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ioaccumulative than the parent molecules. This is partly borne out by the predictive
methods, where (Table 1) it can be seen that certainly the bioaccumulative behaviour is
significantly reduced by 2 to 3 orders of magnitude when compared to the parent
molecule.
2.3 Naphthenics (Cyclo-alkanes)
Cyclic alkanes are relatively resistant to microbial attack, as the absence of an exposed
terminal methyl group complicates the primary attack (Fritsche and Hofrichter, 2000).
The mechanism of cyclohexane degradation is again generally via the alcohol and in
general, alkyl side chains of cyclo-alkanes facilitate degradation. The pattern noted for
the toxicity or biodegradation is variable, and led to more toxic and/or persistent
metabolites. However, without exception, the predicted effect of these changes is to lead
to significant reductions in the bioaccumulative behaviour of the metabolites, of the order
of one to over three orders of magnitude.
Given the properties of certain naphthenic type structures, it was decided to further
investigate the naphthenics in more detail. The results of this are shown in Table 1,
where many more structures were addressed, but instead only at one carbon number, i.e.
C15. Exactly the same result has been obtained in these predictions, with variable
changes being observed to the toxicity and/or persistency of the metabolites, but a
consistent one to three orders of magnitude decline in the bioaccumulative behaviour
when compared with the parent molecules.
It should be concluded therefore that there is no reason to believe that any metabolites of
these structures is likely to be B or vB, and hence they will not be either PBT or vPvB.
2.4 Aromatic compounds
Given that a large number of aromatic compounds are formed by organisms, e.g., as
aromatic amino acids, phenols, or quinines, it is not surprising that many microorganisms
have evolved catabolic pathways to degrade them. It can be assumed that in general
petrochemical molecules can be degraded by microorganisms, when the respective
molecules are similar to other natural compounds and are converted enzymatically to
natural intermediates of the degradation: catechol and protecatechuate (Fritsche and
Hofrichter, 2000). While the introduction of aliphatic substituents will alter the point of
attack, it should not be anticipated that the products of that attack will be significantly
different.
Again the pattern noted for the toxicity or biodegradation is variable, and does in some
cases lead to more toxic and/or persistent metabolites. However, without exception, the
predicted effect of these changes is to lead to reductions in the bioaccumulative
behaviour of the metabolites by two to three orders of magnitude (Table 1).
2.4.1 Polyaromatic compounds
The biochemical pathway for the aerobic biodegradation of PAHs has been extensively
investigated. It is understood that the initial step in the aerobic catabolism of a PAH
molecule by bacteria uses a multicomponent enzyme system to oxidise the PAH to a
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