Ab initio calculation of the carbon isotope signatures of amino acids

Carbon isotope compositions for the 20 standard amino acids are calculated using high-level molecular orbital and density functional theory methods. In agreement with measurements of the isotopic compositions of amino acids in a cyanobacterium [Macko, S.A., Fogel, M.L., Hare, P.E., Hoering, T.C., 1987. Isotope fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms. Chemical Geology 65, 79-92] the calculations give heavy signatures for aspartic acid + asparagine, serine, glutamine + glutamic acid and light signatures for leucine and isoleucine. The relative intramolecular enrichment of 13C in carboxyl groups is often much less than measured [Abelson, P.H., Hoering, T.C., 1961. Carbon isotope fractionation in formation of amino acids by photosynthetic organisms. Proceedings of the National Academy of Sciences, USA 47, 623-632]. Fractionations predicted from density functional calculations often differ by several per mil from those predicted from molecular orbital calculations.