Relationship of azo dye toxicity and structure
The structures of the azo dyes C.I. Food Yellow
and C.I. Acid Orange 7 showing their constituent
aromatic amines are illustrated in Fig. 1. Both
compounds generate sulphanilic acid following
azo bond reduction (decolourisation), but different
amino-naphthols. No decolourisation was recorded
in incubation periods up to 2 h if glucose
was omitted from the assay. Hence glucose was
required in the decolourisation assays as a source
of reducing equivalents (Sweeney et al., 1994). The
toxicity of Acid Orange 7 and Food Yellow were
similar before reduction (Table 1). However, after
azo bond reduction by E. faecalis (10 min incubation)
the toxicity of Food Yellow slightly decreased
(EC50 22.19/2.47 and 32.59/1.53 mg l1)
but the toxicity of Acid Orange 7 increased nearly
100-fold (EC50 15.79/2.68 and 0.29/0.0 mg l1,
respectively). Standard 1-amino-2-naphthol was
very toxic (EC50 0.19/0.03 mg l1) compared
with its sulphonated analogue (1-amino-2-
naphthol-6-sulphonate, EC50 3919/21.92 mg
l1). The toxicity of sulphanilic acid was equivalent
to that of the unreduced dyes. Hence the
increased toxicity of Acid Orange 7 after reduction
was probably due to the liberation of 1-amino-2-
naphthol. The lack of a sulphonate group probably
increases its lipid solubility and hence uptake
by the V. fischeri tester strain. The toxicity of
naphthol compounds varied according to the type
and position of their substitution groups (Table 1).
For example naphthalene sulphonic acid was less
toxic than when the sulphonic group occurs in the