Complexation of diazinon, an organophosphorus pesticide, with α-, β-, and γ-cyclodextrin - NMR and computational studies

Article abstract of DOI:10.1139/V06-053

Complexation of the organophosphorus pesticide, diazinon, with α-, β- and γ- cyclodextrin has been investigated through NMR and computational methodologies. Binding constants (K_b) determined by ¹H and ³¹P NMR follow the order γ-CD > α-CD = β-CD, in contrast with reported K_b data for other pesticides and thus indicative of steric encumbrance by the isopropyl group in diazinon being an important factor influencing binding constants. The interaction of diazinon with the CDs has also been investigated through computational studies via molecular dynamics - molecular mechanics (MD-MM2) and density functional theory (DFT), B3LYP/6-31G*. It is shown that the most favorable orientation in binding corresponds to the hydrophobic heterocyclic residue of diazinon being pulled deepest into the CD cavity, in agreement with the experimentally determined order of binding constants. Moreover, the computations show that it is only with γ-CD that the heterocyclic residue of diazinon and the phosphoryl residue are both largely encrypted in the CD cavity, marking a clear differentiation with α-CD and β-CD where the phosphoryl residue is located largely outside the cavity. Thus, the computational results are in essential agreement with the experimental binding constants where γ-CD stands out with the highest K_b value. Our work could point to the potential usefulness of computational studies to be undertaken in tandem with experimental work in environmental situations such as soil remediation.

Full text of DOI:10.1139/V06-053

702
Complexation of diazinon, an organophosphorus
pesticide, with ␣-, ␤-, and ␥-cyclodextrin — NMR
and computational studies1,2
Doreen Churchill, Jason Chiu Fung Cheung, Yong Sung Park, Vedene H. Smith,
Gary vanLoon, and Erwin Buncel
Abstract: Complexation of the organophosphorus pesticide, diazinon, with α-, β- and γ- cyclodextrin has been investi-
1
gated through NMR and computational methodologies. Binding constants (Kb) determined by H and 31P NMR follow
the order γ-CD > α-CD = β-CD, in contrast with reported Kb data for other pesticides and thus indicative of steric en-
cumbrance by the isopropyl group in diazinon being an important factor influencing binding constants. The interaction
of diazinon with the CDs has also been investigated through computational studies via molecular dynamics – molecular
mechanics (MD–MM2) and density functional theory (DFT), B3LYP/6-31G*. It is shown that the most favorable orien-
tation in binding corresponds to the hydrophobic heterocyclic residue of diazinon being pulled deepest into the CD
cavity, in agreement with the experimentally determined order of binding constants. Moreover, the computations show
that it is only with γ-CD that the heterocyclic residue of diazinon and the phosphoryl residue are both largely en-
crypted in the CD cavity, marking a clear differentiation with α-CD and β-CD where the phosphoryl residue is located
largely outside the cavity. Thus, the computational results are in essential agreement with the experimental binding con-
stants where γ-CD stands out with the highest Kb value. Our work could point to the potential usefulness of computa-
tional studies to be undertaken in tandem with experimental work in environmental situations such as soil remediation.
Key words: organophosphorus pesticides, diazinon complexation, cyclodextrins, computational studies, molecular me-
chanics.
Résumé : Faisant appel à la RMN et à des calculs théoriques, on a étudié la complexation du diazinon, un pesticide
1
organophosphoré, avec les α-, β- et γ-cyclodextrines. Faisant appel à la RMN du H et du 31P, on a déterminé que les
constantes de fixation (Kb) sont dans l’ordre γ-CD > α-CD = β-CD, ce qui contraste avec les valeurs de Kb rapportées
pour d’autres pesticides; ces différences pourraient suggérer que l’encombrement stérique provoqué par le groupe iso-
propyle du diazinon est un facteur qui présente une influence importante dans les constantes de fixation. L’interaction
du diazinon avec les cyclodextrines a été examinée d’une façon théorique en faisant appel à la théorie de la dynamique
moléculaire – mécanique moléculaire (DM–MM2) et à la théorie de la densité fonctionnelle (TDF), B3LYP/6-31G*. On
a montré que l’orientation la plus favorable lors de la fixation correspond à celle où le résidu hétérocyclique hydro-
phobe du diazinon est inséré au point le plus profond de la cavité de la cyclodextrine, ce qui correspond à l’ordre dé-
terminé d’une façon expérimentale par les constantes de fixation. De plus, les calculs montrent le résidu hétérocyclique
du diazinon et le résidu phosphoryle ne sont bien insérés que dans la cavité de la γ-CD, ce qui la différencie claire-
ment de l’α-CD et de la β-CD avec lesquelles le résidu phosphoryle se trouve largement à l’extérieur de la cavité. Les
résultats théoriques sont donc, pour l’essentiel, en accord avec les constantes de fixations expérimentales alors que la
valeur pour la γ-CD est la plus élevée. Ce travail pourrait démontrer l’utilité potentielle d’études théoriques pour étu-
dier, en tandem, avec des expérimentales, des situations environnementales, telles que la récupération des sols.
