Calculations with the Level II Fugacity Model for selected organophosphorus insecticides

The environmental equilibrium distributions and levels of persistency of 20 selected organophosphorus insecticides have been determined using Mackay's Level II Fugacity Model. The model comprised air, water, biota, soil, suspended solids and sediment compartments. Available physicochemical and kinetic data for the insecticides have been compiled. Results suggest that some of the insecticides have tendencies to occur at high concentrations in biota and may be environmentally persistent.     

Inhibition of soil urease by organophosphorus insecticides

The effect of three organophosphorus insecticides on soil urease was examined. Inhibition of urea hydrolysis, some 60 days after application of 1000 parts/10⁶ of insecticide to a sandy clay loam, approached 40% (accothion) and exceeded 50% in the case of malathion and thimet. Similar inhibitory effects were recorded using a silt loam soil with which 200 parts/10⁶ application also produced inhibition ranging from 14% (accothion) to 23% (thimet) after 10 days. With lower concentrations of insecticide (50 parts/10⁶) inhibition, though again significant, was of a more ephemeral nature.All three insecticides, at a concentration of 1000 parts/10⁺⁶, prevented almost any hydrolysis of urea by jack bean urease. Ureolytic microorganisms, isolated from the soils under investigation, were inhibited by the organophosphates to a greater or lesser extent but the development of tolerance was common.It is suggested that the application of insecticides to control soil-borne insect pests may be a factor in determining the efficiency of urea fertilizer mineralization.     

Multiresidue screen for organophosphorus insecticides using gel permeation chromatography--silica gel cleanup.

A multiresidue screen for quantitative determination of 43 organophosphorus insecticides in 5 g of plant and animal tissues is described. The organophosphorus insecticides are extracted with methanol-dichloromethane (10 + 90, v/v) and cleaned up using automated gel permeation chromatography with hexane-ethyl acetate (60 + 40) eluant and in-line silica gel minicolumns. Concentrated extracts are analyzed by gas chromatography with flame photometric detection. The method recovers 43 organophosphorus insecticides in the range of 72 to 115%. Analysis of fortified bovine liver (n = 5) shows an average 95.9 +/- 4.8% recovery at the 0.05 micrograms/g level and 93 +/- 3.8% at the 0.5 micrograms/g level. Analysis of fortified bovine rumen content (n = 5) shows an average 98 +/- 4.2% recovery at the 0.1 micrograms/g level and 98.7 +/- 2.8% at the 1 micrograms/g level. Method detection limits ranged from 0.01 to 0.05 micrograms/g for the compounds studied using a nominal 5 gram sample.     

Persistence and biodegradation of selected organophosphorus insecticides in flooded versus non-flooded soils

Summary The persistence of parathion, methyl parathion and fenitrothion in five tropical soils of varying physicochemical characteristics was compared under flooded and non-flooded conditions. The degradation of all the three insecticides was more rapid under flooded conditions than under non-flooded conditions in four out of five soils. Degradation of these insecticides proceeded by hydrolysis under non-flooded conditions and essentially by nitro group reduction and to a minor extent by hydrolysis under flooded conditions. Kinetic analysis indicated that degradation of the three insecticides followed a first-order reaction irrespective of the soil and water regime. The degradation of these organophosphorus insecticides was accelerated after repeated applications to flooded alluvial soil. Nitro group reduction was the major pathway of degradation for all the three insecticides after the first addition while the rate of hydrolysis increased after each successive addition.     

