Chlorine isotope effects and composition of naturally produced organochlorines from chloroperoxidases, flavin-dependent halogenases, and in forest soil

Abstract

The use of stable chlorine isotopic signatures (δ37Cl) of organochlorine compounds has been suggested as a tool to determine both their origins and transformations in the environment. Here we investigated the δ37Cl fractionation of two important pathways for enzymatic natural halogenation: chlorination by chloroperoxidase (CPO) and flavin-dependent halogenases (FDH). Phenolic products of CPO were highly 37Cl depleted (δ37Cl = -12.6 ± 0.9‰); significantly more depleted than all known industrially produced organochlorine compounds (δ37Cl = -7 to +6‰). In contrast, four FDH products did not exhibit any observable isotopic shifts (δ37Cl = -0.3 ± 0.6‰). We attributed the different isotopic effect to the distinctly different chlorination mechanisms employed by the two enzymes. Furthermore, the δ37Cl in bulk organochlorines extracted from boreal forest soils were only slightly depleted in 37Cl relative to inorganic Cl. In contrast to previous suggestions that CPO plays a key role in production of soil organochlorines, this observation points to the additional involvement of either other chlorination pathways, or that dechlorination of naturally produced organochlorines can neutralize δ37Cl shifts caused by CPO chlorination. Overall, this study demonstrates that chlorine isotopic signatures are highly useful to understand sources and cycling of organochlorines in nature. Furthermore, this study presents δ 37Cl values of FDH products as well of bulk organochlorines extracted from pristine forest soil for the first time. © 2013 American Chemical Society.