Mots clés : pesticides organophosphorés, complexation du diazinon, cyclodextrines, études théoriques, mécanique molé-
culaire.
[Traduit par la Rédaction] Churchill et al. 708
Received 6 December 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 13 May 2006. Reposted
on the Web site with correction on 16 May 2006.
This paper is dedicated to Professor Walter Szarek in recognition of his contributions to chemistry.
D. Churchill. Centre of Environmental Excellence, c/o Sir Wilfred Grenfell College, Corner Brook, NF A2H 6P9, Canada.
J.C.F. Cheung, Y.S. Park, V.H. Smith,3 G. vanLoon, and E. Buncel.4 Department of Chemistry, Queen’s University, Kingston,
ON K7L 3N6, Canada.
1This article is part of a Special Issue dedicated to Professor Walter A. Szarek.
2Part 9 in a series titled “Mechanisms of abiotic degradation and soil–water interactions of pesticides and other hydrophobic organic
compounds”. For part 8, see ref. 1.
3Deceased.
Can. J. Chem. 84: 702–708 (2006)
doi:10.1139/V06-053
© 2006 NRC Canada

Churchill et al.
703
Fig. 1. Chemical structure of α-CD, β-CD, and γ-CD consisting of six, seven, and eight glucose units, respectively.
complexation of the guest (important for the small α-CD);
(3) hydrophobic and van der Waals interactions; (4) hydro-
gen bonding between host and guest; (5) compatability be-
tween guest size and dimensions of the host cavity;
(6) orientation of the guest molecule in the cavity. The com-
puter modeling and NMR studies reported in this paper
bring insight into some of these factors.
CDs have a potentially uniquely important role in the re-
moval of pesticide residues from contaminated soils, for ex-
ample, from localized industrial sites. In such a situation, the
leaching of the pesticide through rainwater, along with other
hydrophobic compounds, causes these to localize below the
ground water table where they accumulate as DNPLs (dense
nonaqueous phase liquids); there they remain virtually im-
mune to degradation through bacterial action over several
decades or longer (10). Now, if a CD solution is pumped
from the surface into the DNPL, it complexes the hydropho-
bic material, which once solubilized, can be carried down-
stream. From there it can be pumped up to the surface where
it is separated into its constituents and degraded, while the
CD solution is recycled upstream. This process is similar to
the traditional “pump-and-treat” procedure using surfactants
(11).
Introduction
The wide usage of pesticides in modern agriculture, cou-
pled with the known toxicity of pesticide residues to humans
and other nontarget organisms, calls for fresh research into
methods for the facile degradation of such residues under en-
vironmentally benign conditions and without generating fur-
ther toxic materials (metabolites) in the process. Our own
approach toward that goal is through the tools of structure–
reactivity relationships in physical organic chemistry and en-
vironmental science. Several papers have been published
from our laboratories concerning the degradation of various
pesticides and other hydrophobic organic compounds (1–7).
In the present work we have explored, through computa-
tional and NMR methodology, the interaction of the organo-
phosphorus pesticide, diazinon (1), with several cyclodextrin
(CD) molecules.
