SYNTHESIS AND ACIDITY CONSTANTS OF (CO2H)-C-13-LABELED DICARBOXYLIC-ACIDS - PK(A)S FROM C-13-NMR

Simple dicarboxylic acids: adipic (1), succinic (2) and malonic (3); m ore complex linear tetrapyrroles with two propionic acid groups: mesob iliverdin-XIII alpha: (4) and mesobilirubin-XIII alpha (5), and a rubi n analog with a gem-dimethyl at C-10 (6) were prepared with 99% C-13-e nrichment in their CO2H groups. Their pK(a) values were determined fro m titration curves in water and in H2O-(CD3)(2)SO solutions: plots of carboxyl carbon C-13-NMR chemical shifts with varying PH. Titration cu rves of diacids 1-3 with known pK(a)'s in H2O served as calibration st andards for determination of pK(a)s 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(a 1) = 4.7, pK(a2) = 5.7 for 6 in H2O. Copyright (C) 1996 Elsevier Scien ce Ltd