The Metallomicelle of Lanthanide Metal (Ce, La) Aza-Macrocyclic Complexes with a Carboxyl Branch: The Catalytic Activity and Mechanism in the Hydrolysis of a Phosphate Diester

Article abstract of DOI:10.1007/s10953-014-0207-y

An aza-macrocyclic ligand, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-N^/- acetic acid (L), was synthesized and characterized. The chemical composition of the complexes (CeL and LaL) was determined by a UV-Vis spectrophotometric

Full text of DOI:10.1007/s10953-014-0207-y

J Solution Chem (2014) 43:1331–1343
DOI 10.1007/s10953-014-0207-y
The Metallomicelle of Lanthanide Metal (Ce, La)
Aza-Macrocyclic Complexes with a Carboxyl Branch:
The Catalytic Activity and Mechanism in the Hydrolysis
of a Phosphate Diester
Fang-zhen Li
•
Famei Feng
•
Lan Yu Jia-qing Xie
•
Received: 19 January 2014 / Accepted: 12 May 2014 / Published online: 6 August 2014
Ó Springer Science+Business Media New York 2014
Abstract An aza-macrocyclic ligand, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-
/
azacyclotetradecane-N - acetic acid (L), was synthesized and characterized. The chemical
composition of the complexes (CeL and LaL) was determined by a UV–Vis spectropho-
tometric method and kinetic Job plot. Two metallomicellar systems made of an lanthanide
metal complex and n-lauroylsarcosine (LSS) micelle were used as catalyst in the hydro-
lysis of bis(4-nitrophenyl) phosphate ester (BNPP), and their catalytic activity was studied
by the comparative method. The interaction between the complex and BNPP in the met-
allomicelle was studied by its fluorescent spectroscopy. The catalytic rate of BNPP
hydrolysis was measured kinetically by UV–Vis spectrophotometry. The results indicate
that CeL systems (aqueous solution and metallomicelle) exhibit higher catalytic activity
than those of the LaL systems in BNPP hydrolysis, and the micelle provides an effective
catalytic environment in the catalytic reaction. A reaction mechanism was also proposed
on the basis of the results.
Keywords Metallomicelle Á Lanthanide complex Á Carboxyl branch Á Catalytic activity Á
BNPP hydrolysis
1
Introduction
Phosphate ester specific hydrolysis plays an important role in the metabolic processes of
living organisms. Research on artificial phosphate diesterases is an important subject in the
F. Li Á L. Yu Á J. Xie (&)
College of Chemistry and Chemical Engineering, Chongqing University of Technology,
Chongqing 400054, People’s Republic of China
e-mail: xjq8686@163.com
F. Feng
College of Chemistry & Pharmaceutical Engineering, Sichuan University of Science & Engineering,
Zigong 643000, Sichuan, People’s Republic of China
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fields of molecular biological technology and drug development. Many researchers have
been working hard to develop biomimetic models of metalloenzyme with high efficiency
and selectivity [1–4]. A variety of different synthetic catalysts for hydrolysis of DNA and
the phosphate diester have been reported, usually based on lanthanide or transition metal
ions [5–9].
The investigations indicate that lanthanide complexes are generally more active than
other transition metal complexes due to the extremely strong Lewis acidity of a lanthanide
cation [10, 11]. Previous studies revealed that the active species was the specific hydroxo-
lanthanum(III) complex in alkali solution [10, 12]. Many lanthanide(III) complexes are
highly active at catalyzing phosphodiester hydrolysis in alkali solution, but not in acidic
solution [12]. These results were also obtained in our experiments. The major drawback of
the hydroxo lanthanum(III) complex system is its very low stability in alkaline solution. A
possible solution to this problem would be to use a lanthanum(III) complex with appro-
priate ligands as catalyst. The investigation also indicated that the structure of the ligand of
the complex plays a key role in designing the mimetic model of the phosphate hydrolase
[13–18]. The aza-macrocyclic ligand is considered as a potential mimetic enzyme because
of its recognition of guests and hosts.