S
OCH2CH3
N
P
N
O
OCH2CH3
1
Measurement of binding constants between a given pesti-
cide and available CDs enables one to quantify and optimize
this methodology. Different methods for the determination
of binding constants have been developed (8, 9, 12), includ-
ing NMR spectroscopy (13, 14). In the case of diazinon,
The CDs being investigated are α-, β-, and γ-CD, contain-
ing six, seven, and eight glucose moieties, respectively, in
the cyclic structure (Fig. 1). The CDs may be shown by the
torus (“bucket”) representation in which the secondary hy-
droxyls are situated on the upper, wider rim, and the primary
hydroxyls on the lower rim (see figures in the Computa-
tional studies section).
1
both H and 31P NMR become particularly convenient.
Experimental
The important characteristic of the CD-type glucose
oligosaccharide structure is a hydrophilic exterior imparted
by the CD hydroxyl groups and an interior lined by H-C-O
linkages that impart hydrophobic character (8, 9). The hy-
drophobic interior is then capable of encapsulating a hydro-
phobic or low polarity molecule, for example, a typical
pesticide containing an aromatic or heteroaromatic moiety,
resulting in the formation of a host–guest complex. The driv-
ing force for CD host–guest complex formation is presumed
to have as its origin the combination of the following fac-
tors: (1) release of “high-energy” H2O molecules from the
CD cavity; (2) relief of strain in the CD molecule upon
Materials
Diazinon was synthesized based on the method of Gysin
and Margot (15). The dried sodium salt of 2-isopropyl-6-
methylpyrimidin-4-ol was heated for 2 days under nitrogen
with diethyl chlorothiophosphate in dry acetone at 35 °C.
After cooling and filtering to remove KCl, then removal of
solvent under vacuo, the residue was chromatographed with
methylene chloride as eluent. Further purification was ef-
fected by dissolving in hexanes and extracting with water.
The hexane layer was dried (MgSO4), and after filtering was
first concentrated and then dried in vacuo to yield pure
diazinon. Purity was checked by 1H, 13C (proton-decoupled),
© 2006 NRC Canada

704
Can. J. Chem. Vol. 84, 2006
and 31P (proton-decoupled) NMR. Spectra were run on a
300 MHz Bruker Avance spectrophotometer using CDCl3 as
solvent. All spectra were consistent with diazinon and con-
Fig. 2. Structure of diazinon illustrating the portions studied by
NMR to determine the binding constant (Kb) with cyclodextrins.
H
CH3
1
tained no impurity peaks. H NMR (ppm) δ: 6.64 (s, 1H),
31P
CH3
S
P
OCH2CH3
OCH2CH3
4.33 (d of q, JH-H = 7.1 Hz, JH-P = 11.2 Hz, 4H), 3.10 (sept,
JH-H = 6.9 Hz, 1H), 2.47 (s, 3H), 1.37 (t, JH-H = 7.1 Hz, 6H),
1.30 (d, JH-H = 6.9 Hz, 6H). 13C NMR (ppm) δ: 176 (s),
170(s), 165 (d, JC-P = 4.6 Hz), 107 (d, JC-P = 6.4 Hz), 65.6
N
N
O
(d, JC-P = 5.1 Hz), 37.7 (s), 24.5 (s), 21.8 (s), 16.2 (d, JC-P
=
CH3
Ar-Me
H
8.0 Hz). 31P NMR (ppm) δ: 61 (s).
Ar-H
α-, β-, and γ-CDs were a generous gift from Cerestar
USA and were used without further treatment. DMSO-d6
and D2O were purchased from Cambridge Isotopes and used
as received.
Kb determination by 31P NMR
Proton-decoupled 31P NMR spectra were recorded for
each tube (see the general Sample preparation section), as
singlets, no other peaks being observed. The change in
chemical shift (∆δ) with increasing CD concentration gave a
curved binding isotherm. Plotting ∆δ vs. [CD] and using the
curve-fitting program GraphPad Prism® yielded Kb accord-
ing to eq. [1]. Kb was again evaluated using the Benesi–
Hildebrand eq. [2]. Each determination was made at least
twice by each method, and all of the acquired values were
averaged to obtain the reported Kb value.