Reactivity of certain organophosphorus insecticides toward hydroxyl radicals at elevated air temperatures

Methods were developed to determine OH reaction rates for medium-weight organophosphorus pesticides in the gas phase. A 57-L chamber was constructed that utilized xenon arc irradiation (>290 nm) to photolyze the OH precursor, methyl nitrite. Experiments were performed at elevated temperatures ranging from 60 to 80 degrees C to minimize wall sorption. Solid-phase microextraction (SPME) sampling of the gas phase was employed to assess the simultaneous rate of loss of the pesticides in relation to the rate of loss of two reference substances with known OH rate constants. An internal gas-phase standard (hexachlorobenzene), relatively stable to hydroxyl radicals, was used to assess other processes, which included dilution and wall sorption. The relative reaction rates of the organophosphorus insecticides, diazinon and chlorpyrifos, when compared to reference compounds, were unaffected by various air temperatures between 60 and 80 degrees C. Although both insecticides were expected to react at similar rates on the basis of structural activity model predictions, diazinon reacted 3 times more rapidly than chlorpyrifos and gave estimated environmental OH half-lives of 0.5 and 1.4 h, respectively. The degree of sorption onto the chamber walls was minimal and similar for each of the compounds examined. Experimental gas-phase determinations at elevated temperatures may provide important information that can be used when in the assessment of the potential of airborne pesticide risks to nontarget and ecologically sensitive areas.     

Zinc chloride-diphenylamine reagent for thin layer chromatographic detection of some organophosphorus and carbamate insecticides

Zinc chloride-diphenylamine reagent, whose use has been reported for the detection of organochlorine insecticides by thin layer chromatography, was further studied for its ability to detect the organophosphorus insecticides phorate, phosphamidon, DDVP, and phosalone and the carbamate insecticide carbaryl and aldicarb. These insecticides give intense blue-green spots with this reagent. The procedure can be applied to the detection of the insecticides in biological materials and thus has a potential use in forensic toxicology.     

Headspace solid phase microextraction applied to the analysis of organophosphorus insecticides in strawberry and cherry juices

A method based on a headspace solid phase microextraction (HS-SPME) technique followed by gas chromatography with flame thermionic and mass spectrometric detection was developed for the determination of seven organophosphorus (OPs) insecticide residues in strawberry and cherry juice samples. The extraction capacities of four fiber coatings, polyacrylate (PA 85 microm), poly(dimethylsiloxane) (PDMS 100 microm), carbowax-divinylbenzene (CW-DVB 65 microm), and poly(dimethylsiloxane)-divinylbenzene (PDMS-DVB 65 microm), have been studied and compared. The method was developed using spiked strawberry and cherry juices in a concentration range of 0.5-50 microg/L. The PDMS 100 microm fiber showed good extraction efficiency for the target compounds. An increase in the extraction efficiency of OP insecticides was observed when the parameters affecting the HS-SPME process such as temperature, extraction time, salt additives, stirring rate, pH, and effect of dilution were optimized. Good linearity of compounds was observed in the tested concentration range. The relative standard deviations were found to be <20%. The limits of detection were between 0.025 and 0.050 microg/L. The mean relative recoveries ranged from 82 to 102%.     

Liquid chromatographic determination of N-methylcarbamate insecticides and metabolites in crops. II. Analytical characteristics and residue findings

Four laboratories obtained 177 carbamate recovery values using a liquid chromatographic method. The average recovery of 11 carbamates (aldicarb, aldicarb sulfone, bufencarb, carbaryl, carbofuran, 3-hydroxy carbofuran, 3-keto carbofuran, methiocarb, methiocarb sulfoxide, methomyl, and oxamyl) from 14 crops was 99% with a coefficient of variation of 8% (0.03-1.8 ppm fortification levels). No statistical difference in recovery was found between oxime and phenyl carbamates, or between parent and metabolite carbamates. Average recovery of aldicarb sulfoxide was 59% due to loss in the liquid-liquid partitioning because of the polarity of this compound. A fifth laboratory contributed 34 carbamate recoveries (average 99%) on table-ready food products for 4 carbamates. Bendiocarb, dioxacarb, isoprocarb, and propoxur are also quantitatively recovered through the method. Previously reported carbamate and noncarbamate recovery data are also discussed. In the Food and Drug Administration's (FDA) analysis of 319 samples (mainly crops), 86 (27%) were found to contain residues of carbamate insecticides and/or toxic carbamate metabolites. Carbaryl and methomyl were the most common carbamate residues found on the food products excluding the aldicarb sulfone and sulfoxide residues found on potatoes. In one FDA Total Diet Program "market basket", 11 of 69 table-ready food commodities contained from 0.005 to 0.094 ppm carbamate residues. Carbaryl was the most prevalent residue. Several laboratories reported adverse effects on the determinative system when inadequately purified reagents were used.     

