Chemical and Structural Implications of 1',2'- versus 2',4'- Conformational Constraints in the Sugar Moiety of Modified Thymine Nucleosides

摘要

In order to understand how the chemical nature of the conformational constraint of the sugar moiety in ON/RNA(DNA) dictates the duplex structure and reactivity, we have determined molecular structures and dynamics of the conformationally constrained 1',2'-azetidine- and 1',2'-oxetane-fused thymidines, as well as their 2',4'-fused thymine (T) counterparts such as LNA-T, 2'-amino LNA-T, ENA-T, and aza-ENA-T by NMR, ab initio (HF/6-31G~(**) and B3LYP/6-31++G~(**)), and molecular dynamics simulations (2 ns in the explicit aqueous medium). It has been found that, depending upon whether the modification leads to a bicyclic 1',2'-fused or a tricyclic 2',4'-fused system, they fall into two distinct categories characterized by their respective internal dynamics of the glycosidic and the backbone torsions as well as by characteristic North-East type sugar conformation (P = 37° ± 27°, φ_m = 25° ± 18°) of the 1',2'-fused systems, and (ⅱ) pure North type (P = 19° ± 8°, φ_m = 48° ± 4°) for the 2',4'-fused nucleosides. Each group has different conformational hyperspace accessible, despite the overall similarity of the North-type conformational constraints imposed by the 1',2'- or 2',4'-linked modification. The comparison of pK_as of the 1-thyminyl aglycon as well as that of endocyclic sugar-nitrogen obtained by theoretical and experimental measurements showed that the nature of the sugar conformational constraints steer the physicochemical property (pK_a) of the constituent 1-thyminyl moiety, which in turn can play a part in tuning the strength of hydrogen bonding in the basepairing.