PROTON MAGNETIC RESONANCE INVESTIGATION OF ASSOCIATION OF ADENINE AND URACIL DERIVATIVES IN CHLOROFORM SOLUTION

The hydrogen-bonded associations of adenine and uracil derivatives in chloroform solutions have been investigated using high-resolution proton magnetic resonance. The adenine-uracil chemical-shift mixing curves for various derivatives confirm that the equilibrium constants for the hydrogen-bonding interaction are directly reflected in the magnitudes of the observed association shifts. Halogenated uracil derivatives interact more strongly with adenine than does uracil. When the nucleoside solutions are concentrated, the data for the association of adenine and uracil is consistent with the formation of 1:2 triplets. Adenine derivatives which can only form pairs with uracil due to steric hinderance or to reduction of the number of hydrogen-bonding groups, have about half the equilibrium constant of association found for adenine itself. A deshielding effect on ring protons of base-pair formation is reported. Hydrogen-bond formation results in down-field shifts of the protons in the planes of the rings. These shifts are greatest for the C(8)-purine protons and are very small for pyrimidine protons. © 1969.