On the other hand, as an enzymatic model, we consider not only the structure of the
active center, but also the microenvironment around the active center [19]. X-ray crys-
tallographic studies showed that the structures of micelles and globular proteins and the
substrate-binding capability of micelle and enzyme are similar [20], so micelles have often
been employed to simulate the microenvironment of enzymes [21]. Based on the factors
above, two lanthanide metal aza-macrocyclic complexes with a carboxyl branch were
chosen as the catalytic activity center, and the metallomicellar system made of the lan-
thanide(III) complex and LSS micelle was designed as a mimetic model of the phosphate
hydrolase for the hydrolytic cleavage of bis(4-nitrophenyl)phosphate (BNPP). The primary
goals of this work are to explore the relation between the catalytic activity of the metallo-
micellar system and the complex structure, pH of the system, and micelle, and to find better
catalytic systems and catalytic conditions for the mimetic phosphate hydrolases.
2
Experimental
2
.1 Instrumentation and Materials
Fluorescence spectra were measured using a Cary Eclipse spectrofluorophotometer (Agi-
lent Technologies Co. USA.). The elemental analysis was performed on a Carlo Erba 1106
elemental analyzer (Carlo-Erba Co. Italy). The pH of the solution was determined by using
a Radiometer PHM 26 pH meter with G202C glass and K4122 calomel electrodes
(Shanghai Photics Apparatus Co. China). The melting point was determined on a Yanaco
MP-500 micro-melting point apparatus (Yanaco-Mat Co. USA.) and is uncorrected. The
kinetic study was carried out and UV–Vis absorption spectra were recorded using a GBC
9
16 UV–Vis spectrophotometer equipped with a temperature holder (Australia).
The n-lauroylsarcosine (LSS, purity C 95.0 %), trishydroxy-methylaminomethane
(Tris, purity C 99.5 %), and bis(p-nitrophenyl) phosphate (BNPP, purity [ 98.0 %) were
purchased from Sigma Chemical Co. Other reagents used in the experiments, unless
otherwise indicated, were of analytical grade (purity C 99.0 %), and were purchased from
Chongqing Chemical Co. The water used for kinetic experiments was doubly distilled.
1
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J Solution Chem (2014) 43:1331–1343
1333
Fig. 1 The structure of the aza-
macrocyclic ligand
H C
3
CH3
O
H C
3
OH
NH
NH
N
2
HBr.H O
2
HN
CH3
CH3
CH3
L
2
.2 Synthesis of the Ligand (L)
An aza-macrocyclic ligand, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-
’
N -acetic (L), was synthesized according to the literature method [22]. Melting point:
-
I
?
2
2
35–236 °C. IR (KBr, cm ): 3400 (OH); 3200 (NH); 2650–2900 (NH ); 1730 (COOH).
‘
(
H NMR (D O): d 0.95–1.04 (3H, d, CH -C–N), 1.13–1.20 (3H, d, CH -C–N), 1.30–1.41
2 3 3
12 H, t, CH -C–N), 1.72–1.93 (4 H, m, C–CH -C), 2.85 (2H, s, CH COOH), 2.90–3.20 (8
2
3
2
H, m, CH -N), 3.64 (1 H, s, C–CH-N), 3.85 (1 H, s, C-CH-N). Analysis, calculated for
2
C H Br N O (H O): C, 41.39; H, 8.10; N, 10.73. Found: C, 41.38; H, 8.09; N, 10.57.
42
18
2
4
3
2
The structure of the ligand (L) is shown in Fig. 1.
2
.3 Preparation of the Solutions and Metallomicelle of the Lanthanide
Metal Aza-Macrocyclic Complex
-
2
-3
A concentrated solution (5 9 10 molÁdm ) of lanthanide metal nitrate and the ligand
was prepared at pH = 8.0. The lanthanide metal aza-macrocyclic complex solution of
5
-
3
-3
9 10 molÁdm was prepared by mixing the concentrated lanthanide metal nitrate
(
10 mL) and the concentrated ligand solution (10 mL) at 100 °C and stirring for 10 h.
Then the mixture solution was transferred to a volumetric flask of 100 mL, and diluted
with trishydroxy-methylaminomethane (Tris) buffer solution of pH = 8.0.
-2
-3
The concentrated surfactant LSS solution of 4.6 9 10 molÁdm was prepared in Tris
buffer solution of pH = 8.0. The surfactant LSS of the desired concentration was prepared
by adding the concentrated surfactant LSS solution to the buffer solution of pH = 8 with
-
5
-3
stirring. The metallo-micellar system of 5 9 10 molÁdm was prepared by adding the
-
3
-3
lanthanide metal aza-macrocyclic complex solution of 5 9 10 molÁdm (1 mL) to LSS
solution of the desired concentration (100 mL) and pH = 8 with stirring.
