Effect Of Cyclodextrins On Environmental Behavior Of Selected Chiral Pesticides

Fenoxaprop-ethyl (FE) and malathion are two important chiral pesticides. Effect of other substances on environmental behavior and enantioselectivity of FE and malathion was not involved in most of researches. Cyclodextrins (CDs) are a kind of very important remediation agents for environmental contamination. When CDs were applied in environmental remediation, their remedial effects were related with types of CDs and contaminants character. CDs have also chiral recognition ability, so they might have an anisomerous impact on enantioselectivity of chiral contaminants. There are few published papers about the effect of CDs on environmental behavior of FE and malathion at present, especially, the effect of CDs on enantioselective toxicity of FE and malathion. This work evaluated the effect of CDs on achiral and chiral environmental behavior of FE and malathion, the detailed information is as follows:(1) The complexation of Fenoxaprop-p-ethyl (FE) and various cyclodextrins (p-CD, HP- p-CD, RAMEB) in solution was studied by UV spectra and phase solubility technique. The results showed that all cyclodextrins could form 1:1 inclusion complexes with FE in water. The binding constants of three types cyclodextrins toward FE was in the order, K_RAMEB>>K_HP-β-CD > K_β-CD. Between 298.15 and 318.15 K, the binding constants decrease with temperature increasing. All Gibbs free energy AG, enthalpy change AH, and entropy change AS determined were negative, suggesting the inclusion complexation is exothermic and can spontaneously occur by balance of enthalpy driven and entropy opposed. The complexation was confirmed by the differential scanning calorimetry (DSC), the Fourier transform infrared spectroscopy (FTIR) and the X-ray diffraction (XRD), where the FE melting peaks in DSC, the FTIR bands and the XRD peaks were generally weakened. Within the tested time up to 60 min, the dissolution of the FE-CD complexes resulted in higher FE concentrations than did that of FE by itself. The dissolution of FE-RAMEB complex was much faster than other complexes and FE alone. These results indicate that RAMEB was a better inclusion complexation agent for FE in terms of bothsolubility enhancement and dissolution rate.(2) CDs have a impact on interaction between fenoxaprop acid (it is a very important degradation product of FE in environment) and Scenedesuus obliquus, this can be concluded by the effect of CDs on EC50 value for FA to S. obliquus and degradation of FA by S. obliquus, the effect was related with types of CDs. There were not obvious difference between toxicity of RS-FA and i?-FA to S. obliquus, because their EC50 value were equal in general. Compared with their control EC50 value, the EC50 value corresponding to every stage increased in the presence of CDs. Among three CDs, p-CD, RAMEB and HP-p-CD, p-CD had the most outstanding ability not only on detoxicity of RS-FA and 7?-FAto S. obliquus, but also on enantioselectivity of FA. The degradtion of FA by S. obliquus was not very visible in the absence of CDs, but it increased in the presence of CDs. Compared with RAMEB and HP-P-CD, P-CD had the most distinct effect on degradation of FA by S. obliquus.(3) The effects of CDs on hydrolysis of malathion was also evaluated in this work. a-CD, P-CD and its two derivatives, randomly methylated P-CD (RAMEB) and hydroxypropyl P-CD (HP-P-CD), were tested. It was found that RAMEB could inhibit the hydrolysis of malathion, however, a-CD had a catalytic effect on the hydrolysis of malathion, and this was the function of pH and temperature. The inhibitory effects of RAMEB on hydrolysis of malathion increase with increasing concentration of RAMEB and elevating temperature between 15 and 35°C, similarly, the catalytic effects of a-CD on hydrolysis of malathion increase with increasing concentration of a-CD. On the other hand, P-CD and HP-P-CD have little or no stabilizing effects on malathion at all pH and temperature studied, except that the large concentration of p-CD and HP-P-CD can mildly reduce hydrolysis of malathion. Both 2 M and 5M urea increase the inhibitory effects of RAMEB on hydrolysis of malathion, but decrease the catalytic effect of a-CD on hydrolysis of malathion at 25°C,atpH9.0.(4) Baseline separation of the enantiomers of malathion, malaoxon and four isomers of isomalathion on CHIRALCEL OJ-H and CHIRALCEL AD-H wasrespectively obtained on using hexane-isopropanol(V/V, 91:9) mobile phase at 40°C. HPLC could run in the good state at such separation condition, this was very favored to prepare the enantiomers of malathion, malaoxon and four isomers of isomalathion. The thermodynamic data indicated that the chiral resolution of malathion, malaoxon and isomalathion were controlled by enthalpy change AH between 288.15-313.15K. All isomers of malathion, malaoxon and isomalathion possess outstanding toxicity against D. magna, with LC5o<2Oug/L, the R enantiomers were more active than S enantiomers by 2-3 times in all test chemicals. The bimolecular reaction constants between enantiomers of malaoxon and acetylcholinesterase was related with enzyme sources and temperature, the bimolecular reaction constants between i?-malaoxon and electric eel acetylcholinesterase was a 3.7-fold bigger than that between S-malaoxon and electric eel acetylcholinesterase. The enantioselective inhibition of malaoxon to electric eel acetylcholinesterase can be regulated by CDs, the effect was dependent on types of CDs. According IC50 value for enantioners of malaoxon to electric eel acetylcholinesterase, a-CD could reduce the inhibition of both i?-malaoxon and S-malaoxon to electric eel acetylcholinesterase, however, the reducing degree was different. RAMEB was able to decrease the inhibition of i?-malaoxon to electric eel acetylcholinesterase, but it had no obvious effect on inhibition of S-malaoxon to electric eel acetylcholinesterase. In general, RAMEB reduced the enantioselective inhibition between R-malaoxon and S-malaoxon to electric eel acetylcholinesterase.