Conformation of adenosine 5′‐monophosphate in aqueous solution as studied by nmr‐DESERT method

The molecular conformation of adenosine‐5′‐monophosphate (5′‐AMP) in D2O (pH 8.1) has been investigated by the method of “DESERT” proposed earlier by Akasaka et al. [J. Magn. Reson. 18, 328 (1975)]. The reciprocal sixth‐power‐averaged distances r̄i′8 between H(8) of the adenine ring and H(i′) (i′ = 1′, 2′, and 3′) of the ribose moiety have been deduced directly from the differential spin‐lattice relaxation rates of these ribose protons upon deuterium substitution for H(8) and the rotational correlation times of the molecule estimated from carbon‐13 T1 values, by utilizing the pulse Fourier transform nmr method. The r̄i′8 values have been found to change significantly with the concentration of 5′‐AMP. The values of r̄1′8, r̄2′8, and r̄3′8 at infinite dilution have been determined to be 3.14, 2.45, and 3.37 Å, respectively, whereas those corresponding to the infinite concentration have been determined to be 3.84, 2.80, and 3.01 Å, respectively. The interproton distances obtained for infinite dilution are not consistent with any single conformation with respect to the rotation about the glycosidic torsional angle and to the ribose puckering, but can be explained well by approximately an equal mixture of three groups of conformers showing an intercorrelation between the ribose puckering and the glycosidic torsional angle, i.e., 3′‐endo‐anti, 2′‐endo‐syn, and 2′‐endo‐intermediate. The increase of r̄1′8 and r̄2′8 and the decrease of r̄3′8 with increasing concentration of 5′‐AMP can be explained well quantitatively by the preferential stabilization of anti conformers for both ribose states with formation of a base‐stacked dimer. The present results are compared with those obtained previously by other workers using a variety of methods.