Migration of organophosphorus insecticides cyanophos and prothiofos residues from impregnated paper bags to Japanese apple-pears (Pyrus pyrifolia nakai cv. Nijisseiki)

Bags impregnated with the organophosphorus pesticides prothiofos and cyanophos in three levels were used to cover Japanese apple-pears to protect them from insects. The amounts of prothiofos residue in the bags collected 4 months after application ranged from 0 to 34% of the amounts (81, 148, and 333 microg) found in the bags prior to a bagging. The amounts of cyanophos residue in bags collected 4 months after application ranged from 11 to 29% of the amounts (335, 2860, and 3740 microg) present prior to a bagging. Amounts of prothiofos found in fruits after 4 months ranged from 0 to 0.076 ppm. Amounts of cyanophos found in fruits after 4 months ranged from 0 to 0.011 ppm. Results of the present study indicate that these two pesticides migrated from pesticide-impregnated bags to fruits during the growing season.     

NMR and molecular mechanics study of pyrethrins I and II.

Bioassay-directed fractionation of the organic extract of the Kenyan pyrethrum flowers (Chrysanthemum cinerariaefolium Vissiani) resulted in the isolation of two natural pyrethrin esters, pyrethrin I (PI) and pyrethrin II (PII) as the major constituents. These esters elicited inhibition of the multiple drug resistant (MDR) Mycobacterium tuberculosis. The high-field (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of PI and PII were unequivocally assigned using modern two-dimensional (2D) proton-detected heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) experiments. The conformations of both esters were deduced from (1)H-(1)H vicinal coupling constants and confirmed by 2D nuclear Overhauser effect spectroscopy (NOESY). Computer molecular modeling (MM) studies revealed that PI and PII molecules adopt a "love-seat" conformation in chloroform (CDCl(3)) solution.     

Soil organic matter beyond molecular structure Part II: Amorphous nature and physical aging

Glassy, rubbery, and crystalline phases are representatives of supramolecular structures which strongly differ in order, density, and other characteristics. In this contribution, the amorphous nature of soil organic matter (SOM) is reviewed with respect to the glassy/rubbery model, glass transition mechanisms, interactions of SOM with water, and physical aging. Glass‐transition behavior and physical aging are inherent properties of amorphous solids, and numerous spectroscopic investigations give insights into different domain mobilities of humic substances (HS). The correlation between sorption nonlinearity and glassiness of polymers and HS supports a relation between sorption and amorphicity in Aldrich humic acid. Further evidence is still required for the transfer to soil HS and SOM. Sorption and differential scanning calorimetry (DSC) data suggest a correlation between aromaticity and glassiness in HS, and the available data do currently not allow to decide unambiguously between specific sorption and hole filling as explanation. This needs to be verified in future research. Although parts of the investigations have up to now only been conducted with humic substances, the collectivity of available data give strong support for the glassy/rubbery conception of SOM. They clearly indicate that amorphous characteristics cannot be excluded in SOM. This is further supported by the observation of different types of glass‐transition behavior in samples of whole humous soil. In addition to classical glass transitions in water‐free soil samples, water surprisingly acts in an antagonistic way as short‐term plasticizer and long‐term antiplasticizer in a second, nonclassical transition type. Latter is closely connected with physico‐chemical interactions with water and suggests water bridges between structural elements of SOM (HBCL‐model). The gradual increase of T_g* in SOM indicates physico‐chemical aging processes, which are not restricted to polymers. They may be responsible for contaminant aging, changes in surface properties and increased soil compaction in agricultural soils.     

