The Effects of Monovalent Cations Li+, Na+, K+, NH4+, Rb+and Cs+on the Solid and Solution Structures of the Nucleic Acid Components. Metal Ion Binding and Sugar Conformation

Abstract

The interactions of the monovalent ions Li⁺, Na⁺, K⁺, NH4⁺, Rb⁺ and Cs⁺ with adenosine–5′—monophosphoric acid (H₂-AMP), guanosine-5′-monophosphoric acid (H₂-GMP) and deoxy- guanosine-5′-monophosphoric acid (H₂-dGMP) were investigated in aqueous solution at physiological pH. The crystalline salts M₂-nucleotide.nH₂O, where M=Li⁺, Na⁺, K⁺ NH₄ ⁺, Rb⁺ and Cs⁺, nucleotide=AMP, GMP and dGMP anions and n=2–4 were isolated and characterized by Fourier Transform infrared (FTIR) and ¹H-NMR spectroscopy. Spectroscopic evidence showed that these ions are in the form of M(H₂O)n⁺ with no direct metal-nucleotide interaction, in aqueous solution. In the solid state, Li⁺ ions bind to the base N-7 site and the phosphate group (inner-sphere), while the NH₄ ⁺ cations are in the vicinity of the N-7 position and the phosphate group, through hydrogen bonding systems. The Na- nucleotides and K-nucleotides are structurally similar. The Na⁺ ions bind to the phosphate group of the AMP through metal hydration shell (outer-sphere), whereas in the Na₂-GMP, the hydrated metal ions bind to the base N-7 or the ribose hydroxyl groups (inner-sphere). The Na₂-dGMP contains hydrated metal-carbonyl and metal-phosphate bindings (inner- sphere). The Rb⁺ and Cs⁺ ions are directly bonded to the phosphate groups and indirectly to the base moieties (via H₂O). The ribose moiety shows C2′-endo/anti conformation for the free AMP acid and its alkali metal ion salts. In the free GMP acid, the ribose ring exhibits C3′- endo/anti conformer, while a C2′-endo/anti sugar pucker was found in the Na₂-GMP and K₂- GMP salts and a C3′-endo/anti conformation for the Li⁺, NH₄ ⁺, Rb⁺ and Cs⁺ salts. The deoxyribose has C3′-endo/anti conformation in the free dGMP acid and 04′-endo/anti in the Na₂-dGMP, K₂-dGMP and a C3′-endo/anti for the Li⁺, NH₂ ⁺, Rb⁺ and Cs⁺ salts. An equilibrium mixture of the C2′-endo/anti and C3′-endo/anti sugar puckers was found for these metal -nucleotide salts in aqueous solution.