Synthesis and Acidity Constants of 13CO2H-Labelled Dicarboxylic Acids. pKas from 13C-NMR

Article abstract of DOI:10.1016/0040-4020(96)00153-6

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 gemdimethyl at C₁₀ (6) were prepared with 99% ¹³C-enrichment in their CO₂H groups. Their pK_a values were determined from titration curves in water and in H₂O-(CD₃)₂SO solutions: plots of carboxyl carbon ¹³C-NMR chemical shifts with varying pH. Titration curves of diacids 1-3 with known pK_a's in H₂O served as calibration standards for determination of pK_as of tetrapyrrole diacids 4-6: pK_a1 = 3.9, pK_a2 = 5.3 for 4; pK_a1 = 4.2, pK_a2 = 4.9 for 5, and pK_a1 = 4.7, pK_a2 = 5.7 for 6 in H₂O. Copyright