Stock solutions of BNPP and p-nitrophenyl phosphate (NPP) were prepared at
-
2
-3
5
.0 9 10 molÁdm in water. The final concentration of the substrate (BNPP and NPP)
-4
-3
was 5.0 9 10 molÁdm in the kinetic experiments of hydrolysis of phosphate ester.
2
.4 Analysis of the Characteristic Spectrum
The UV–Vis spectra of the aza-macrocyclic ligand and the mixture solution (Ln(NO ) and
3
3
the ligand) were examined by UV–Vis absorption spectrophotometry in the wavelength
3
range 190–500 nm and at pH = 8.0, 25 °C. Tris buffer solution (3 cm ) of pH = 8.0 was
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J Solution Chem (2014) 43:1331–1343
added to the reaction cuvette and the reference cuvette, respectively, and then the absor-
bance of the solution was set to zero. The absorbance measurement of the solution was
-
3
-3
initiated by injecting 30 lL of the ligand or the mixture solution (5.0 9 10 molÁdm
)
into the reaction cuvette. The fluorescence change of the metallomicelle system upon
-
6
-3
addition of BNPP was monitored at pH = 8.0, 25 °C, [complex] = 5.0 9 10 molÁdm
,
-
6
-3
[
BNPP] = 5.0 9 10 molÁdm , kex = 254 nm, kem = 290–490 nm, voltage = 650 V,
and slit width = 5 nm.
2
.5 Kinetic Measurement Method
Kinetic studies were carried out using a GBC 916 UV–Vis spectrophotometer equipped
-5
3
with a temperature controlled cell holder. The metallomicellar solution (3 cm , 5 9 10
-
3
3
molÁdm ) was added to the reaction cuvette and reference cuvette (4 cm ), respectively,
and then the absorbance of the solution was set to zero. The reaction was initiated by
-3
)
-
2
injecting 30 lL of the substrate (BNPP and NPP) stock solution (5.0 9 10 molÁdm
into the reaction cuvette. The characteristic spectrum of p-nitrophenol (the product of
BNPP and NPP hydrolysis) was detected at the wavelength of 400 nm. Therefore, the
kinetics of the substrate cleavage was monitored by the absorbance change of the p-
nitrophenol at 400 nm. To obtain reliable trends of BNPP and NPP catalytic hydrolysis, the
testing was performed on one day with the same reaction cuvette and stock solutions. The
catalytic hydrolysis was observed to be a first-order reaction. The observed first-order rate
constant (kobs) of BNPP catalytic hydrolysis was obtained by the mapping method with the
equation ln(A? - A ) = k t ? ln(A? - A ). A , A and A are the initial absorbance,
t
obs
0
0
t
?
absorbance at time t and final absorbance of the product at 400 nm. The data were obtained
from the kinetic curves followed up to 95 % or higher conversion of the substrate.
3
Results and Discussion
3
.1 Composition of the Binary Complex in the Metallomicelle
The UV–Vis spectra of the aza-macrocyclic ligand and the mixture solution (Ln(NO ) and
3
3
the ligand) are shown in Fig. 2. From this figure, it can be seen that the characteristic peak
(
k = 201 nm) of the ligand is lower in the mixture solution than in the ligand solution.
Generally, the change of UV–Vis absorption intensity in the mixture solution can be used
to confirm the binding between metal ions and ligands. This may be ascribed to the energy
transfer between metal ions and ligand. Apparently, the results in Fig. 2 indicate the
formation of new complex Ln L between lanthanide metal ions and the ligand in the
x
y
mixture solution.
-1
To determine the chelating stoichiometry of metal complex, kobs (s ) of BNPP cata-
lytic hydrolysis on different the mole fraction (x) of metal ion was examined and is shown
in Table 1. The relationship (kinetic Job’s plot) between the observed rate constants (kobs
)
and the mole fraction (x) is plotted at constant total concentration of the metal ion and
ligand [23] and the results are shown in Fig. 3. It be seen that the x value corresponding to
the maximum kobs is about 0.5 for the two metal complexes investigated at the desired pH,
which indicates that the 1:1 complex (x:y = 1:1 in complex Ln L ) is the active species,
x
y
and this result also verifies the presence of the metal complex. Because the coordination
number of a lanthanide metal ion is usually 8 or 9, and one molecule of the ligand bromide
can supply only 7 donor atoms to the lanthanide metal ion, the binary complex contains at
1
23

J Solution Chem (2014) 43:1331–1343
1335
0
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
ligand
3
+
La +ligand
3
+
Ce +ligand
190
200
210
220
230
240
250
λ
(nm)
3?.