NMR determination of binding constants
Sample preparation
To determine each binding constant, 10 samples were pre-
pared in NMR tubes; the first with diazinon (plus DMSO
cosolvent, 0.2% v/v) in D2O, and the following containing
successively more CD. First, a 0.99 mol/L stock solution of
diazinon was prepared in DMSO-d6, then a 0.006–
0.06 mol/L stock solution of CD in 2.5 mL D2O. Ten NMR
tubes were filled with various amounts of the CD solution
and D2O to give a range of CD concentrations from zero
(D2O only) to that of the pure CD stock solution (no added
D2O). To each tube was added 2.5 µL of the diazinon stock
solution, giving a final diazinon concentration of 4.93 ×
10–3 mol/L in each. The NMR spectra were recorded using a
400 or 500 MHz Bruker Avance spectrophotometer.
Results and discussion
CD–diazinon binding constants
1
Kb from H NMR
The change in chemical shift of the Ar-H and Ar-Me
peaks in the NMR spectra of diazinon on increasing the CD
concentration was followed for α-, β-, and γ-CD. The
change in chemical shift was downfield, except for the Ar-H
peak in γ-CD, where it shifted upfield with increasing [CD].
The data were used to produce binding isotherms that fit a
1:1 binding model for all three CDs. Figure 3 shows results
for α-CD. Figure 3a gives the binding isotherm generated
for the Ar-H peak, which yields Kb = 183 (mol/L)–1, while
Fig. 3b provides the plot according to the Benesi–
Hildebrand treatment, yielding Kb = 221 (mol/L)–1. Plots for
β-CD and γ-CD are qualitatively similar.
1
Kb determination by H NMR
1H NMR spectra were run on all 10 tubes using a water
presat to remove the HOD peak from the spectra. Two pro-
ton signals were examined, the aromatic proton (Ar-H) on
the pyrimidinol ring and the CH3 protons (Ar-Me) located
on the pyrimidinol ring, as illustrated in Fig. 2. Both singlets
appear in an area of the spectrum with no interference from
CD signals. The change in chemical shift (∆δ) for the Ar-Me
and Ar-H peaks of diazinon with increasing CD concentra-
tion, when plotted against [CD], yielded a curved binding
isotherm. The curve-fitting program GraphPad Prism® was
used to determine Kb according to eq. [1]
Kb from 31P NMR
Downfield changes in the 31P NMR chemical shift for
diazinon were observed on increasing the CD concentration
for α-, β-, and γ-CD. Figure 4a shows the binding isotherm
for α-CD, while Fig. 4b gives the Benesi–Hildebrand plot of
the data. β-CD and γ-CD yield similar plots.
∆bKb [CD]
1 + [CD]Kb
[1]
∆ =
where ∆ is the observed change in chemical shift, ∆b is the
maximum change in chemical shift for 100% bound sub-
strate, and Kb is the binding constant. Kb values were also
obtained by the Benesi–Hildebrand eq. [2], the linear double
reciprocal plot of the binding isotherm equation.
Structure–stability relationships
The overall average Kb values obtained from the 1H (Ar-H
and Ar-Me) and 31P NMR data are given in Table 1. Clearly,
the largest Kb value is found for γ-CD, around 400 (mol/L)–1,
while the values for α-CD and β-CD are not significantly
different, at around 200 (mol/L)–1.
1
∆
1
1
[2]
=
+
∆bKb [CD]
∆
A partial summary of existing literature Kb data for pesti-
cides is given in Table 2. The values generally vary from
100 to 600 (mol/L)–1. Thus, the present results for diazinon
(Table 1) fall within the range of existing data for other pes-
ticides (Table 2). Data for all three CDs are available for
parathion (2), methyl parathion (3), and paraoxon (4), all
three organophosphorus pesticides with an aromatic moiety
b
On plotting 1/∆ vs. 1/[CD], Kb was found by dividing the
intercept by the slope, as determined by linear regression us-
ing Microsoft Excel®. Each determination was made at least
twice by each method, and all of the values acquired were
averaged to obtain the reported Kb value.
© 2006 NRC Canada

Churchill et al.
705
1
Fig. 3. (A) Binding isotherm and (B) Benesi–Hildebrand plots for the binding of diazinon with α-CD generated from H NMR data
for the Ar-H peak.
0.25
(B)
15
10
5
(A)
y = 0.020x + 4.402
R2= 0.992
0.20
0.15
0.10
0.05
0.00
Kb = 183 (mol/L)–1
R2 = 0.998
Kb = 221 (mol/L)–1
0
100
200
300
400
500
600
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1/[α-CD] (mol/L)–1
[α-CD] (mol/L)
Fig. 4. (A) Binding isotherm and (B) Benesi–Hildebrand plots for the binding of diazinon with α-CD generated from 31P NMR data.
