Analysis of OPs of degradation of MC-LR was carried out by high resolution liquid chromatography–mass spectrometry/mass spectrometry technique (Jiang et al. 2014). The proposed structures for the OPs were based on the molecular formula and are summarized in Table 2. Basically, four primary reactions occurred from the attacks of Fe(VI) on the aromatic ring, diene, enone, and amide functionalities of MC-LR by Fe(VI) (Figure 2). In hydroxylation of the aromatic ring, mono, di and trihydroxylation of the aromatic ring were obtained with corresponding m/z = 1011.5510, 1027.5459, and 1043.5408 (Table 2). Monohydroxylation involved the loss of a hydrogen atom to yield a highly stabilized aromatic product (M + 16). Further hydroxylation thus formed di- and tri- hydroxylation products. Hydroxylation of the carbon–carbon double bond in the MHDA moiety also occurred, which resulted in the formation of an enol functional group through the elimination of an H atom. Tautomerization of the enol group gave a chiral center at the alpha position. A pair of diastereotopic isomers consistent with m/z 1011.5510 (M + 16) were thus obtained (Table 2). Fe(VI) also oxidized the diene group of the Adda moiety of MC-LR via dihydroxylation to yield products with M + 34 (Table 2). This corresponded to addition of two HO groups without loss of H atoms. Hydroxylation yielded 1,2-, 3,4-, and 1,4-diol products (Table 2). Significantly, the products seen from the attack on diene moiety were also reported in oxidation performed by photocatalytic and electrochemical process (Antoniou et al. 2008; Zhang et al. 2013; Zong et al. 2013; Liao et al. 2014). The cleavage of peptide bonds in MC-LR by Fe(VI) was also seen, which caused the hydrolysis of amide bonds of D-glu-MDHA and the l-Arg-Methyl d-Asp of the MC-LR by Fe(VI) (Jiang et al. 2014). This step of the attack of Fe(VI) on the peptide bond was similar to the transformation of -NH = C- amino acid functionality by Fe(V) (Bielski et al. 1994; Rush & Bielski 1995).
OPs observed during hydroxylation of moieties of MC-LR by Fe(VI)