Fig. 2 Absorption spectra of ligand with increasing amounts of Ce
3?
25.00 °C, pH = 8.0, [La ] =
3
?
-5
-3
[Ce ] = [ligand] = 5 9 10 molÁdm
least two water molecules directly coordinated to the lanthanide metal ion. Therefore, the
complex LnL in the metallomicellar system as catalyst can coordinate water molecules and
then form the hydrated complex LnLBr (OH)(H O) or LnLBr (H O) or LnLBr (OH) at
2
2
2
2
2
2
2
different acidities.
3
.2 The Comparison of the Catalytic Activity of Different Systems
To analyze the product of BNPP catalytic hydrolysis, the spectra of the standard sample of
p-nitrophenol and the product of BNPP were measured in the metallomicelle of CeL; these
are shown in Fig. 4. The experimental results show that the standard p-nitrophenol has a
strong absorption peak at 400 nm in the metallomicelle. From Fig. 4, it can also be seen
that the absorbency of kmax (290 nm) of BNPP decreased and the absorbency of kmax
(400 nm) of hydrolytic product increased with passage of time. The results indicate that
one of the products of BNPP is p-nitrophenol.
The catalytic activity of different systems is shown in Table 2. The experimental results
show that the ligand and LSS have no activity. From Table 2, it can be seen that the rate of
7
BNPP catalytic hydrolysis is accelerated by about 2 9 10 or 2 9 10 times those for CeL
6
9
or LaL aqueous solution and by about 1 9 10 or 1 9 10 times in CeL or LaL metal-
8
lomicelles, respectively, when compared with the rate of BNPP spontaneous hydrolysis
[
24]. Obviously, the catalytic activity of the CeL system is higher than that of the LaL
system. This can be attributed to differences of the central metal ion radius in the two
systems. Although the ligands of the complexes are the same in the two catalytic systems,
the metal ions are different and then the Pauling radius of the metal ion Ce(III) is smaller
than that of the metal ion La(III) due to the lanthanide contraction. The smaller the metal
ion radius is, the stronger is the interaction between the metal ion and BNPP molecule in
the reaction, which is conducive to stability of the intermediate containing BNPP and the
complex molecule. Therefore, the catalytic efficacy of the CeL system is higher than that
of LaL system.
Moreover, the experimental results also indicate that the rate of BNPP hydrolysis cat-
alyzed by LnL complex in the metallomicelle system is about fifty times faster than that in
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23

J Solution Chem (2014) 43:1331–1343
1337
Fig. 3 Job’s plot of BNPP
catalytic hydrolysis (filled circle
LaL catalysis, filled down
0
.012
0.010
0.008
pointing triangle CeL catalysis):
-4
3
?
[
L] ? [Ln ] = 1 9 10
-3
-4
molÁdm , [BNPP] = 5 9 10
0
.006
-3
molÁdm , pH = 8.0, 25.00 °C,
3
x = [Ln ]/[Ln ] ? [L]
?
3?
0.004
0
0
.002
.000
-
0.002
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
r
1
0
.0
.8
Fig. 4 The spectral scan plot of
p-nitrophenol and the product of
BNPP catalytic cleavage in the
metallomicelle of CeL at
0.8
.6
0.4
p-nitrophenyl
0.6
0.4
0
2
5.00 °C, pH = 8.0,
-4
-3
[
[
LSS] = 4.95 9 10 molÁdm
,
-
4
BNPP] = 5.0 9 10
0
.2
-3
-5
molÁdm , [CeL] = 5.0 9 10
-3
0.0
molÁdm , [p-
0
0
.2
.0
-5
nitrophenol] = 5.0 9 10
250
300
350
400
450
500
-3
λ
(nm)
molÁdm
hydrolysis of BNPP
250 300 350 400 450 500
λ
(nm)
-1
Table 2 kobs (s ) of BNPP hydrolysis catalyzed by the different systems
System Ce complex La complex Ce complex ? LSS La complex ? LSS
H
2
O
-
1
-4
-5
-2
-3
-11
k
obs (s
)
2.72 9 10
2.35 9 10
1.16 9 10
1.25 9 10
1.2 9 10
[24]
and
-
4
-3
-4
-3
2
[
5.00 °C, pH = 8.0, [LSS] = 4.95 9 10
LnL] = 5.0 9 10 molÁdm
molÁdm
,
[BNPP] = 5.0 9 10
molÁdm
,
-5
-3
the aqueous solution under the same conditions. It is well known that micelles play very
important roles in the catalytic process owing to the effect of the high local concentration
[
25]. The zwitterionic surfactant LSS (isoelectronic point of 5.2 [26]) shows anionic
characteristics in the range of pH investigated in this paper, so the complex is solubilized
easily in the Stern layer of LSS micelles because of the electrostatic attraction between the
electropositive complex and the anionic micelle (see Scheme 1). This will increase the
local concentration and the collision frequency of the reaction molecules due to the sol-
ubilizing effect of the LSS micelle, and so facilitate the BNPP catalytic hydrolysis.