4
(A)
(B)
1.0
0.5
0.0
y = 0.006x + 1.050
R 2= 0.994
3
2
1
Kb = 163 (mol/L)–1
R2 = 0.999
Kb = 191 (mol/L)–1
0
100
200
300
400
500
600
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1/[α-CD] (mol/L)–1
[α-CD] (mol/L)
in place of the heteroaromatic moiety in diazinon, but other-
wise similar structures.
highest Kb. A plausible explanation for this variance lies
with the sterically demanding isopropyl group in diazinon,
which together with the Ar-methyl cannot easily be accom-
modated in the α-CD or β-CD cavity, but can be readily ac-
commodated in the γ-CD cavity. The computational study
that follows sheds further light on this aspect.
S
OCH2CH3
P
O2N
O
OCH2CH3
Computational studies
2
The manifold applications of computational chemistry
have made enormous impact in the field of cyclodextrin
chemistry, including modeling studies of the host–guest
type. The article by Kenny Lipkowitz on this subject, which
appeared in 1998 in a special issue of Chemical Reviews fea-
turing cyclodextrin chemistry, provides an excellent over-
view of this area up to that time; it lists 214 references (24),
while many have appeared since then. In this study we pres-
ent results of two approaches in computational chemistry as
applicable to the present system: molecular mechanics
(MM) and ab initio, density functional theory (DFT).
S
OCH3
P
O2N
O
OCH3
3
O
OCH2CH3
OCH2CH3
B3LYP energy optimized structures
P
Figure 5 presents the B3LYP/6-31G** minimized struc-
ture of diazinon in the form of the space-filling model and as
ball-and-stick representation. These are used in visualizing
the orientations of diazinon bound in the α-, β-, and γ-CD
cavities presented in the following.
O2N
O
4
A major difference between our results and earlier ones
now becomes apparent in that Kb values in Table 2 are low-
est for γ-CD, while in our study (Table 1), γ-CD has the
Binding orientations of diazinon into α-, β-, and γ-CDs
The dimensions of the diazinon – CD bound complex
have been evaluated using MD–MM2 and the results are
© 2006 NRC Canada

706
Table 1. Diazinon–CD binding constants (Kb, (mol/L)–1) determined by H and 31P NMR.
Can. J. Chem. Vol. 84, 2006
1
Average Kb ((mol/L)–1)a
determined by Ar-H H NMR
Average Kb ((mol/L)–1)a
determined by Ar-Me H NMR
Average Kb ((mol/L)–1)a
1
1
determined by 31P NMR
Binding
isotherm
Binding
isotherm
Binding
isotherm
Average Kb from
all methods
CD
Benesi–Hildebrand
Benesi–Hildebrand
Benesi–Hildebrand
α-CD
β-CD
γ-CD
181 3
200 0
422 32
220 1
156 14
368 41
200 0
200 0
456 16
283 54
215 45
335 30
157 8
134 35
454 33
179 18
121 31
328 38
203 46
170 45
390 63
aError limits in the Kb value were taken as the standard deviation (three or more values) or the average deviation (only two values).
Table 2. Binding constants of different pesticides with α-CD, β-CD, and γ-CD.
α-CDa Kb
β-CDa Kb
γ-CDa Kb
Pesticide
((mol/L)–1)
((mol/L)–1)
((mol/L)–1)
Ref.
Diazinon
Parathion
203 46
—
170 45
526
390 63
—
This work
16
—
286
—
17
588
123
285
—
192
182
526
625
83
18
18
18
16
Paraoxon
Methyl parathion
110
119
—
—
526
—
17
Fenitrothion
—
417
—
16
—
227
—
17
Pirimiphos-methyl
Chlorpyriphos
DCPEb
—
—
—
171 34
90 28
555
—
—
—
19
19
20
(S)-(+)-Sarin
(S)-(–)-Sarin
Warfarin
167 25
25 2
—
—
—
160
—
—
—
21
21
22
Coumatetraly
Carbaryl
Deltamethrin
Fenvalerate
—
—
—
—
565 42
289 21
627 104
316 46
—
—
—
—
19
23
19
19
aKb error limits listed as available in the referenced paper.
bα-(Diethoxyphosphinoximino)dicyclopropylmethane (DCPE).
