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Perturbation of the NH 2 p K a Value of Adenine in Platinum( II ) Complexes: Distinct Stereochemical Internucleobase Effects
Author(s) -
Garijo Añorbe Marta,
Lüth Marc Sven,
Roitzsch Michael,
Morell Cerdà Marta,
Lax Patrick,
Kampf Gunnar,
Sigel Helmut,
Lippert Bernhard
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305509
Subject(s) - nucleobase , chemistry , protonation , deprotonation , tautomer , stereochemistry , ligand (biochemistry) , amide , hydrogen bond , molecule , dna , organic chemistry , biochemistry , ion , receptor
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 p K a value from 16.7 in the free base to 12–8. The lowest p K 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 N 7, N 6‐metalated form of the rare imino tautomer of 9MeA, which has a p K a value of 5.03. These findings demonstrate a principle that is of potential relevance to the topic of “shifted p K 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 p K a value can be sufficiently lowered to reach near‐neutral values and that the p K a value of the protonated base does not necessarily have to be increased to accomplish this effect.