Effect of molecule size on carbon isotope fractionation during biodegradation of chlorinated alkanes by Xanthobacter autotrophicus GJ10

The effect of the number of carbon and chlorine atoms on carbon isotope fractionation during dechlorination of chlorinated alkanes by Xanthobacter autotrophicus GJ10 was studied using pure culture and cell-free extract experiments. The magnitude of carbon isotope fractionation decreased with increasing carbon number. The decrease can be explained by an increasing probability that the heavy isotope is located at a non-reacting position for increasing molecule size. The isotope data were corrected for the number of carbons as well as the number of reactive sites to obtain reacting-site-specific values denoted as apparent kinetic isotope effect (AKIE). Even after the correction, the obtained AKIE values varied (on average 1.0608, 1.0477, 1.0616, and 1.0555 for 1,2-dichloroethane, chloropentane, 1,3-dichloropentane and chlorobutane, respectively). Cell-free extract experiments were carried out to evaluate the effect of transport across the cell membrane on the observed variability in the AKIE values, which revealed that variability still persisted. The study demonstrates that even after differences related to the carbon number and structure of the molecule are taken into account, there still remain differences in AKIE values even for compounds that are degraded by the same pure culture and an identical reaction mechanism.