NH-PROTON EXCHANGE OF PURINE IN AQUEOUS SOLUTION - EFFECT OF MOLECULAR COMPLEXING ON REACTION RATE

The effect of molecular complex formation on the kinetics of NH-proton exchange of purine was studied by the nmr method in aqueous solution at pH 4.2-7.0. At low purine concentrations, the following fast reactions were characterized by kinetic analysis: (a) proton transfer with second-order kinetics involving the reactants purine, water, and purinate ion; rate constant k2 = 1.0 X 108 sec−1 M−1at 20°; (b) reaction of purine with hydrogen ion: rate constant kH= 2.4 X 1010 sec−1 M−1 at 20°. At 0.85 M, where the average degree of association of purine is 2.01, the rate constants are nearly the same: k2 = 1.0 X 108 and kH = 3.1 X 1010 sec−1 M−1 at 20°. The high value of kH - implies that purinium ion can undergo very rapid proton exchange at N-9, perhaps by a mechanism of reversible ionization. The purine NH-water OH proton chemical shift was found to be 12.4 ppm at 20°. © 1969, American Chemical Society. All rights reserved.