3
.3 The Acid Effect and Real Active Species
The experimental results at various pH values are shown in Table 3 and Fig. 5. From this
figure, it can be seen that the pH-rate curve presents a ‘‘bell–shape’’ profile with the acidity
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J Solution Chem (2014) 43:1331–1343
Scheme 1 The interaction
solubilization effect of LSS
micelle in the metallomicelle
H C
3
CH3 Aqueous
O
H C
3
bulk phase
N
N
N
O
Br
H O
2
Ln
N
OH
CH3
N
N
CH3
Br CH3
Stern layer
N
OH
N
2
H O
2
H O
2
H O
Corn
change of the catalytic system from pH = 7.0 to 9.0, and the best pH values of the BNPP
catalysis are 8.2 and 8.0 in the LaL and CeL metallomicelles, respectively. This indicates
that the kobs of BNPP catalytic hydrolysis is correlated to the acidity of the reaction system.
The experiment results also show that the metallomicelle is remarkably stable over a week
at pH = 8.0. Therefore, in spite of the optimal acidity for stabilization of the metallomi-
celle being at pH = 7.0 in the measured range of pH values, the major experiment of
BNPP catalytic hydrolysis was performed at pH = 8.0 because the activity of the metal-
lomicelle is much higher at pH = 8.0 than that at pH = 7.0.
From the structure of the lanthanide metal complex, the ionization of H O in the
2
complex can be induced by the acidity of the metallomicelle system. Therefore, the
complex exists in three different protonated states through the first and second acidic
dissociation of H O molecules coordinated to lanthanum(III) ion in solution. The two-
2
hydroxy complex LnLBr (OH) is easily generated in the strongly alkaline solution, and
2
2
the hydrated complex LnLBr (H O) is easily generated in the strongly acidic solution, but
2
2
2
both are unfavorable for the formation and stabilization of the mono-hydroxy complex
LnLBr (H O)(OH). So, the mono-hydroxy complex is most likely to be the active species
2
2
in BNPP catalytic hydrolysis since the maximum kobs of the pH-rate curve in Fig. 5 is at ca.
pH = 8.0 or 8.2.
3
.4 Mechanism of BNPP Hydrolysis Catalyzed by the Metallomicelle
To clarify the interaction between the complex and BNPP in the metallomicelle, the
fluorescence spectra of the cerium complex was determined and analyzed in the presence
and absence of BNPP. The fluorescence spectra of the cerium complex under different
conditions are shown in Fig. 6. This figure shows that the emission intensities of the CeL
complex decrease with increasing concentration of BNPP. The obvious decrease of fluo-
rescence intensity of the complex caused by BNPP indicates strong binding between the
complex and BNPP molecules, since the binding can lead to the photoelectron transfer
from BNPP to the excited state of the complex [27] and then cause a configuration change
of the complex.
