Synthesis and acidity constants of 13CO2H-labelled mono and dipyrrole carboxylic acids. pKa from 13C-NMR

Article abstract of DOI:10.1016/0040-4020(94)01052-2

Six monocarboxylic acids were prepared highly enriched with ¹³C in their CO₂H groups, and their pK_a values were determined at low concentrations 10^-4-10^-5 M in H₂O and in H₂O-(CD₃)₂SO mixtures by analysis of pH-dependent ¹³CO₂H NMR chemical shifts. Plots of the variation of CO₂H(CO₂^-) ¹³C-NMR chemical shift vs pH gave a typical titration curve from which pK_a's for [1-¹³C]-phenylpropionic (1) and [1-¹³C]-phenylacetic (2) acids were determined to be 4.60 and 4.16 respectively in H₂O, and 4.67 and 4.31 respectively in H₂O-27% vol (CD₃)₂SO. Bilirubin analogs, xanthobilirubic acid (5) and nor-xanthobilirubic acid (6) were determined to have pK_a values of 4.76 and 4.64 respectively in H₂O-27% vol (CD₃)₂SO, and extrapolated to pK_a values and 4.62 and 4.51 in H₂O.