Fig. 5. Ball-and-stick (left) and space-filling model (right) of diazinon optimized with B3LYP/6-31G**.
© 2006 NRC Canada

Churchill et al.
707
Fig. 6. α-CD–, β-CD–, and γ-CD–diazinon binding orientation, energy optimized with MD–MM2. Top row shows the diazinon in the
ball-and-stick model and the α-CD, β-CD, and γ-CD in the wire-frame model; bottom row shows the torus representation of α-CD, β-
CD, and γ-CD.
Table 3. Primary OH rim radius, secondary OH rim radius, and height of the diazinon–CD complexes, with errors,
optimized with MD–MM2.
Primary
OH rim
radius (Å)
Error in
primary rim
radius (Å)
Secondary
OH rim
radius (Å)
Error in
secondary
radius (Å)
Error in
height
(Å)
Height
(Å)
α-CD + diazinon
β-CD + diazinon
γ-CD + diazinon
4.97
5.77
5.57
1.25
1.62
1.45
5.13
6.25
6.76
1.00
0.85
1.31
5.24
5.40
5.86
0.41
0.47
0.39
given in Table 3 as the primary OH rim and secondary OH
rim radii, and the height of the complexed CDs. The
diazinon-complexed γ-CD is found to have the largest sec-
ondary OH rim radius, as expected. Interestingly however,
the primary OH rim radius in the complexed CD is about
equal for β- and γ-CD.
We have also calculated (using MM2) the distance from
the phosphorus atom of diazinon to the secondary OH rim,
as the average distance to the three nearest secondary oxy-
gen neighbours. This gives a measure of the depth of the
diazinon molecule inside the CD cavity. The results given in
Table 4 clearly show that this distance is greatest with γ-CD,
i.e., diazinon is found deepest in the CD cavity with γ-CD.
Table 4. Calculation of average distance from
the phosphorus atom to the secondary oxygen
plane optimized with MM2 with three nearest
oxygen neighbours.
Distance (Å)
α-CD + diazinon
β-CD + diazinon
γ-CD + diazinon
2.52
0.55
6.37
Note: Alternatively one can indicate distance mea-
surements above the rim as + and below the rim as
–, giving the values for α-CD (+2.52), β-CD (+0.55),
γ-CD (–6.37).
Figure 6 shows the different orientations for the binding
of diazinon in α-, β-, and γ-CD optimized with MD–MM2.
The illustrations given are for both ball-and-stick models
showing diazinon inside the wire-frame CD and the corre-
sponding CD torus representation (Fig. 6), and also space-
filling models (Fig. 7).
Several important observations follow on visual examina-
tion of these models. (1) The heterocyclic residue of
diazinon appears inside the cavity for all three CDs. (2) The
methyl substituent is inside the cavity and points downwards
for α- and β-CD, while for γ-CD it is situated largely out-
© 2006 NRC Canada

708
Can. J. Chem. Vol. 84, 2006
Fig. 7. Space-filling model of diazinon with α-, β-, and γ-cyclodextrins complex optimized with MD–MM2.
5. J.E. Omakor, I. Onyido, G.W. vanLoon, and E. Buncel. J.
side, pointing upwards. (3) The isopropyl substituent is
largely outside pointing upwards for α- and β-CD, while be-
ing situated on the secondary rim with γ-CD (4). The main
contrast comes with the phosphoryl residue that is essen-
tially outside the cavity for α- and β-CD, while with γ-CD it
is inside the cavity, with the exception of one ethoxy
substituent that protrudes from the lower rim.
Thus, the computations reveal a clear contrast in the three
computed orientations. In the case of γ-CD, the bulk of the
hydrophobic diazinon molecule, i.e., both the heterocyclic
residue and the phosphoryl residue, are situated inside the
CD cavity, whereas with α- and β-CD only the heterocyclic
residue is present inside the cavity. This, then, provides a ra-
tionale for the experimental observation of γ-CD having the
greatest binding constant compared with α-CD and β-CD,
which possess much smaller and virtually equal Kb values.
Any attempts at environmental remediation of pesticides,
e.g., from soils, must be cognizant of these findings.
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