Connectivity patterns and rotamer states of nucleobases determine acid-base properties of metalated purine quartets.

Potentiometric pH titrations and pD dependent (1)H NMR spectroscopy have been applied to study the acidification of the exocyclic amino group of adenine (A) model nucleobases (N9 position blocked by alkyl groups) when carrying trans-a2Pt(II) (with a=NH3 or CH3NH2) entities both at N1 and N7 positions. As demonstrated, in trinuclear complexes containing central A-Pt-A units, it depends on the connectivity pattern of the adenine bases (N7/N7 or N1/N1) and their rotamer states (head-head or head-tail), how large the acidifying effect is. Specifically, a series of trinuclear complexes with (A-N7)-Pt-(N7-A) and (A-N1)-Pt-(N1-A) cross-linking patterns and terminal 9-alkylguanine ligands (9MeGH, 9EtGH) have been analyzed in this respect, and it is shown that, for example, the 9MeA ligands in trans-,trans-,trans-[Pt(NH3)2(N7-9MeA-N1)2{Pt(NH3)2(9EtGH-N7)}2](ClO4)6·6H2O (4a) and trans-,trans-,trans-[Pt(NH3)2(N7-9EtA-N1)2{Pt(CH3NH2)2(9-MeGH-N7)}2](ClO4)6·3H2O (4b) are more acidic, by ca. 1.3 units (first pKa), than the linkage isomer trans-,trans-,trans-[Pt(CH3NH2)2(N1-9MeA-N7)2{Pt(NH3)2(9MeGH-N7)}2](NO3)6·6.25H2O (1b). Overall, acidifications in these types of complexes amount to 7-9 units, bringing the pKa values of such adenine ligands in the best case close to the physiological pH range. Comparison with pKa values of related trinuclear Pt(II) complexes having different co-ligands at the Pt ions, confirms this picture and supports our earlier proposal that the close proximity of the exocyclic amino groups in a head-head arrangement of (A-N7)-Pt-(N7-A), and the stabilization of the resulting N6H(-)⋯H2N6 unit, is key to this difference.

Multiple metal binding to the 9-methyladenine model nucleobase involving N1, N6, and N7: discrete di- and trinuclear species with different combinations of monofunctional Pd(II) and Pt(II) entities.

Several di- and trinuclear metal complexes consisting of the model nucleobase 9-methyladenine (9-MeA) or its mono-deprotonated form (9-MeA(-)) and monofunctional (dien)Pd(II), (dien)Pt(II), (NH(3))(3)Pt(II), or (trpy)Pd(II) in different combinations have been prepared and/or studied in solution by NMR spectroscopy: [{Pd(dien)}(3)(9-MeA(-)-N1,N6,N7)]Cl(3.5)(PF(6))(1.5)·3H(2)O (1), [(dien)Pd(N1-9-MeA-N7)Pt(NH(3))(3)](ClO(4))(4)·9.33H(2)O (2), [(dien)Pt(N1-9-MeA-N7)Pt(NH(3))(3)](ClO(4))(4)·H(2)O (3), and [{(trpy)Pd}(2)(N1,N6-9-MeA(-)-N7)Pt(NH(3))(3)](ClO(4))(5)·3H(2)O (4). A migration product of 3, [(dien)Pt(N6-9-MeA(-)-N7)Pt(NH(3))(3)](3+) (3a), has been identified in solution. Unlike Pt-adenine bonds, Pd-adenine bonds are substantially labile, and consequently all Pd-containing complexes discussed here (1, 2, 4) exist in aqueous solution in equilibria of slowly interconverting species, which give rise to individual resonances in the (1)H NMR spectra. For example, 1 exists in an equilibrium of five adenine-containing species when dissolved in D(2)O, 2 undergoes dissociation to [Pt(NH(3))(3)(9-MeA-N7)](2+) or forms the migration product [(dien)Pd(N6-9-MeA(-)-N7)Pt(NH(3))(3)](3+) (2a), depending on pD, and 4 loses both (trpy)Pd(II) entities as the pD is increased. In no case is Pd binding to N3 of the adenine ring observed. A comparison of the solid-state structures of the two trinuclear complexes 1 and 4 reveals distinct differences between the Pd atoms bonded to N1 and N6 in that these are substantially out of the nucleobase plane in 1, by ca. 0.6 Å and -1.0 Å, respectively, whereas they are coplanar with the 9-MeA(-) plane in 4. These out-of-plane movements of the two (dien)Pd(II) units in 1 are not accompanied by changes in hybridization states of the N1 and N6 atoms.

Perturbation of the NH2 pKa value of adenine in platinum(II) complexes: distinct stereochemical internucleobase effects.

The degree of acidification of the exocyclic N6 amino group of the model nucleobase 9-methyladenine (9MeA) in relation to the number and site(s) of Pt(II) binding has been studied in detail. It is found that twofold Pt(II) binding to N1 and N7 lowers the pK(a) value from 16.7 in the free base to 12-8. The lowest pK(a) values are observed when the resulting N6H(-) amide group is intramolecularly stabilized by an H-bond donor such as the N6H(2) group of a suitably positioned second 9MeA ligand. Deprotonation of the N6 amino group facilitates Pt migration from N1 to N6, and subsequent reprotonation of the N1 position yields a twofold N7,N6-metalated form of the rare imino tautomer of 9MeA, which has a pK(a) value of 5.03. These findings demonstrate a principle that is of potential relevance to the topic of "shifted pK(a)" values of adenine nucleobases, which is believed to be important with regard to acid-base catalysis of RNAs at physiological pH values. The principle states that a nucleobase pK(a) value can be sufficiently lowered to reach near-neutral values and that the pK(a) value of the protonated base does not necessarily have to be increased to accomplish this effect.