Neonicotinoid insecticides: highlights of a symposium on strategic molecular designs

Neonicotinoids are the newest of the five major classes of insecticides (the others are chlorinated hydrocarbons, organophosphorus compounds, methylcarbamates, and pyrethroids), and they make up approximately one-fourth of the world insecticide market. Nithiazine was the lead compound from Shell Development Co. in California later optimized by Shinzo Kagabu of Nihon Tokushu Noyaku Seizo to increase the potency and photostability, resulting in imidacloprid and thiacloprid. These discoveries are the basis for the International Award for Research in Agrochemicals of the American Chemical Society presented in 2010 to Professor Shinzo Kagabu. Five other neonicotinoids were added by others for the current set of seven commercial compounds. This symposium considers the progress in discovery and development of novel chemotype nicotinic insecticides with enhanced effectiveness, unique biological properties, and maximal safety. Chemorational approaches considered include physicochemical properties, metabolic activation and detoxification, and chemical and structural biology aspects potentially facilitating receptor structure-guided insecticide design.     

Crystal and molecular structure of piceatannol; scavenging features of resveratrol and piceatannol on hydroxyl and peroxyl radicals and docking with transthyretin

The mechanism by which the naturally occurring polyphenolic compounds resveratrol (RES), C(14)H(12)O(3), and its metabolite piceatannol (PIC), C(14)H(12)O(4), scavenge free radicals is studied using experimental and density functional theory (DFT) methods. PIC's crystal structure shows a strong intermolecular hydrogen bond network, which, through a concerted motion of the hydroxyl hydrogen atoms, can produce a second hydrogen bond chain. This reorganization offers a low-energy pathway for the transfer of hydrogen atoms and is a contributing factor to PIC's biological activity. Additionally, DFT calculations describing the entire reaction mechanism of RES, PIC, and 3,3',4',5,5'-pentahydroxystilbene with hydroxyl and peroxyl radicals agree with experimental results, showing that increased hydroxylation aids in scavenging activity. PIC is more efficient than RES because (i) by sharing its 3'-OH hydrogen atom with its adjacent neighbor, O-4', the abstraction and transfer of the 4'-H atom to the free radical becomes easier and (ii) the resulting PIC semiquinone radical is more stable. As a result of the reaction with OH(*), both RES and PIC form water; with the peroxyl radical, both RES and PIC form hydrogen peroxide. Also, docking of PIC onto the protein transthyretin suggests better performance than RES and confirms its possible application in neurodegenerative conditions such as Alzheimer's disease.     

Fulvic acids: structure and metal binding. I. A random molecular model

We have investigated a model of fulvic acid based on randomized positioning of functional groups and aromatic rings which can be used to deduce the types of metal chelating sites and provide statistical estimates of their concentrations. A FORTRAN program ‘RANDOM’ has been written which generates random molecules of fulvic acid. For each molecule generated the program also searches for and counts 14 types of binding site and calculates average concentrations over one thousand such molecules. A formula relating percentage aromatic carbon to the molar hydrogen/carbon ratio is also derived. The results show that the predominant bidentate sites are likely to be phthalate- and salicylate-type sites with a significant proportion of aromatic carboxyl and phenolic hydroxyl not participating in chelating sites.     

Neonicotinoid insecticides: molecular features conferring selectivity for insect versus mammalian nicotinic receptors

The favorable selective toxicity of neonicotinoid insecticides (represented here by imidacloprid, thiacloprid, and a nitromethylene analogue) for insects versus mammals is not shared by three of their N-unsubstituted imine derivatives or by nicotine or epibatidine. The same selectivity pattern is evident at the receptor level, i.e., the insect nicotinic acetylcholine receptor (nAChR) versus mammalian nAChR subtypes (alpha1, alpha3, alpha4, and alpha7) assayed independently. The insect-selective compounds are not protonated with a nitroimine, cyanoimine, or nitromethylene group and the mammalian-selective compounds are ionized at physiological pH. We propose that the negatively charged tip of the nitro or cyano group (not a partial positive charge at imidazolidine N-1 as suggested earlier) interacts with a putative cationic subsite of the insect nAChR. This contrasts with the mammalian nAChRs where the iminium cation (+C-NH2 <--> C =+NH2) of the neonicotinoid imine derivatives or ammonium nitrogen of nicotine or epibatidine interacts with the anionic subsite.