How does the 3′-phosphate drive the sugar conformation in DNA?

Abstract

While the ³J_C(4′)P, ³J_C(2′)P, ³J_H(3′)P, ³J_CH3P and ³J_CH2P coupling constants in simple model systems 13–16 remain unchanged over 278–358 K, considerable changes of endocyclic ³J_HH coupling constants have been found to take place, which, for the first time, unequivocally show that the change of the North (3′-endo-2′-exo)⇄ South (3′-exo-2′-endo) sugar pseudorotamer equilibrium is independent of the change of the phosphate backbone torsion, it is also found that the gauche effects between O(4′) and 3′-OPO₃H- in 9–12 and between O(4′) and 3′-OPO₃Et^– in 13–16 are responsible for the stabilization of the South sugar conformer by ΔH°=–1.5 kJ mol^–1 compared to 3′-OH in 5–8.