SYNTHESIS AND ACIDITY CONSTANTS OF (CO2H)-C-13-LABELED MONO AND DIPYRROLE CARBOXYLIC-ACIDS - PK(A) FROM C-13-NMR

Six monocarboxylic acids were prepared highly enriched with C-13 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)(2)SO mixtures by analysis of pH-dependent (CO2H)-C-13 NMR chemical shifts. Plots of the variation of CO2H(CO2-) C-13-NMR chemical shift vs pH gave a typical titration curve from which pK(a)'s for [1-C-13]-phenylpropionic (1) and [1-C-13]-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)(2)SO. 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)(2)SO, and extrapolated to pK(a) values similar to 4.62 and 4.51 in H2O.