3.3  Constituents of the Tissues: Which Are the Biological and Biochemical Targets?

Without action

5

Irritant

4

Irritant Corrosive

0

Fig. 3.46  pK, normality, and corrosivity

Acid1 + Base2 Base1 + Acid2

Fig. 3.47  Acido–basic solution

u1 Ox1 + u2 Red2 u1 Red1 + u2 Ox2

Fig. 3.48  Reaction for oxidizing and reducing agents

E° (V)

Oxidant

0.34

Cu2+ / Cu

0.76

Zn2+ / Zn

Reductant

Fig. 3.49  Copper sulfate solution

R O H RO- + H+

H+ + Base- BH

Fig. 3.50  Acidic potential of an alcohol

•For chelating agents, we will use the complexation constant.

•For addition and substitution, it will be the electrophilic/nucleophilic character.

•For solvents, finally, it will be a combination of physical parameters such as the partition coefficient coupled with the potential of chemical reactivity. For example, it is easy to understand that, for alcohols, this chemical potential may be relative to a certain degree of acidity due to the possible break of the connection between the hydrogen atom and the oxygen atom, when the molecule is in a basic environment (Fig. 3.50).

For solvents, the value of this constant is expressed in logarithmic unit for the same practical reasons as for the acid–base scale:

log P = log Ko/w

It is the differential measurement of the solubility of a product in two solvents (with O figuring octanol and W figuring water). When log P is very high, the chemical compound under study reveals much more soluble in octanol than in water. Therefore, the chemical compound has a much more lipophilic character. Conversely, when log P is low, the chemical substance is more hydrophilic. When log P equals zero, the chemical is equally split in two solvent phases.

Therefore, the partition coefficient (log P) enables to foresee with figures the behavior of a given substance (the solute) in a hydrophilic or lipophilic solvent environment.

A further step can be reached by linking log P with pH and pKa: log D is a measure of log P for a certain pH value with a product having a pK value.

log D = log P −[1+ 10(pH−pKa ) ].

3.3  Constituents of the Tissues:

Which Are the Biological

and Biochemical Targets?

The reaction develops from left to right when E°1 > E°2. A good illustration of this rule is a blade of zinc covered with lead, disaggregated and oxidized in a copper sulfate solution (Fig. 3.49).

As an organ, the eye is constituted by various structures each formed by a specifically differentiated cellular tissue: the transparent cornea, the aqueous humor

of the anterior chamber, the lamellar lens, etc. Each of these structures is constituted by a large number of cells. Every cell presents substructures concentrating one very large number of assemblies of more or less complex molecules.

The histological description is to be studied in Chap. 4 and anatomy in Chap. 7.

But, for us, everything is naturally molecular chemistry in an ultrastructural scale. This mechanistic and materialistic approach might surprise any doctor? Nevertheless not only this molecular organization of the living substance is real but it even helps to understand the intimacy of the eye chemical danger and the ultimate consequences of chemical burns.

The living substance is constituted by six major atomic elements entering the architecture of the biochemical molecules. They are carbon, which defines the organic chemistry (that is the chemistry of what’s alive), hydrogen, oxygen, nitrogen, phosphorus, and sulfur. It is necessary to add to these five other elements of mainly ionic nature included in trace elements: Na (Na+), Mg (Mg++), K (K+), Ca (Ca++), Cl (Cl−). These ions are essential to keep the balance of aqueous biological environments whether they are in intraor extracellular environments.

The three kinds of biological substrates are results of the combination of these elements:

•Lipids are mostly more or less long hydrocarbon chain molecules, constituting particularly the cell membranes and the mitochondria membrane (Figs. 3.51 and 3.52).

•Glucides or carbohydrates combine carbon, hydrogen, and oxygen (carbohydrates) inside isolated molecular structures, gathered in macromolecules (glycogen) or connected to proteins (glycoproteins). Figure 3.53 illustrates the example of a sugar: glucose.

•Proteins may have a gigantic size and are constituted of the chaining of tens to several hundred various amino acids. They contribute to the support of the cell structural architecture, to the transport of numerous compounds by blood and most of all they have enzymatic properties of catalysis of biochemical reactions (Fig. 3.54).

As studied above, chemical burns are always the result of an elementary chemical reaction during the contact of two entities, one “the aggressor,” the other

Fig. 3.51  Phospholipid chemical structure

one “the assaulted,” in chemistry, the reagent and the reactant.

We understand easily that the corrosive chemical agents can give very quickly a fatal blow to the cells constituing organic tissues of organs, in different ways:

•Damaging cytoplasmic membranes

•Disintegrating the three-dimensional configuration of proteins, or by coagulating them

•Even by chelating trace elements indispensable to the cellular functioning (as the calcium or the magnesium)

The immediate or secondary necrosis (by more progressive diffusion into depth or by concomitant toxic effect) explain the macroscopic lesions of the tissues of the various eye structures (eyelids, conjunctiva, cornea, even iris or the ocular lens) that are specific to the chemical burn.