2′-O-Methyl-cis-adenosine 3′,5′-cyclic methyl monophosphate, a new model system for cAMP. Aspects of structure and reactivity

Abstract

This study is focussed on 2′-O-methyl-cis-adenosine 3′,5′-cyclic methyl monophosphate (cis-4; cis describes the relationship between OMe (bound to phosphorus) and the adenine base), which is regarded a model for enzyme-bound cyclic adenosine monophosphate (cAMP). In this complex the negative phosphate charge is shielded in part via complexation with cationic sites on the enzyme surface. We report synthesis, crystal structure, solution conformation (400 MHz 1H NMR), and kinetic-mechanistic aspects of the alkaline hydrolysis of cis-4. The methanol solvate of cis-4 crystallizes in the orthorhombic space group P212121 and the cell dimensions are a = 8.170(2) Å, b = 9.249(1) Å, c = 23.699(4) Å; V = 1788.5(6) Å3; Z = 4 molecules per cell. Least-squares refinement converged at R = 0.062 for 2006 observed reflections. The adenine bases are linked via [Formula: see text] and [Formula: see text] hydrogen bonds in such a way that infinite one-dimensional chains are formed. This hydrogen bond scheme is very similar to that observed in the structure of 2′-deoxy-3′,5′-di-O-acetyl adenosine. An additional hydrogen bond is formed between methanol, incorporated in the crystal structure, and the adenine base. The conformational preferences of the cis-4 dissolved in methanol have been determined with 400 MHz 1H NMR. It is found that the conformations observed in the solid and solution states are practically the same. Hydrolysis of the title compound yields two acyclic phosphate diesters (a 3′- and a 5′-phosphate diester). The intermediates formed during the hydrolysis reaction are most likely five-coordinated phosphorus (PV) compounds with a trigonal bipyramidal geometry and an equatorial-axial located 3′,5′-dioxaphosphorinane ring.