(1+) (BAD and N6-furfuryl-adeninium(l+) (FAD" cations to yield the compounds [Cd(p2-C1)2C1(H2(N3,N7)BAD)] and [Cd(p2-C1)2C1(H2(N3,N7) EAD)] [15]. In these polymers the coordination number of cadmium is six. Both N6-adenine derivatives bind the metal via the Cd-N9 bond (2.348 or 2.361 A, respectively) reinforced by an intramolecular interligand N3-H—(p-Cl) interaction (3.188 A, 159.3° or 3.154 A, 165.1°, respectively). The protonation sites (N3, N7) of these N6-substituted adenines should be related to the aforementioned intramolecular N3-H—(p-Cl) bond and the crystal packing. Indeed, the N7-H bond is involved in bifurcated H-bonding with two different chlorido ligands. 

Third, density functional theory calculations, including the presence of one [M(ox)2] fragment in all of its possible dispositions around the cytosinium cation, seem to indicate that the structural synthons found in the adeninium compound are also quite robust for assembling the cytosinium and oxalato-containing complexes. 

In the case of lH,9H-adeninium compound, the protonated Watson-Crick face of the cations (H6-N6-C6-N1-H) is alternately placed in both sides of the one-dimensional aggregate with a distance of 11.649 A between two... 

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