Speciation 98: Abstracts

The ionised form of malic acid is an intermediate of the Krebs cycle, the central cross-road of metabolic pathways. Malate is also important in photosynthetic energy-producing processes in some plants. Its pure scientific impact is also significant, since the stereochemical "Walden-inversion" is closely related to malic acid.

Despite its importance, a profound interpretation of its acid-base chemistry has not yet appeared in the literature. It has long been qualitatively known that its less basic carboxylate group is the one adjacent the electron-withdrawing hydroxyl group. The protonation macroconstants of the dibasic malate ion at 25 ^oC and I=1,00 are logK₁ = 4.56 and logK₂ = 3.20; which are, as macroconstants not characteristic of the specific groups. Unfortunately the spectroscopic techniques (NMR, CD, UV) that can sensitively monitor the protonation stage of the specific groups, can not be used here, because all of the spectroscopic signals are to some extent functions of the changing protonation stage of both group. We have therefore determined the microconstants by synthesising derivative compounds, and an appropriate deductive method, based upon the protonation constants of the derivatives. Our derivatives were the a-, and b-amides. Thus, having a redundant set of equations, the accuracy of the microconstants could also be estimated. Every microspecies is a composite of different rota-microspecies, which are in equilibrium with the proton and each other. The rotamer and conformation-specific constants were determined from three-bond ¹H-¹H and ¹H-¹³C NMR coupling constants.

The site- and rotamer-specific constants show that significant, rotamer-dependent differences of the basicities (Dlog k ~ 0.5) occur at the first malate protonations. Rotamer-dependence of the second malate protonation is in the range of Dlog k ~ 0.1 only. The proton-binding ability is enhanced when the carboxylates are in gauche position, especially when the hydroxyl is in trans.

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