Mass isotopomer pattern and precursor‐product relationship

The synthesis of a homonucleus polymer from its labeled precursor will lead to the formation of molecules with different masses. The distribution of these mass isotopomers is strictly a function of the enrichment of the ¹³C‐labeled precursor, and can thus be used for the determination of the precursor enrichment and product dilution in the de novo synthesis of the polymer. We present here a study of the isotopomer pattern of a polymer of acetate in the form of glucose pentaacetate synthesized from ¹³C‐enriched acetic anhydride. The molecular ion contains four acetyl units. Its synthesis is analogous to that of octanoic acid from acetyl coenzyme A. The process of obtaining the mass isotopomer distribution in the tetraacetyl moiety from the ion cluster of m/z 331 of glucose pentaacetate is illustrated. After correcting for the contribution of ¹³C natural abundance, the plot of the ratio of mass isotopomers (m4/m2) against the observed enrichment of the tetraacetate moiety yielded a straight line with a slope of 1.45. The ratio was not altered by dilution with pre‐existing unenriched product, as predicted. The slope of the observed linear relationship agreed with the general formula (N ‐ (j ‐ 1))/j for the ratio of any two consecutive mass isotopomers (m_j/m_j‐1). Theoretical and practical aspects of determining precursor enrichment from isotopomer pattern in polymers are discussed.