A study on the hydrolysis of bis(nitrophenyl) phosphate catalyzed by N-methyl-diethanolamine-Ce(III) complex

Article abstract of DOI:10.1002/kin.20045

The hydrolysis of bis(p-nitrophenyl) phosphate (BNPP) catalyzed by N-methyl-diethanolamine-Ce(III) complex in the presence and absence of cetyltrimethylammonium bromide (CTAB) and Brij35 surfactants at pH 7.20 and 303 K has been studied. The experimental

Full text of DOI:10.1002/kin.20045

A Study on the Hydrolysis of
Bis(nitrophenyl)phosphate
Catalyzed by N-methyl-
diethanolamine-Ce(III)
Complex
ZHANG YUANQIN, LIU FAN, XIANG QINGXIANG, XIONG JUNRU
Department of Chemistry, Leshan Teachers College, Sichuan Leshan 614004, People’s Republic of China
Received 20 October 2003; accepted 12 August 2004
DOI 10.1002/kin.20045
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) catalyzed by N-methyl-
diethanolamine-Ce(III) complex in the presence and absence of cetyltrimethylammonium bro-
mide (CTAB) and Brij35 surfactants at pH 7.20 and 303 K has been studied. The experimental
results indicate that N-methyldiethanolamine-Ce(III) complex remarkably accelerates the hy-
drolysis of BNPP. The observed first-order rate constant of the hydrolysis of BNPP catalyzed
−
2
−1
s
by N-methyldiethanolamine-Ce(III) complex at pH 7.20 and 303 K is 1.22 × 10
, which is
.09 × 10 times of that of spontaneous hydrolysis of BNPP at pH 7. It is close to the activity
of natural enzyme. A general quantitative treatment of the catalytic reaction involved a ternary
9
1
complex as MmL S has also been proposed in this paper. Applying this method to the catalytic
l
hydrolysis of BNPP, we have obtained its thermodynamic and kinetic parameters. CTAB and
Brij35 surfactant micelles obviously influence the rate constants of the catalytic hydrolysis of
BNPP. Brij35 micelles promote the catalytic hydrolysis of BNPP, while CTAB micelles inhibit it.
ꢀ
C
2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 687–692, 2004
INTRODUCTION
artificial research restriction enzymes since they have
much greater sequence specificity than their natural
counterparts [4]. In addition, the treatment of incurable
diseases by the in-vivo silencing of the genetic code of
pathogenic proteins on the RNA or DNA level has been
attempted [5]. RNA is more susceptible to hydrolysis
than DNA, and various low-molecular weight cleaving
agents are available for this. The efficient hydrolysis
of linear DNA [6] under mild conditions is possible
with Ce(IV), a hard Lewis acid with a high charge/size
ratio that will strongly activate the phosphate group.
Heterogeneous systems (Ce(IV-hydroxide gels)) are
particularly efficient and accelerate the rate of DNA
Nonenzymatic hydrolysis of the phosphodiester back-
bone of nucleic acids is an attractive research aim
in molecular biology [1–3]. Bioconjugates of hy-
drolytically active metal complexes and antisense-
oligonucleotides may have important applications as
Correspondence to: Zhang Yuanqin; e-mail: yqzhang18@
hotmail.com.
Contract grant sponsors: Science and Technology Department
and Education Department of Sichuan Province.
Contract grant numbers: 01GY051-63 and 01LA52.
ꢀ
c 2004 Wiley Periodicals, Inc.