Because one mole of BNPP can liberate two moles of p-nitrophenol, the catalytic
hydrolysis rate of the p-nitrophenyl phosphate ester (NPP) is examined in the
1
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J Solution Chem (2014) 43:1331–1343
1339
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J Solution Chem (2014) 43:1331–1343
0
0
0
0
0
0
0
0
0
.012
.011
.010
.009
.008
.007
.006
.005
.004
Ce complex
0.0014
0
0
0
0
0
0
.0012
.0010
.0008
.0006
.0004
.0002
La complex
6.9 7.2 7.5 7.8 8.1 8.4 8.7 9.0
pH
6.9
7.2
7.5
7.8
8.1
8.4
8.7
9.0
pH
Fig. 5 pH-rate profile for BNPP catalytic hydrolysis in the metallomicelle system: 25-.300 °C,
-
4
-3
-4
-3
-5
[
LSS] = 4.95 9 10 molÁdm , [BNPP] = 5.0 9 10 molÁdm , [LnL] = 5.0 9 10 molÁdm
Fig. 6 Fluorescence spectra of
the complex with increasing
amounts of BNPP: pH = 8.0,
t = 25.00 °C,
8
7
00
00
-
---Ce complex
600
500
400
-
6
[
complex] = 5.00 9 10
-3
molÁdm , kex = 254 nm, 650 V
------BNPP:complex=1:2
and 5 nm slit
3
2
1
00
00
00
0
------BNPP:complex=2:1
-
100
2
50
300
350
400
450
500
λ (nm)
metallomicelle of the lanthanide(III) metal complex. Fast hydrolysis of the monoester has
been confirmed by a separate experiment on NPP hydrolysis. The experimental results
show that hydrolysis rate of the p-nitrophenyl phosphate ester (NPP) is ten times higher
than that of BNPP. Therefore, it may be deduced that the release of the first p-nitrophenol
molecule of BNPP is the rate-determining step in BNPP catalytic hydrolysis.
According to the experimental results and the acid effect of BNPP catalytic hydrolysis
above, the mechanism of BNPP hydrolysis catalyzed by the title complex is proposed as
Scheme 2. This Scheme shows that the complex binds the phosphodiester through elec-
trostatic interaction between lanthanide(III) metal ion center and the BNPP molecule,
which is proved by the obvious change of fluorescence intensity of the complex in the
fluorescence experiment, and this will facilitate the formation of the reaction intermediate
containing BNPP and the complex LnLBr (OH)(H O) in the metallomicelle. In this pro-
2
2
cess, the intermediate is stabilized and the negative charge of the substrate molecule is
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J Solution Chem (2014) 43:1331–1343
1341
CH3
CH3
Br
H C
H C
3
3
H C
H C
3
3
Br
NO2
NO2
N
N
N
N
N
N
N
N
O
P
Br
O
OH
K
Br
O
O
Ln
Ln
O
+
BNPP
O
O
H
S
O
O
O
H
H
CH3
CH3
H C CH3
H C CH3
LnLS
3
3
LnL
slow
kcat
fast
H PO
3
4
H O
H O
2
2
CH3
CH3
Br
H C
H C
3
H C
3
OH
3
H C
OH
3
Br
N
N
N
N
N
N
N
O
O
+
OH
fast
H O
OH
Br
O
Br
O
Ln
+
Ln
O P
O P
NO2
2
OH
O
N
NO2
O
O
H
O
O
NO2
H
CH3
CH3
H C CH3
3
H C CH3
3
Scheme 2 The reaction pathway of the BNPP catalytic hydrolysis
dispersed by the coordination of phosphoryl oxygen and lanthanide metal ion. Then the
intramolecular Ln(III)–bound OH, as a nucleophile, attacks the electropositive P atom of
the BNPP molecule, which is the rate-determining step of the whole reaction. Scheme 2
also shows that the second p-nitrophenol is released rapidly and the catalyst is rapidly
regenerated in the BNPP catalytic hydrolysis cycle. Furthermore, the intermediate of the
transition state of the reaction can also be stabilized by interaction with the polar head
groups of the micelle [28], which is conducive to the stability of the intermediate.
The relationship between the kobs and kcat can be obtained by the method described
previously [29]; that is 1/kobs = 1/kcat ? 1/K kcat [LnL]. Here, kcat is first order rate con-
stant and is pH-dependent, K is the association constant between LnL and the BNPP
molecule, and [LnL] is the concentration of the catalyst.
4
Conclusion
In summary, two metallomicellar systems made of the lanthanide metal complexes and n-
lauroylsarcosine (LSS) micelle can promote BNPP cleavage, and exhibit high catalytic
activity in the hydrolysis of BNPP at 25 °C; the catalytic efficiency of the metallomicellar
system is dependent on the pH value of the reaction system, and the best catalyst acidity for
the CeL and LaL metallomicellar systems are pH = 8.0 and 8.2, respectively; the LSS
micelle palys two important roles in the BNPP catalytic hydrolysis, one is to promote the
solubilization of the solute and the other is to stabilize the intermediate containing BNPP
and the complex molecule.
Acknowledgments The authors gratefully acknowledge financial support from Chinese National Natural
Science Foundation (21173274) and Major Science and Technology Project of Zigong city (2013X03), and
the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education
(LYJ1303).
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