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Dockery and, Wesley G. Free Radical Chain Reactions of [1. Evidence for the High Reactivity of the Bicyclo[1. Journal of the American Chemical Society , 6 , Kami K. Thoen,, Rebecca L. Smith,, Jaakko J. Nousiainen,, Eric D. Nelson, and, Hilkka I. Charged Phenyl Radicals. Journal of the American Chemical Society , 36 , Alok K. The inclusion of the apolar heterocyclic ring into the CD cavity is the most likely situation to occur.

As the inclusion phenomena occur, it is reported that the fluorescence properties of the guest are improved compared to its free form in aqueous solution. The stability of these complexes were kinetically analyzed through the DCPE degradation monitoring in aqueous solution in the presence or absence of CD derivatives [49,50]. Additionally, C—N stretching vibration band shifted from Additionally, the presence of a guest might weaken the hydration water binding, as revealed in DSC experiments by the lower endothermic fusion peak, specific to a dehydration process.

X-ray powder diffraction analysis finally confirmed the complex formation. All these studies proved that organophosphorus pesticides are able to interact with CDs and their derivatives. The structure of the pesticide also plays a major role in the mode of inclusion.

1997, Vol.70, No.12

For instance, aromatic ring as substituent of the phosphorus atom are often included into the cavity, while steric hindrance is able to impair the complex formation. Finally, without aromatic groups, the mode of entry of the pesticide depends on the specific affinity of the phosphorus substituents for the CD cavity.

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According to the above mentioned results, CDs provide interesting scaffolds able to bind organophosphorus compounds. The complex stabilization between the substrate and the oligosaccharide relies on weak interactions such as hydrophobic interactions, hydrogen bonding and van der Waals forces.

The inclusion of organophosphorus pesticides into the CDs cavity could either promote or prevent their hydrolysis and photodegradation. Figure 4: Structures of pesticides sensitive to the presence of CDs. In buffer solution pH 8. Scheme 1: The hydrolysis mechanism of substrate S in presence of a cyclodextrin CD.


However, the outcome of this complexation strongly depends of the host—guest system. It was previously established that the catalytic effect of CDs strongly depended on the interactions between the included pesticide and the secondary hydroxy groups of CDs. The correlation between inclusion parameters and catalytic properties of CDs was studied by induced circular dichroism ICD of the inclusion complexes using rotational-strength analysis method. ICD analysis underlines a change of the environment in the cavity due to the formation of the intermolecular complex. Positive and negative bands observed are respectively attributed to the axial and equatorial positioning of the organophosphorus compound.

For the three hosts, the para -nitrophenoxy group is included with its long-axis parallel to the cavity. It was reported that the catalytic activity is affected by the hydrogen bonding between the secondary OH groups of the CD and the reactive site of the guest.

Indeed, such interactions increase the nucleophilicity of the secondary hydroxy groups and facilitate their attack onto the phosphorus atom. It might be generalized that the inhibitive or promotive effect of the native CDs strongly depends on the inclusion depth of the guest, which can modify its interaction with the catalytically active secondary hydroxy groups or the hydroxide ions in the solvent.

Acting as the active site, the substitution of secondary hydroxy groups might impact the hydrolysis rate. However it appears clearly that the three studied CDs do not share the same pesticide hydrolysis inhibition level as in all cases, fenitrothion is hydrolyzed faster than parathion and methyl parathion. It was shown that the methyl group, adjacent to the nitro function slightly impairs both deep and axial inclusion of fenitrothion into the cavity. These unpredictable results revealed that much more complex parameters than the easiness of the guest entry into the methylated CD are involved in the hydrolysis.

Indeed, the methylation degree could also affect the steric hindrance of the secondary rim, resulting in CD geometrical changes with modifications of the complex stability, the orientation and inclusion depth of the pesticide within the cavity. However, the two main parameters to take into account remain the methylation degree and the pesticide inclusion depth. Table 3: Kinetic and equilibrium parameters for CD-organophosphorothioate substrates systems [53,54]. Table 4: Association constants K a and ratio of the observed rate constants of the fenitrothion hydrolysis.

Furthermore, the methyl groups also protect the phosphate moiety against external nucleophiles attack by impairing the entry of hydroxide ions. Most of the studies about the pesticides hydrolysis were carried out in alkaline aqueous solutions. However, in the natural environment, the medium pH is usually nearly neutral and the promoting or protecting effects of CDs on hydrolysis might be pH-dependant. In sharp contrast to the inhibition of pesticides hydrolysis in alkaline media reported above, at neutral pH, the three native CDs display a catalytic effect on the hydrolysis of all the studied pesticides, highlighting the importance of pH [53,54].

As some hydroxy groups of CDs might be ionized in alkaline media, the authors proposed that the resulting electrostatic repulsions could expand the size of the cavities, resulting in a better hydrolysis inhibition of the pesticides through its deeper inclusion. Table 5: Inclusion effect of CDs on the initial photolysis of parathion and paraoxon [59,60]. Following the same trend as for pesticide hydrolysis, the photolysis outcome is related to the inclusion depth of the pesticide within the CD cavity.

Later, the effect of native cyclodextrins on the photodegradation of organophosphorus pesticides was evaluated in water containing humic acids [61]. Indeed, the ability of CDs to relay radicals towards the phosphorus atom might be tuned by the geometrical disposition of the hydroxy groups of CDs. A common feature of these agents is the chiral phosphorus atom and as CDs and their derivatives are also chiral, a stereoselective interaction with the nerve agents is possible. The stereochemistry of organophosphates is to be considered important insofar as the individual stereoisomers of a neurotoxic agent do not share the same level of toxicity [20].