688
YUANQIN ET AL.
1
1
cleavage up to 10 -fold at pH 7 and 303 K [7–10].
Homogeneous Ce(IV) solutions also display high reac-
tivity [11–13]. Other metal ions, for example trivalent
lanthanides, hydrolyze linear DNA either very slowly
or only at high temperature [14]. While many studies
on phosphodiester hydrolysis by hard Lewis acids with
high charges have been reported, little is known about
the Ce(III) and its complexes. No evidence of Ce(III)
complexeshavingoutstandingreactivityhassofarbeen
discussed.
into 10 mL calibrated flask, and diluted to 10 mL with
−
3
+
0.02 mol dm pH 7.20 Tris-TrisH buffer.
A V-530 spectrophotometer with a thermostatic cell
compartment from Japan and a PH-3C meter from
Shanghai, China, were used in our experiment for the
absorption measurements under the experimental
conditions.
Kinetics
The hydrolysis of BNPP catalyzed by N-methyl-
diethanolamine-Ce(III) complex has been studied at
pH 7.20 and 303 K in this paper. Rapid hydrolysis
of BNPP has been observed in our experiment. The
observed first-order rate constant of the hydrolysis of
BNPP catalyzed by N-methyldiethanolamine-Ce(III)
Kinetic measurements were made spectrophotomet-
rically at 303 K by employing a V-530 spectropho-
tometer with a thermostatic cell compartment. Three
milliliters of the solution of N-methyldiethanolamine-
Ce(III) complex was added to a cuvette and ther-
mostated at 303 K. After the equilibration time, 20 ꢀL
of a 5 × 10 mol dm BNPP stock solution was
injected to initiate the hydrolysis of BNPP. The release
of p-nitrophenol was monitored by following the
absorbance change against time at 400 nm for at least
three half-lives. The solution’s pH was measured after
each run and all kinetic runs in which pH variation was
smaller than 0.02. The observed first-order rate con-
stants were obtained from the spectrophotometer with
a computer data processing system. Each observed
first-order rate constant was the average of three deter-
minations. Excellent pseudo-first-order kinetics under
the experimental condition was observed throughout.
The rate constants calculated were reproducible within
± 5%.
−2
−1
9
−3
−3
complex is 1.22 × 10
s
, which is 1.09 × 10 times
of that of spontaneous hydrolysis of BNPP at pH 7.
Similarly, we have developed a general treatment for
the catalytic reaction involved the ternary complex as
MmLlS in order to obtain the thermodynamic and ki-
netic parameters.
Many chemical reactions are affected by the con-
centration effect, electrostatic effect, orientation, po-
larity effect, cage effect, and microviscosity [15–17]
of surfactant micelles. We have also studied the ef-
fects of CTAB and Brij35 micelles on the hydrolysis of
BNPP catalyzed by N-methyldiethanolamine-Ce(III)
complex. The experimental results indicate that the ob-
served first-order rate constant of the catalytic hydrol-
ysis of BNPP is obviously influenced by CTAB and
Brij35 micelles. Brij35 micelles promote the catalytic
hydrolysis of BNPP, while CTAB micelles inhibit it.
RESULTS AND DISCUSSION
The Ratio of Metal Ion to Ligand
in the Complex
EXPERIMENTAL
A convenient method to estimate the ratio of metal ion
to ligand in the complex is the kinetic version of a Job
plot [18], in which the rate constants are plotted as a
function of the mole fraction of metal ion or ligand,
keeping their total concentration constant. The results
of the hydrolysis of BNPP are shown in Fig. 1. Figure 1
indicates the necessity of the coexistence of the ligand
and metal ion for obtaining a higher rate. It also indi-
cates that the maximum rate is at ꢁ = 0.25, indicating
that ML3 is the active species.
Reagents and Apparatus
Ce(NO3)3·6H2O, tri(hydroxymethyl)aminomethane
(Tris), CH3CN, HNO3, and absolute ethanol were of
analytical grade. Cetyltrimethylammonium bromide
(
(
CTAB) and Brij35, bis(p-nitrophenyl)phosphate
BNPP), and N-methyldiethanolamine were obtained
from Sigman. Deionized distilled water was used in all
solutions. The pH of Tris-TrisH buffer was adjusted
+
by adding analytically pure nitric acid in all runs.
The solution of N-methyldiethanolamine-Ce(III)
complex for the catalysis of BNPP was prepared by
the following procedure: To a solution with a known
amount of N-methyldiethanolamine in 1 mL abso-
lute ethanol, a desired amount of Ce(NO3)3·6H2O was
added. After the addition, the solution was further
stirred 15 min at room temperature, then transferred
The observed first-order rate constants of the hy-
drolysis of BNPP at different conditions are listed in
Table I.
From Table I, it can be seen that the effects of the
pure buffer and the solution of N-methyldiethanol-
amine on the hydrolysis of BNPP can be neglected.
The effect of Ce(III) solution on the hydrolysis of
BNPP is not remarkable. However, the coexistence

