Metal Coordination and Imine−Amine Hydrogen Bonding as the Source of Strongly Shifted Adenine pKa Values
dataset
posted on 2004-03-03, 00:00 authored by Michael Roitzsch, Bernhard LippertThe X-ray crystal structure of a PtII complex of composition trans-[(NH3)2Pt(1,9-DimeA) (1,9-DimeAH)](ClO4)3 (2) with 1,9-DimeA = 1,9-dimethyladenine and 1,9-DimeAH+ = 1,9-dimethyladeninium)
is presented. Complex 2 forms upon deprotonation of one of the exocyclic amino groups of the adeninium
ligands in trans-[(NH3)2Pt(1,9-DimeAH)2](ClO4)4 (1), where the two nucleobases are in a head−tail
arrangement. The low pKa1 of 1 (4.1 ± 0.2) is a consequence of a combination of the effects of metal
coordination to N7 of the purine base and efficient stabilization of the deprotonated species. This feature
is supported by the results of the structure determination of 2, which displays a head−head orientation of
the two bases and intramolecular H-bonding between the imine group of 1,9-DimeA and the amino group
of 1,9-DimeAH. In the fully deprotonated species trans-[(NH3)2Pt(1,9-DimeA)2](ClO4)2 (3), the two
nucleobases are again in a head−tail arrangement. The findings are of relevance with regard to the concept
of “shifted pKa values” of nucleobases. This concept is applied to rationalize acid−base catalysis reactions
involving nucleobases of DNA and RNA which occur in the near-physiological pH range.
