Mechanism-based quantitative structure-biodegradability relationships for hydrolytic dehalogenation of chloro- and bromo-alkenes

Quantitative Structure-Biodegradability Relationships (QSBRs) have been developed for microbial hydrolytic dehalogenation of small chlorinated and brominated alkenes. Two different reaction mechanisms, S(N)2 and S(N)2', were considered for the dehalogenation of small haloalkenes by hydrolytic dehalogenases. Multivariate QSBRs based on quantum-chemically calculated descriptors were used to distinguish between these dehalogenation mechanisms. A QSBR based on atom valence and partial atomic charge on electrophilic site, together with the energy of the lowest unoccupied molecular orbital, molecular weight and dipole moment were able to explain 92% (81% cross-validated) of the quantitative variance in the dehalogenation rates. This QSBR supported a S(N)2 reaction mechanism. This reaction mechanism was also confirmed by GC-MS identification of the reaction products for the compounds which should give discriminating reaction products depending on mechanism. The substrate specificity of hydrolytic dehalogenases for unsaturated halogenated substrates is discussed in the light of chemical structure and properties of the substrate molecules.