A STUDY ON THE HYDROLYSIS OF BIS(NITROPHENYL)PHOSPHATE
689
Figure 2 The effect of complexing time on the hydrolysis of
−
3
−3
3+
Figure 1 Job plots for N-methyldiethanolamine (L) and
Ce(III) (M) complexation measured by the rates of hydrol-
ysis of BNPP at pH 7.20 and 303 K, [M] + [L] = 1.50 ×
BNPP catalyzed by 3.78 × 10 mol dm CeL complex
3
at pH 7.20 and 303 K.
−
−
2
5
−3
1
0
0
mol dm , ꢁ = [M] / ([M] + [L]), [BNPP] = 3.33 ×
−3
1
mol dm
.
the complexing time before 13 min, then keeps con-
stant. The appropriate complexing time between N-
methyldiethanolamine and Ce(III) can be determined
to be 15 min in our experiment.
of N-methyldiethanolamine and Ce(III) leads to a
remarkable acceleration effect on it. The observed
first-order rate constant of the hydrolysis of BNPP
when N-methyldiethanolamine and Ce(III) coexist is
General Treatment of Catalytic Reactions
Involved in Ternary Complexes
−2
−1
1
.22 × 10
s , which is 72 times of that only with
Ce(III). This result indicates that the coexistence of N-
methyldiethanolamine and Ce(III) has a cooperation
effect on the hydrolysis of BNPP. This cooperation ef-
fect comes from the formation of the ternary complex
with the ligand, metal ion, and the substrate. The for-
mation of the ternary complex leads to the intramolec-
ular nucleophilic substitution reaction, which greatly
promotes the hydrolysis of BNPP.
The process of a reaction catalyzed by a metal com-
plex is described as follows: An “m” metal ion or ions
(
(
M) forms a binary complex (MmLl) with “l” ligands
L) with an association constant (KM), and this binary
complex forms a ternary complex (MmLlS) with a sub-
strate (S) with association constant (KS). Then an intra-
complex nucleophilic substitution reaction in the rate-
limiting step takes place in this ternary complex with a
first-order rate constant (kN) to afford the products (P).
The products are also formed through the processes
The Effect of Complexing Time Between
Ligand and Metal Ion on Hydrolysis
of BNPP
(3)–(5). The processes can be expressed as
KM
The complexing time between N-methyldiethanol-
amine and Ce(III) in absolute ethanol has a remarkable
effect on the rate of the hydrolysis of BNPP, as shown
in Fig. 2. From Fig. 2, it can be seen that the observed
first-order rate constant increases with the increase in
[
MmL1]
mM + 1L ꢁꢀ MmL1 KM =
(1)
[M L S]
m
1
[M] [L]
KS
k
N
m
1
MmL1 + S ꢁꢀ MmL1S −→ P KS =
[
MmL1][S]
(
2)
k0
Table I Observed First-Order Rate Constants of the
Catalytic Hydrolysis of BNPP at 303 K
S −→ P
(3)
(4)
(5)
kM
3
−1
)
S + M −→ P
No.
Catalytic System
kꢂ (10 s
kL
11 [19]
1
2
3
4
Buffer
L
1.12 × 1– 0−
S + L −→ P
Ce(III)
0.170
12.2
where kN, k0, kM, and kL are the rate constants of the in-
tramolecular nucleophilic substitution reaction of the
ternary complex, the hydrolytic reaction of the sub-
strate in the pure solvent, and the reactions catalyzed
by metal ion and ligand, respectively.
3
+
CeL3
−
3
−3
Conditions: [Ce(III)] = 3.75 × 10 mol dm , [L] = 1.13 ×
−
2
−3
−5
−3
−3
10
mol dm , [BNPP] = 3.33 × 10 mol dm , 0.02 mol dm
+
pH 7.20 Tris-TrisH buffer.