Figure 5: Structures of the different stereoisomers of G agents. CDs can tune a reaction mechanism in two ways: a by creating a covalent bond between the macrocycle and the reagent, first forming an intermediate and then the product; b by providing the reactants a new environment with novel reactivity, i.

In alkaline medium hydrolysis of organophosphorus species, an intermediate mechanism is usually involved. The formation of an inclusion-complex first occurs and then the secondary alkoxide of the host pseudo-intramolecularly attacks the phosphorus atom to form a covalent bond. Stereospecificity, both in the formation of the inclusion complex and in the nucleophilic reaction, are also observed to some extent. Thus, a better affinity with the CD cavity does not invariably imply a higher reaction rate.

The comparison of the dissociation K d and rate constants k 2 of the two sarin enantiomers clearly shows that the stereospecificity mainly occurs in the nucleophilic attack rather than in the complex formation. Nevertheless, the optical rotations attributed to each enantiomer of sarin are in contradiction to those described by Benschop and De Jong in [20]. Indeed, they determined that the R -sarin is dextrorotatory whereas the S -sarin is levorotatory. Nevertheless, at pH 7.

The formation of a stable inclusion complex between CDs and the organophosphorus nerve agent is postulated as the first step in the catalytic process. After its complexation into the cavity, soman reacts with a nucleophilic oxyanion formed from a secondary hydroxy group of the CD torus. As a result of this pseudo-intramolecular process, at pH 7. The complexation of the P R -isomers into the CD cavity is followed by the formation of a hydrogen bond between a C-3 hydroxy and the phosphoryl oxygen atom of soman. Then a C-2 ionized hydroxy function attacks the phosphorus atom to form a covalent bond with the release of the leaving group fluoride ion.

The different orientations of the phosphorus atom substituents for the P S -isomers prevent the optimal interaction between the ionized hydroxy group and the phosphorus atom and thus reduce the hydrolysis rate. The substrate is therefore not sufficiently enclosed in the cavity to allow the optimal nucleophilic attack by a hydroxy group of CD. The study also reports the influence of the pH on the degradation rate.

The Chemistry of Organophosphorus Compounds I | L. Maier | Springer

The different alkoxy substituents on the phosphonate group for the three methylfluorophosphonates do not result in fundamental changes of the dissociation constants. Sarin is the smallest of the three methylfluorophosphonates studied. Thus, in all the studies of Cabal, the P R -stereoisomers of soman, sarin and cyclosarin are faster hydrolyzed than the P S -stereoisomers in presence of CDs.

However, we previously underlined the incoherencies between the optical activity and the absolute configurations of the enantiomers of sarin described by van Hooidonk and Breebaart. Consequently, it seems that Cabal inversed all the absolute configuration of soman, sarin and cyclosarin enantiomers, which would imply a faster degradation of the P S -stereoisomers.

Several studies discussed the interactions between cyclodextrins and organophosphorus compounds and their influence on the hydrolysis rates, but the reaction pathways are not always clearly understood. In , Kranawetvogl et al. Then, two reaction pathways are possible. To efficiently catalyse the NOPs hydrolysis, CDs and their derivatives have to be able to stabilize the reaction intermediate.

To this regard, if the formation of a stable inclusion complex is likely to improve the reaction rate, the binding mode of the substrate and CD must match to observe a faster reaction. On the contrary, a non-reactive complex would decrease the hydrolysis rate. The design of more reactive CD derivatives is thus a challenging approach to degrade NOPs more effectively.

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Nevertheless, improvements remained relatively weak considering the results obtained under physiological conditions pH 7. The design of novel macrostructures displaying a higher hydrolytic efficiency under mild conditions is then a stimulating challenge. The main strategies described rely on the introduction of one or several reactive groups against nerve agents on these cyclic oligosaccharides.

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As CDs exhibit a large number of primary C-6 position of glucose units and secondary C-2 and C-3 positions alcohols, selective modifications are therefore difficult. Most of them are based on the slight differences in the reactivity of these three kinds of hydroxy groups. This representation was also chosen for their derivatives in order to point out the modified units. This seminal work proved unequivocally that modified CDs are able to accelerate the hydrolysis rate [74].

Regarding these preliminary results based on the C-6 position functionalization, Kubik et al. Following this second approach, Kubik et al. Scheme 4: Synthetic pathways to 6-monosubstituted CD derivatives. Firstly, the less active compound is the weak acid oxime derivative 20a. Then, the formation of the more nucleophilic aldoximate is favored with the pyridinium derivatives 20b — d , explaining their high activity.

The moderate efficiency of ketoximes 20d and 20e , compared to that of the corresponding aldoximes 20b and 20c , strengthen this hypothesis. Finally, the para -substituted pyridinium derivative 20c is much less reactive than the meta compound 20b. This tends to prove that even slight structural differences have a major impact on the hydrolytic reactivity of the cyclodextrin. Based on these results, Kubik et al. Contrary to sarin derivatives, soman detoxification by 20b was not as effective, suggesting a non-reactive position of the nucleophilic oxime to efficiently react with this toxic.

Worek et al. A dose of cyclosarin s. For them, the systemic signs of poisoning almost completely disappeared. Reiter et al. As previously described, the formation of an intermediate inclusion complex is likely to occurred.