Simple dicarboxylic acids: adipic (1), succinic (2) and malonic (3); more complex linear tetrapyrroles with two propionic acid groups: mesobiliverdin-XIIIα (4) and mesobilirubin-XIIIα (5), and a rubin analog with a gem-dimethyl at C10 (6) were prepared with 99% 13C- enrichment in their CO2H groups. Their pKa values were determined from titration curves in water and in H2O-(CD3) 2SO solutions: plots of carboxyl carbon 13C-NMR chemical shifts ...