690
YUANQIN ET AL.
Suppose v is the total rate of the hydrolytic reaction,
we have
Let
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
1
b1 c1
a1 b1 c1
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢁ
v = kꢂ [S]t = k [S] + kN[MmLlS]
(6)
(7)
ꢀ = ꢂ 1 b2 c2 ꢂ , ꢀ1 = ꢂa2 b2 c2 ꢂ (15)
0
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
1
b3 c3
a3 b3 c3
[
S]t = [S] + [MmLlS]
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
1
a1 c1
1
b1 a1
ꢂ
ꢂ
ꢂ
ꢂ
where [S]t and [MmLlS] are the total concentrations
of S and ternary complex at reaction time t, respec-
tively, kꢂ is the observed first-order rate constant,
ꢂ
ꢂ
ꢂ
ꢂ
ꢀ
2 = ꢂ 1 a2 c2 ꢂ , ꢀ3 = ꢂ 1 b2 a2 ꢂ (16)
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
1
a3 c3
1
b3 a3
ꢁ
k = k0 + kM[M]T + kL[L]L ([M]T and [L]T are the to-
0
From Eqs. (14)–(16), we have
tal concentrations of M and L, respectively).
From Eqs. (1), (2), (6), and (7), we have



x = ꢀ1/ꢀ
y = ꢀ2/ꢀ
z = ꢀ3/ꢀ
ꢁ
k + kN KS[MmL1]
(17)
0


kꢂ =
(8)
1
+ KS[MmL1]
Rearrangement of Eq. (8) leads to
From Eqs. (12) and (17), we have
kN = ꢀ/ꢀ1 + kꢁ
KS = ꢀ1/ꢀ2
KM = ꢀ1/ꢀ3
(18)
(19)
(20)
1
1
1
1
0
=
+
·
(9)
kN − kꢁ
KS(kN − k ) [MmL1]
ꢁ
kꢂ − k0
0
0
Inserting [M] = [M]T—m[MmLl] and [L] = [L]T—
l[MmLl] into Eq. (1) and neglecting the high powers
of [MmLl] leads to
Equations (18)–(20) are the general formulae for cal-
culating kN, KS, and KM, respectively. They are suit-
able not only for the hydrolytic reactions catalyzed by
mononuclear metal complexes but also for those cat-
alyzed by polynuclear metal complexes. Adopting this
method, the values of kN, KS, and KM can be estimated
with only three measurements of kꢂ . It is much sim-
pler than the other methods [20–22]. It does not require
a consecutive graphic method, and the values of the
parameters are more accurate.
In our experiment, m and l are 1 and 3, respectively.
When the observed first-order rate constants of the re-
actions with three different initial concentrations of
metal ion and ligand are obtained, the values of kN,
KS, and KM of the hydrolysis of BNPP catalyzed by
N-methyldiethanolamine-Ce(III) complex at pH 7.20
and 303 K can be calculated according to Eqs. (11),
(15), (16) and ( 18), (19). They are listed in Table II.
From Table II, we can see that the relative average er-
rors of kN, KM, KS are 10%, 6%, and 10%, respectively.
Therefore, Eq. (8) is validated.
1
1
1
=
+
+
ꢁ
kN − kꢁ
ꢁ
kꢂ − k
KS(kN − k )
0
0
0
ꢀ
ꢁ
m2
l
2
1
×
+
(10)
KM[M]T [L]T1
m
[
M]T
[L]T
Let
1
m2
l2
1
a =
,
,
b =
+
,
c =
[M]T [L]T
kꢂ − kꢁ
[M]
[L]T
m
1
0
(
11)
1
1
x ,
x =
z =
y =
=
KS
ꢁ
ꢁ
ꢁ
ꢁ
k − k
KS(k − k )
N
0
N
0
1
y
=
(12)
ꢁ
ꢁ
KS KM(k − k )
KM
N
0
Combining Eq. (10) with Eqs. (11) and (12) leads to
a = x + by + cz
(13)
The Effects of Surfactants
on the Hydrolysis of BNPP
The following equations can be obtained by changing
the initial concentrations of metal ion and ligand:
We have also studied the effects of Brij35 and CTAB
surfactants on the hydrolysis of BNPP catalyzed by
N-methyldiethanolamine-Ce(III) complex. These re-
sults are shown in Fig. 3 and 4. From Fig. 3, it can be
seen that Brij35 obviously accelerates the hydrolysis of



a1 = x + b1 y + c1z
a2 = x + b2 y + c2z
a3 = x + b3 y + c3z
(14)



