A Chemical Relaxation Study of Base Pairing through Self‐Association of Uracil and its Hetero‐Association with Adenine

A chemical relaxation study of base pairing through self‐association of 1‐isobutyl‐6‐methyl‐uracil (I) and its hetero‐association with 5‐O‐acetyl‐2′,3′‐O‐isopropylidene‐adenosine (II) in benzene has been carried out by means of the dipolar chemical field effect. The measurements have been interpreted in terms of kinetic parameters as well as with respect to the structures of these hydrogen bonded complexes. The rate constants and dipole moments are: magnified image the self‐association being a dimerization process. The dimerization constant (K0 = 1151/mol) has been checked by i.r. spectrophotometric measurements, and a comparison has been made between I and 5′‐O‐acetyl‐2′,3′‐O‐isopropylidene‐uridine in tetrachloromethane, chloroform, and benzene. In benzene, the dimer of I acquires most probably structure IV, and the A = U‐base pair will be the so‐called Hoogsteenassociate X.