Synthesis and acidity constants of 13CO 2H-labelled dicarboxylic acids. pK as from 13C-NMR

Abstract

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 C 10 ( 6) were prepared with 99% 13C-enrichment in their CO 2H groups. Their pK a values were determined from titration curves in water and in H 2O-(CD 3) 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 H 2O 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 H 2O.