A STUDY ON THE HYDROLYSIS OF BIS(NITROPHENYL)PHOSPHATE
691
Table II The Values of kN, KM, and K of the
S
Hydrolysis of BNPP Catalyzed by
N-methyldiethanolamine-Ce(III) Complex at pH 7.20
and 303 K
kN (s 1
−
)
KM (mol dm9) KS (mol dm3)
−3
−1
No.
8
1
2
3
32.2
31.5
34.3
32.7
1333
1200
1390
1307
1.96 × 10
1.67 × 10
2.26 × 10
1.96 × 10
8
8
8
Average
BNPP catalyzed by N-methyldiethanolamine-Ce(III)
complex when the concentration of Brij35 surfactant
is greater than its critical micelle concentrations (cmc)
−5
−3
(
[
for Brij35, cmc = 5.22 × 10 mol dm at 298 K
Figure 4 Plots of concentrations of CTAB vs. kꢂ of
3
+
23]). From Fig. 4, we can see that CTAB ob-
the catalytic hydrolysis of BNPP ([CeL ] = 3.78 ×
3
−
3
−3
viously inhibits the hydrolysis of BNPP catalyzed
by N-methyldiethanolamine-Ce(III) complex when
the concentration of CTAB is greater than its crit-
ical micelle concentrations (for CTAB, cmc = 9 ×
10 mol dm , 303 K, pH 7.20).
negative charges, and on the other hand they repel the
complex with positive charges, therefore, BNPP is sep-
arated from N-methyldiethanolamine-Ce(III) complex
due to the presence of CTAB micelles. This factor de-
creases the observed first-order rate constant of the hy-
drolysis of BNPP.
− −3
4
1
0
mol dm [24] at 298 K). The accelerating ef-
fect of Brij35 micelles on the catalytic hydrolysis of
BNPP might result from the local concentration effect
of the micelles. The solubilizations of BNPP and N-
methyldiethanolamine-Ce(III) complex in Brij35 mi-
cellar phase lead to their concentrations in the micellar
phase being greater than those in the bulk phase. This
result increases the observed first-order rate constant
of the hydrolysis of BNPP. When the concentration
of Brij35 goes beyond a certain value, the solubiliza-
tions of BNPP and N-methyldiethanolamine-Ce(III)
complex become saturated, consequently, the observed
first-order rate constant of the hydrolysis of BNPP
keeps constant. For CTAB micelles, the electrostatic
effect might be the main factor affecting the hydrol-
ysis of BNPP. On the one hand, the positive charges
on CTAB micellar surface strongly adsorb BNPP with
Dependence of Rate of the Catalytic
Hydrolysis of BNPP on pH
The pH is an important factor to affect the rate of the
catalytic hydrolysis of BNPP. The pH-rate curve of the
hydrolysis of BNPP catalyzed by N-methyldiethanol-
amine-Ce(III) complex is shown in Fig. 5. From Fig. 5,
we can see that the observed first-order rate constant of
the hydrolysis of BNPP increases in parallel with the
pH value before pH 7.2, when the pH goes beyond 7.2,
Figure 5 Plots of pH-rate for the catalytic hydrolysis
of BNPP by N-methyldiethanolamine-Ce(III) complex at
Figure 3 Plots of concentrations of Brij35 vs. kꢂ of
3+
−
3
−3
the catalytic hydrolysis of BNPP ([CeL ] = 3.78 ×
303 K ([Ce(III)] = 3.75 × 10
mol dm , [L] = 1.13 ×
3
−3
−3
−2
−3
−5
−3
1
0
mol dm , 303 K, pH 7.20).
10 mol dm , [BNPP] = 3.33 × 10 mol dm ).

692
YUANQIN ET AL.
it decreases due to the hydrolysis of Ce(III), indicat-
ing that the active nucleophile is the activated bound
water molecule in N-methyldiethanolamine-Ce(III)
complex.
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N-methyldiethanolamine-Ce(III) complex remarkably
accelerates the hydrolysis of BNPP in neutral pH
aqueous solutions. Activating a bound water molecule
through the cooperation of the ligand and metal ion is
essential to the rapid hydrolysis of BNPP. A general
quantitative treatment for modeling the catalytic reac-
tions involved metal complexes has been developed in
the present paper. This model may be used to obtain the
thermodynamic and kinetic parameters of the catalytic
reactions. The presence of surfactant micelles has an
obvious effect on the hydrolysis of BNPP. CTAB mi-
celles inhibit the catalytic hydrolysis of BNPP, while
Brij35 micelles promote it.
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