Lipophilicity of zwitterions and related species: a new insight.

The experimental determination of microscopic partition coefficients for protonation isomers is elaborated for the first time, and applied for niflumic acid, an ampholytic, mainly zwitterionic drug for pains in joints and muscles. The acid-base microequilibria of niflumic acid are also characterized by NMR-pH and deductive methods using auxiliary compounds of reduced complexity. The results show that 16 times as many zwitterionic than non-charged microspecies exist in aqueous solution. Partition of the individual microspecies was mimicked by model compounds of the closest possible similarity, then correction factors were also determined and introduced. Thus the long-awaited intrinsic partition coefficients of the non-charged vs. zwitterionic species could be calculated. The non-charged microspecies is 390 times as lipophilic as its zwitterionic protonation isomer. The microscopic partition coefficients are also in line with the experimentally determined distribution coefficients. These results make evident that contribution of the zwitterionic microspecies to the overall lipophilicity is not negligible, especially at the isoelectric pH region of the compound.