160
2
The Behaviour of Metals as Related to
Some Organic Chemical Groups
A.C.A. da Costa
To better understand the interaction of heavy metals with biological molecules, it
is interesting to mention something about the chemistry of these elements
regarding their reaction with some specific ligands.
Pearson (1966), studying chemical reactions with heavy metals, classified these
elements as hard and soft Lewis acids, depending on the strength of their
reactiviness with certain types of ligands. Prom that study, it was possible to
estimate the affinity of heavy metal elements for different biomolecules based on
these results. Remacle (1990) performed a summarized review of Pearson's
observations defining a hardness scale based on metals' binding strength with
different ligands.
Remacle (1990) described the essential metals Na+, MgH and Ca H as hard
acids, both interacting with P- through strong binding forces; on the other hand,
the toxic HgH, Cd H and Pb H interacted with that anion through weak binding
forces.
These observations can be closely correlated to the interaction of essential and
toxic elements with biomolecules; of course, the interaction of essential metals
with Lewis bases such as OH-, HP0 4 =, CO F ' -COO- and =C=O is of a stronger
nature than that with CN-, -S-, -SH- and NH2 _. One has only to review the basic
composition of macromolecules in biological structures to see that its nature
involves the presence of carbon- and oxygen-containing molecules, mainly. On
the other hand, these elements would not so effectively interact with sulphur and
nitrogen compounds, this interaction being, however, feasible for toxic heavy
metals elements. In this context, it was Pearson's and Remacle's purpose to
present an approach of the most energetically feasible reactions occurring
between different metals.
In general, hard acids (essential metals, usually) prefer to associate with hard
basis (oxygen compounds interacting through ionic bonds), and soft acids (toxic
metals, usually) prefer to associate with soft basis (nitrogen and sulphur
compounds interacting through covalent bonds). This does not necessarily mean
that hard acids could not interact with soft basis and vice versa; if so, the toxic
effect of heavy metals would not be observed. The primary mode of action of
toxic elements considers the non-specific interaction of heavy metals substituting
essential metals in biological molecules, and this interaction is usually driven
through a rule different than that established by Pearson. As previously stated, if
this kind of interaction were not made possible, the primary toxic mode of action
of heavy metals would not occur.
2
The Behaviour of Metals as Related to
Some Organic Chemical Groups
A.C.A. da Costa
To better understand the interaction of heavy metals with biological molecules, it
is interesting to mention something about the chemistry of these elements
regarding their reaction with some specific ligands.
Pearson (1966), studying chemical reactions with heavy metals, classified these
elements as hard and soft Lewis acids, depending on the strength of their
reactiviness with certain types of ligands. Prom that study, it was possible to
estimate the affinity of heavy metal elements for different biomolecules based on
these results. Remacle (1990) performed a summarized review of Pearson's
observations defining a hardness scale based on metals' binding strength with
different ligands.
Remacle (1990) described the essential metals Na+, MgH and Ca H as hard
acids, both interacting with P- through strong binding forces; on the other hand,
the toxic HgH, Cd H and Pb H interacted with that anion through weak binding
forces.
These observations can be closely correlated to the interaction of essential and
toxic elements with biomolecules; of course, the interaction of essential metals
with Lewis bases such as OH-, HP0 4 =, CO F ' -COO- and =C=O is of a stronger
nature than that with CN-, -S-, -SH- and NH2 _. One has only to review the basic
composition of macromolecules in biological structures to see that its nature
involves the presence of carbon- and oxygen-containing molecules, mainly. On
the other hand, these elements would not so effectively interact with sulphur and
nitrogen compounds, this interaction being, however, feasible for toxic heavy
metals elements. In this context, it was Pearson's and Remacle's purpose to
present an approach of the most energetically feasible reactions occurring
between different metals.
In general, hard acids (essential metals, usually) prefer to associate with hard
basis (oxygen compounds interacting through ionic bonds), and soft acids (toxic
metals, usually) prefer to associate with soft basis (nitrogen and sulphur
compounds interacting through covalent bonds). This does not necessarily mean
that hard acids could not interact with soft basis and vice versa; if so, the toxic
effect of heavy metals would not be observed. The primary mode of action of
toxic elements considers the non-specific interaction of heavy metals substituting
essential metals in biological molecules, and this interaction is usually driven
through a rule different than that established by Pearson. As previously stated, if
this kind of interaction were not made possible, the primary toxic mode of action
of heavy metals would not occur.