Background theory

Guidelines for Aquatic Toxicity

This experiment was inspired by a recent article published in the journal Green Chemistry (Voutchkova, et al., 2011). Adelina Voutchkova and Paul Anastas (one of the fathers of green chemistry) worked together to define parameters that correlate with whether or not a molecule would be toxic to aquatic organisms. Typical toxicology studies focus on measuring the effects of exposure of organisms to specific chemicals. This recent work by Voutchkova et al. attempts to correlate physical properties of compounds to determine if they are likely to be toxic to aquatic organisms. Many different chemical and physical properties of over 500 compounds were studied and compared to existing data on toxicity for three species: the fathead minnow, Japanese medaka, and Daphnia magna. One property that correlated with toxicity was the octanol-water partition coefficient, which you explored last week. The other significant property was the HOMO-LUMO gap, ΔE, for the molecules. Molecules with a HOMO-LUMO gap < 9eV tend to be reactive with biological molecules.

Octanol-water Partition Coefficient

Octanol and water do not mix, and the interface between the two solutions is a good model for the lipid bilayers (Baird & Cann, 2012) in cells. If a molecule is more soluble in the octanol or fatty layer, the compound can penetrate into the cellular tissue and bio-accumulate. If the molecule is more soluble in the water layer, the compound is more likely to be excreted as waste before causing damage to the organism. This property is useful to predict the fate of pollutants as well as in designing pharmaceuticals to maximize efficacy.

The octanol-water partition coefficient is an equilibrium constant that describes the relative concentrations of a solute in each layer of an octanol-water mixture.

If a compound has a high K_ow, the molecule is more soluble in the octanol and will tend to be more fat soluble in the body. If a compound has a low K_ow, the molecule is more water soluble.

You may have heard the term fat soluble and water-soluble vitamins before and may even have been cautioned against ingesting high levels of certain vitamins. Vitamin A is an example of a very fat-soluble molecule, and you can actually build up a toxic amount in the body if you are not careful. Vitamin C, on the other hand, is water-soluble. Excess Vitamin C in the body is usually just excreted harmlessly (Table 1).

Because the relative concentrations of solutes in each layer can vary widely over several orders of magnitude, it is often easier to express the partition coefficient as a logarithm of the K_ow.

NOTE: logK_ow is often also called logP, where P is the partition coefficient.

Voutchkova Aquatic Toxicity Guidelines

In the molecules studied by Voutchkova and Anastas, those with a logK_OW above two tended to be toxic to aquatic species. Thus, a chemist synthesizing new molecules can try to target a logK_ow < 2 and ΔE > 9 eV for the safety of aquatic life (Figure 1).

In this experiment module, you will test the Voutchkova model with atrazine (Table 2). Approximately 76.4 million pounds of Atrazine, an herbicide, is applied to crops annually. Atrazine is a contaminant in drinking and surface water with a Maximum Contaminant Level (MCL) of 3 ppb in water. Recently, Atrazine was listed as one of the 12 harmful chemicals found to disrupt cancer-related pathways by the Environmental Working Group (http://www.ewg.org/research/ewgs-dirty-dozen-cancer-prevention-edition?inlist=Y).

This week, you will measure the octanol-water partition coefficient for atrazine. To compute logK_ow from experimental data, you need to be able to determine the concentration of the solute of interest in each layer of solvent. This can be done using spectroscopy and calibration curves just like you did in last week’s experiment.

You will also begin an ecotoxicity assay using a strain of green algae (Pseudokirchneriella subcapitata), which you will analyze next week. We will explore this assay in more detail in the next experiment.