William E. White
QSAR of Phosphorus Fluoridates: Toxicity vs Reactivity
Research & Technology Directorate Edgewood Research, Development, & Engineering Center Aberdeen Proving Ground, MD 21010, USA. E-mail: wewhite@CBDCOM-EMH1.APGEA.ARMY.MIL
The toxicity of nerve agents and organophosphorus pesticides results from inhibition of acetylcohlinesterase. Compounds, which are too reactive (i.e. have a low energy of activation for nucleophilic substitution), hydrolyze prior to reaching the enzyme and therefore possess low toxicity. Compounds, which are extremely stable, do not react in a timely manner and fail to inhibit the enzyme If a reliable and simple method could be developed for determining the relative reactivity of cholinesterase inhibitors, the procedure could be used to explain the toxicity of perplexing compounds and predict the toxicity of unknown structures. The technique must involve calculations on some unstable or metastable structure because chemical reactivity cannot be explained sole by examining the reactants.
During nucleophilic substitution of organophosphorus compounds, the reactants pass through a transition structure and form a pentavalent intermediate that subsequently dissociates into the products after passing through a second transition. The hypothesis of this project is that the stability (i.e. free energy) of the metastable intermediate is indicative of the energy of activation and could be used as a measure of reactivity. The free energy in aqueous solution of the intermediate for the hydrolysis reaction was determined by quantum calculations at the semiempirical level using the PM3 Hamiltonian. Because the total free energy depends on the composition of the molecule, the relative free energy was determined by subtracting the energy of the intermediate from that of the substrate itself. The free energy of the hydroxyl was not included because the hydroxyl was used in all reactions and therefore would not change the relative positions of the compounds.
A wide range of G type nerve agents (i.e. O=P-F moiety) were studied containing a variety of carbon, nitrogen, and oxygen substituents attached directly to the phosphorus. As expected, phosphinates were more reactive than phosphonates, which were more reactive than phosphates. In general, amino substituents increased the reactivity relative to carbon analogs; however, there were examples of the opposite effect. Electron withdrawing groups on the alkyl substituents decreased reactivity.
A plot of toxicity (iv, mouse) vs. relative reactivity produced a parabolic curve with GD, GB, GF having median reactivity and high toxicity. Phosphates and phosphinates were at the extremes of the reactivity scale and had lower toxicity. Among the compounds examined, the size of the substituents had little relation to reactivity or toxicity.
The parabolic shape of the curve implies that toxicity resulting from cholinesterase inhibition does not increase indefinitely. There is an optimal reactivity because of the competition between hydrolysis and enzyme inactivation. For cholinesterase inhibition, reactivity of the substrate is the important determinant of toxicity as long as minimal structural requirements are met.
The shape of the graph has more general implications for biochemistry. Enzymes reduce the energy of activation by a specific amount. Regardless of the substrate binding characteristics, the reaction will not occur if the energy of activation for a particular compound exceeds the catalytic capacity of the enzyme.
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