Equilibrium D/H Fractionation of Organic H

Interpretations of D/H ratios in ancient organic molecules must generally address the possibility of isotopic exchange with ambient water. A common observation used to argue against exchange is a lack of systematic shifts in δD values with age or thermal maturity. This assumes that the organic molecules are far from isotopic equilibrium with water, an assertion that remains untested. The goal of our study is to experimentally measure the relevant equilibrium fractionation factors (αeq) in model organic compounds. To facilitate experimental measurements, we exploit the rapid tautomerism of ketones under basic or acidic catalysis that leads to isotopic equilibration of hydrogen adjacent to the carbonyl (α-H) on timescales of days. Compounds tested so far include cyclohexanone, 4-heptanone, and 2, 4-dimethyl-3-pentanone. Substrates are incubated with four waters of varying δD values, and exchange is quantified by measuring organic δD values by GC-IRMS at discrete time intervals. Isotopic equilibrium is approached from both directions and confirmed via attainment of constant substrate δD values. The average fractionation for α-H in cyclohexanone over the temperature range 25-70°C is -182 ± 7‰ (n = 14) relative to water. The magnitude of fractionation appears to increase slightly with rising temperature, though the effect may not be statistically significant. Fractionation of α-H in 2, 4- dimethyl-3-pentanone is -59 ± 9‰ and in 4-heptanone is -197 ± 9‰. The uncertainty of fractionation is reduced by about one order of magnitude compared to previous estimates based on spectroscopic analyses. α-H on tertiary carbon atoms exchanges ~10 times slower than those on secondary carbon atoms, which is inconsistent with the trend of enolate ion stability but probably related to the spatial configuration of the molecule. The measured fractionations are similar to the range of δD values (-150 to -200‰) observed in n- alkyl lipids of living organisms. We suggest that the biosynthetic fractionation of hydrogen in n-alkyl lipids is probably affected by the rapid equilibration of ketonic α-H in the acetogenic pathway. Moreover, constant δD values of n-alkyl lipids over time cannot be used as evidence for a lack of exchange with environmental water.