THE EFFECTS OF MG-2+, HYDROGEN-BONDING, AND STERIC FACTORS ON RATE AND EQUILIBRIUM-CONSTANTS FOR PHOSPHORYL TRANSFER BETWEEN CARBOXYLATE IONS AND PYRIDINES

The reaction of bicarbonate ion with phosphorylated-picoline monoanion (PicP), which presumably gives carboxyphosphate, is faster than the reactions of acetate and carbonate by factors of 40 and 6, respectively (25 °C, I = 1.5). The rate increase is attributed to hydrogen bonding of the bicarbonate hydroxyl group to a phosphoryl oxygen atom; phosphate monoanion and dianion show similar increases. The reaction of acetate ion with PicP and the reverse reaction of γ-picoline with acetyl phosphate dianion are catalyzed ~ 20-fold by Mg2+. The reaction of PicP with formate ion is 20-fold faster than with acetate ion and shows no catalysis by Mg2+. This indicates that the rate increases with bicarbonate and with Mg2+ arise from overcoming unfavorable electrostatic, solvation, or steric interactions. Catalysis of the reaction of -picoline with acetyl phosphate (AcP), in both directions, is described by chelation of Mg2+ to the transition state, with Ka, * = 97 M-1, and binding to the ground states with Ka = 5 and 4.4 M-1 for PicP and AcP, respectively. Equilibrium constants are reported for phosphoryl transfer between pyridines, imidazole, ammonia, and acetate ion; the favorable transfer from pyridine to acetate, by ~ 100-fold, shows that the P-O bond is stronger than the P-N bond, relative to H-O and H-N bonds. © 1990, American Chemical Society. All rights reserved.