Half-life of chlorpyrifos oxon and other organophosphorus esters in aqueous solution

Abstract

Aqueous solutions of chlorpyrifos oxon are used to study the ability of chlorpyrifos oxon to catalyze protein crosslinking. Assays for protein crosslinking can avoid artifacts by using information on the stability of chlorpyrifos oxon in solution. We undertook to determine the half-life of chlorpyrifos oxon in aqueous solution because literature values do not exist. The rate of conversion of chlorpyrifos oxon to 3,5,6-trichloro-2-pyridinol was measured at 23 °C in 20 mM TrisCl pH 8 and pH 9 by recording loss of absorbance at 290 nm for chlorpyrifos oxon and increase in absorbance at 320 nm for 3,5,6-trichloro-2-pyridinol. The half-life of chlorpyrifos oxon was 20.9 days at pH 8 and 6.7 days at pH 9. Literature reports for the stability of other organophosphorus toxicants were summarized because our current studies suggest that other organophosphorus toxicants are also crosslinking agents.

Keywords: Absorbance spectra; Chlorpyrifos oxon; Extinction coefficient; Half-life; Stability.

Figure 1.

Absorbance spectra of 20 μM chlorpyrifos oxon (CPO) and 20 μM 3,5,6-trichloro-2-pyridinol (TCP) in 20 mM TrisCl pH 8.0 at 23°C, measured in a single-beam Gilford spectrophotometer in 4 ml quartz cuvettes. The symbols indicate the wavelengths at which absorbance was measured manually. Curves were drawn through the points with Excel software. Diamond symbols are absorbance values for 20 μM chlorpyrifos oxon. Squares are absorbance values for 20 μM 3,5,6-trichloro-2-pyridinol.

Figure 2.

Hydrolysis of 28.8 μM chlorpyrifos oxon (CPO) to 3,5,6-trichloro-2-pyridinol (TCP) by 10 mM sodium hydroxide at 23°C. Arrows indicate disappearance of CPO at 290 nm and appearance of TCP at 320 nm. Conversion to TCP was complete in 103 min. The half-life of CPO at alkaline pH was 14 min. The extinction coefficient for TCP at 320 nm pH 11.8 was 7100 M⁻¹ cm⁻¹.

Figure 3.

Rate of decay of chlorpyrifos oxon to 3,5,6-trichloro-2-pyridinol (TCP) at 23°C. Chlorpyrifos oxon (20 μM) in 100 mL of 20 mM TrisCl, 0.01% sodium azide pH 8.0 and pH 9.0 was stored in the dark. Spontaneous decay of chlorpyrifos oxon to TCP was monitored by the increase in absorbance at 320 nm. The half-life for chlorpyrifos oxon at pH 8.0 was 20.9 days, and at pH 9.0 was 6.7 days. Duplicate absorbance readings were essentially identical for each point.

Figure 4.

Rate of decay of chlorpyrifos oxon to 3,5,6-trichloro-2-pyridinol at pH 11.8 and 23°C. Absorbance change at 320 nm as a function of time in 10 mM sodium hydroxide was from Figure 2. The half-life of chlorpyrifos oxon was 13.9 min at pH 11.8.

Figure 5.

Chlorpyrifos (Panel A), parathion ethyl, and paraoxon ethyl (Panel B) are stable at acid to neutral pH (4 to 7) for days, but decay rapidly at alkaline pH (Gomaa and Faust, 1972; Freed et al., 1979; Chapman and Cole, 1982; Macalady and Wolfe, 1983; Solomon et al., 2014).

Figure 6.

Diazinon and diazoxon (Panel A), sarin and soman (Panel B) are unstable at pH 3 and pH 10. Sarin and soman are most stable at pH 4-6 where sarin has a half-life of 461 hours, and soman 144 hours. Note the half-life data for sarin and soman are in hours in the figure, but in days in Table 1. Diazinon and diazoxon are most stable at pH 7-8. Figure 6A was constructed from information in (Gomaa et al., 1969). Figure 6B was constructed from information in (Epstein, 1974) and (Clark, 1989).