Carbon isotopic compositions of carboxyl groups of biosynthesized fatty acids

The carbon isotopic compositions (±0.001% ¹³C) of carboxyl groups in fatty acids were determined by measuring the isotopic composition of the carbon dioxide released quantitatively from the acids by employing a modified Schmidt decarboxylation. The adequacy of the decarboxylation technique was tested by comparing the predicted value of the isotopic compositions of the carboxyl carbons in isotopic standard fatty acids with those experimentally obtained by means of the Schmidt reaction. Results showed that the accuracy and precision of the method were approximately 0.0008 and 0.0002% carbon-13. No significant isotope fractionation was observed when test acids were carried through sample isolation and purification procedures. The isotopic compositions of the carboxyl carbons of hexadecanoic acid (C16), 9-octadecenoic acid (C18:1), and 9,12-octadecadienoic acid (C18:2) extracted from corn and soybeans, as well as tetradecanoic acid (C14) from nutmeg, revealed that the carboxyl carbons of the fatty acids were enriched in carbon-13 in the order C18:2 > C18:1 > C16 ≈ C14. The carboxyl carbon of C18:2 from both corn and soybeans and that of C18:1 from corn were observed to be enriched in carbon-13 relative to the parent molecule as a whole. Carboxyl carbons in fatty acids derived from a mixed microbial source did not display the same isotopic heterogeneity within carbon-number homologs. These results demonstrate that the isotopic composition of the carboxyl group in fatty acids does not represent accurately the isotopic composition of the carbonyl carbon in the precursor acetyl-CoA.