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Multinuclear NMR Study of the Conformational Changes in Me p dG and dG p Me upon Interaction with Mg 2+ , Zn 2+ and Hg 2+ Ions Reveals the Strengthening of the Anomeric Effect by Soft M 2+ Ions
Author(s) -
Polak Matjaž,
Plavec Janez
Publication year - 1999
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199903)1999:3<547::aid-ejic547>3.0.co;2-1
Subject(s) - chemistry , metal ions in aqueous solution , crystallography , ion , moiety , anomer , proton nmr , stereochemistry , organic chemistry
A conformational study is reported on 2′‐deoxyguanosine 5′‐methylmonophosphate [Me p dG ( 1 )] and 2′‐deoxyguanosine 3′‐methylmonophosphate [dG p Me ( 2 )] and their interactions with Mg 2+ , Zn 2+ and Hg 2+ ions. The conformation of Me p dG ( 1 ) and dG p Me ( 2 ) in D 2 O solution was inferred from vicinal proton‐proton, proton‐phosphorus and carbon‐phosphorus NMR coupling constants and nuclear Overhauser effects (NOE). The chemical shift changes showed that hard Mg 2+ ions interact preferentially with the phosphate oxygen atoms in Me p dG ( 1 ), whereas in the case of dG p Me ( 2 ) the interaction with the phosphate oxygen atoms competes with the interaction to the C6=O carbonyl group. Softer Zn 2+ and Hg 2+ ions were found to show strong binding affinity towards N7 in both Me p dG ( 1 ) and dG p Me ( 2 ). Analysis of J coupling constants and NOEs measured as a function of metal ion concentration revealed that: (i) N → S pseudorotational equilibria are biased towards C2′‐ endo pseudorotamers in M 2+ ‐free Me p dG ( 1 ) and dG p Me ( 2 ) by 68% and 75% at 298K, respectively. Titration of Me p dG ( 1 ) and dG p Me ( 2 ) with Mg 2+ ions caused no observable changes in N → S pseudorotational equilibrium, whereas the interactions of Zn 2+ and Hg 2+ ions with N7 resulted in the shift towards N‐type pseudorotamers which can be explained by the strengthening of the anomeric effect as softer metal ions bind to N7 and make the imidazole moiety less electron‐rich. (ii) The binding of divalent metal ions to Me p dG ( 1 ) and dG p Me ( 2 ) causes a shift of the syn → anti equilibrium towards anti , which is larger for softer Zn 2+ than for harder Mg 2+ ions. (iii) The conformational equilibrium across the C4′–C5′ bond (γ torsion) in dG p Me ( 2 ) is not affected by the increased concentration of M 2+ ions. (iv) β t conformers are preferred by ca. 77% in aqueous solution of Me p dG ( 1 ) and only small changes of ca. 1 percentage point in β t population have been found upon metal ion binding to Me p dG ( 1 ). (v) The two‐state ϵ t → ϵ − conformational equilibrium is biased towards ϵ t rotamers by 63.5% in dG p Me ( 2 ). Interaction of hard Mg 2+ and softer Zn 2+ or Hg 2+ ions with dG p Me ( 2 ) resulted in the minor increase (< 3 percentage points) in the population of ϵ t conformers.