Biotic and Abiotic Transformation Pathways of Synthetic Musks in the Aquatic Environment

Abstract

Synthetic musks are being transformed to intermediates or relatively stable metabolites that may exhibit a strong potential for environmental harm. For example, microbial metabolisation of nitro musks in aquatic ecosystems preferentially gives rise to the formation of the amino derivatives, and, as a consequence, benzene-amine derivatives were found in environmental samples in higher concentrations than their parent compounds. Transformation of musk xylene (MX) chiefly results in 1-amino-4-tert-butyl-2,6-dimethyl-3,5-dinitrobenzene (4-amino-MX) and a minor product, 1-amino-2-tert-butyl-4,6-dimethyl-3,5-dinitrobenzene (2-amino-MX). Musk ketone (MK) is transformed by microbial processes to one main transformation product only, 1-(3-amino-4-tert-butyl-2,6-dimethyl-5-nitrophenyl)ethanone (2-amino-MK). In higher organisms, such as male Wistar rats the main metabolic pathway included, as a first step, reduction of the 4-nitro group of MX to the amino group, followed by acetylation of the resulting amino function and oxidation of one of the methyl groups. Alternatively, oxidation of the tert-butyl group appeared to be possible. The reduction of the 2-nitro group to the amino function was assumed to be less probable due to increased sterical hindrances induced by the adjacent tert-butyl group. Detailed investigations on the mechanism and kinetics of enzymatic MX reduction in mice and humans revealed that induction and inhibition of cytochrome P-450 2B (CYP2B) enzymes as well as the binding of 4-amino-MX to haemoglobin in blood appears to play an important role. Additional polar transformation products of the nitro musks were discussed in the literature for higher organisms, which are capable of both N- and para-hydroxylation of aniline derivatives. Recently, transformation products of polycyclic musks were also identified in considerable concentrations in environmental samples. The structure of the most abundant metabolite of HHCB was identified as a lactone (HHCB-lactone). Potential structures of photochemical transformation products of the polycyclic musks HHCB and AHTN are presently under investigation. Furthermore, in this chapter, analytical methods for the determination of the metabolites together with the parent compounds of nitro and polycyclic musks as well as environmental levels and the ecotoxicological potential of the musk metabolites are discussed.