270
Ecotoxicological Issues
aromatics and phenols are likely to be phototoxic only if the parent aromatic
structure is phototoxic. It is also likely that the principal influence of these
substituents on toxicity will be through a change in bioaccumulation of the
chemicals. Nitro, alkene and chloro substituents have a more pronounced effect on
HOMO-LUMO gap. The p-1t and 1t-1t interactions permit hyperconjugation
between the electronic structures and aromatic rings which causes a decrease or
"red-shift" to gap energies. The maximum reduction of HOMO-LUMO gap for
monosubstituted derivatives was approximately 0.42 eV, and that for disubstituted
derivatives was approximately 0.74 eV. While the magnitude of substituent effects
is not large, derivatives of parent aromatic chemicals which have a gap energy
between 7.S and 8.0 eV could be found to be phototoxic after substitution by
nitro, alkene and chloro substituents.
5 CONCLUSION
Photo-induced toxicity of PAHs was found to be a result of competing internal
(structural) and external (light characteristics) factors. The molecular descriptor of
the energy difference between the highest occupied molecular orbital and the
lowest unoccupied molecular orbital (HOMO-LUMO gap) discriminated between
chemicals which either were or were not phototoxic to Daphnia magna in the
presence of UV radiation. A bilinear (bell-shaped) curve described the
relationship, and QSAR models that predict toxicity were developed for both the
ascending and descending components of this curve. The same type of
relationship between HOMO-LUMO gap and toxicity was observed with PAHs in
both the ground and triplet excited states.
Most phototoxic PAHs have HOMO-LUMO gap energies of 7.2 ± 0.4 eV. The
additions of alkane or hydroxy substituents to parent PAHs had negligible effects
upon phototoxicity. Phototoxicity in such cases is likely only if the parent
compounds are already phototoxic. However, substitutions of nitro, alkene and
chloro substituents can reduce the HOMO-LUMO gaps of parent PAHs. This
could result in a non-phototoxic compound becoming phototoxic with the
addition of these substituents.
KEYWORDS
Toxicity, PAH toxicity, global change, solar photo-induced toxicity, hydrocarbons,
chemistry alkene, orbital energy, polycyclic aromatic hydrocarbon
REFERENCES
[1]
Veith, G.D., Call, DJ. and Brooke, L.T. "Structure-toxicity relationships for the fathead
minnow", Pimephales promelas: Narcotic industrial chemicals, Can. J. Fish. Aqua. Sci. 40,
743-748 (1983).
[2]
Thakker, D.R., Yagi, H., Levin, W., Wood, A.W., Conney, A.H., and Jerina, D.M.
"Polycyclic aromatic hydrocarbons: metabolic activation to ultimate carcinogens", In,
Bioactivation of Foreign Compounds, M.W. Anders, Ed., Academic Press, NY pp. 177-242
(1985).
Ecotoxicological Issues
aromatics and phenols are likely to be phototoxic only if the parent aromatic
structure is phototoxic. It is also likely that the principal influence of these
substituents on toxicity will be through a change in bioaccumulation of the
chemicals. Nitro, alkene and chloro substituents have a more pronounced effect on
HOMO-LUMO gap. The p-1t and 1t-1t interactions permit hyperconjugation
between the electronic structures and aromatic rings which causes a decrease or
"red-shift" to gap energies. The maximum reduction of HOMO-LUMO gap for
monosubstituted derivatives was approximately 0.42 eV, and that for disubstituted
derivatives was approximately 0.74 eV. While the magnitude of substituent effects
is not large, derivatives of parent aromatic chemicals which have a gap energy
between 7.S and 8.0 eV could be found to be phototoxic after substitution by
nitro, alkene and chloro substituents.
5 CONCLUSION
Photo-induced toxicity of PAHs was found to be a result of competing internal
(structural) and external (light characteristics) factors. The molecular descriptor of
the energy difference between the highest occupied molecular orbital and the
lowest unoccupied molecular orbital (HOMO-LUMO gap) discriminated between
chemicals which either were or were not phototoxic to Daphnia magna in the
presence of UV radiation. A bilinear (bell-shaped) curve described the
relationship, and QSAR models that predict toxicity were developed for both the
ascending and descending components of this curve. The same type of
relationship between HOMO-LUMO gap and toxicity was observed with PAHs in
both the ground and triplet excited states.
Most phototoxic PAHs have HOMO-LUMO gap energies of 7.2 ± 0.4 eV. The
additions of alkane or hydroxy substituents to parent PAHs had negligible effects
upon phototoxicity. Phototoxicity in such cases is likely only if the parent
compounds are already phototoxic. However, substitutions of nitro, alkene and
chloro substituents can reduce the HOMO-LUMO gaps of parent PAHs. This
could result in a non-phototoxic compound becoming phototoxic with the
addition of these substituents.
KEYWORDS
Toxicity, PAH toxicity, global change, solar photo-induced toxicity, hydrocarbons,
chemistry alkene, orbital energy, polycyclic aromatic hydrocarbon
REFERENCES
[1]
Veith, G.D., Call, DJ. and Brooke, L.T. "Structure-toxicity relationships for the fathead
minnow", Pimephales promelas: Narcotic industrial chemicals, Can. J. Fish. Aqua. Sci. 40,
743-748 (1983).
[2]
Thakker, D.R., Yagi, H., Levin, W., Wood, A.W., Conney, A.H., and Jerina, D.M.
"Polycyclic aromatic hydrocarbons: metabolic activation to ultimate carcinogens", In,
Bioactivation of Foreign Compounds, M.W. Anders, Ed., Academic Press, NY pp. 177-242
(1985).