Kinetics and mechanism of promoted hydrolysis of 4-nitrophenyl acetate by Zinc(II)-tripod: Different roles of mononuclear and trinuclear species

Article abstract of DOI:10.1002/kin.10165

Coordination studies on Zn(II) complexes of 1,3,5-tri(2,5-diazahexyl)benzene (L) show that by comparison with the non-deprotonation of complex ZnL in a 1:1 system, the three-dimensional complexiaton decreases the pK_a of the Zn-bound water molec

Full text of DOI:10.1002/kin.10165

Kinetics and Mechanism
of Promoted Hydrolysis
of 4-Nitrophenyl Acetate
by Zinc(II)-tripod: Different
Roles of Mononuclear
and Trinuclear Species
1
1
2
1
1
YAN-HE GUO, QING-CHUN GE, HAI LIN, HUA-KUAN LIN, SHOU-RONG ZHU
1
Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
2
State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Nankai University, Tianjin 300071,
People’s Republic of China
Received 18 September 2002; accepted 8 August 2003
DOI 10.1002/kin.10165
ABSTRACT: Coordination studies on Zn(II) complexes of 1,3,5-tri(2,5-diazahexyl)benzene (L)
show that by comparison with the non-deprotonation of complex ZnL in a 1:1 system, the three-
dimensional complexiaton decreases the pKa of the Zn-bound water molecule, that is, pKa =
7
.47 for trinulclear complex Zn L in a 3:1 metal–ligand ratio. These two types of zinc(II) com-
3
plexes have been examined as catalysts for the hydrolysis of 4-nitrophenyl acetate (NA) in 10%
−
3
(
v/v) CH CN at 298 K, I = 0.10 mol dm KNO at pH range 6.5–8.2 and 8.5–10, respectively.
3
3
Kinetic studies show that the second-order rate constants of NA-hydrolysis catalyzed by com-
plexes ZnL, Zn L, and Zn LH are 0.021, 0.0082, and 0.342 mol dm s , respectively. In all
the cases, the pH-dependent observed first-order rate constant, kobs, shows sigmoidal pH–rate
−
1
3
−1
3
3
−1
profile. The 1:1 complex ZnL–H O undergoes NA hydrolysis by direct rate-determining hydroly-
2
−
sis to produce 4-nitrophenol(ate) (NP ) and ZnL(OOCCH ); while in the 3:1 system the oxygen
3
atom of acetic group forms a H-bond with the Zn(II)-bound water of the second branch of tri-
pod indicating that the polynuclear centers are associated and bi-functional. ꢀC 2003 Wiley
Periodicals, Inc. Int J Chem Kinet 36: 41–48, 2004
INTRODUCTION
Correspondence to: Hua-Kuan Lin; e-mail: hklin@nankai.
Zinc(II) is found at the active site of a large number of
edu.cn.
Contract grant sponsor: National Natural Science Foundation of
China.
metallo-enzymes, and most zinc enzymes are capable
of catalyzing the hydrolysis of such condensed bonds
as those in esters (both phosphate and carboxylate) and
amides (carboxypeptide and aminopeptide), control-
Contract grant number: 29971018.
Contract grant sponsor: Natural Science Foundation of Tianjin.
Contract grant number: 023605811.
c 2003 Wiley Periodicals, Inc.
ꢀ
ling the interconversion between CO and bicarbonate
2

4
2
GUO ET AL.
as well [1]. Thus, various types of zinc(II) complexes
have been designed to investigate or mimic the func-
tions of the central zinc(II) ions in bio-enzymes [2]. It
has shown that the unique roles that zinc(II) play are
concerning the Lewis acidity and coordination flexibil-
Synthesis
Synthesis of L [9] was neatly performed by conden-
sation of 1,3,5-benzene-tricarboxaldehyde [10] with 3
equivalents of N-methyl-ethylenediamine and subse-
quent reduction by NaBH4. Zinc complexes of L in
1
0
ity of its d configuration [3].
1:1 or 3:1 metal–ligand ratios were prepared by the
The low pKa value of Zn(II)-coordinated water in
enzymes (about 7 and 6 in carbonic anhydrase and car-
boxypeptidase respectively) is critical to an effective
hydrolysis process catalyzed by zinc(II)-containing
metalloenzymes as well as by mono- or multinuclear
model complexes [4], as it facilitates the formation
of the nucleophilic metal-bound hydroxide species at
physiological pH. On the other hand, several groups
studied some zinc compounds as enzyme models for
catalyzing the hydrolysis of carboxylic esters and phos-
phate esters, using external H2O or internal alcoholic
residues as nucleophiles to react with electrophilic sub-
strates [5–7]. All the model studies show that prior ac-
tivation of the nucleophiles is essential.
reaction of the tripod with corresponding amounts of
Zn(ClO4)2 in methanol.
Potentiometric Determination
Potentiometric titrations were carried out with an auto-
titrator Beckman pH meter Model ꢀ71. The apparatus
and procedure have been described before [10]. Com-
putation of equilibrium constants was done using the
TITFIT [11] program.
Kinetics of 4-Nitrophenyl Acetate
Hydrolysis
Cooperativity of metal ions at multinuclear active
sites is usually utilized to enhance the catalytic ca-
pacity of the model complexes, since the cleavage of
the ester bond in the center of enzymes relies on the
number of metal ions bound to each substrate. For
example, an early study of the promoted hydrolysis
A kinetic study was carried out by the UV spectral
method using a Shimadzu UV-160A spectrophotome-
ter equipped with a thermostatic cell (298 K). The hy-
drolysis rate of NA in aqueous solution was measured
by an initial slope method following the increase in 400
nm absorption of the released 4-nitrophenolate. The re-
action solution was maintained at 298 K and the ionic
strength was adjusted to 0.10 M with KNO3. Good’s
III
of diphosphate to orthophosphate by Co (tn)2(aq) re-
vealed that the reactive species at pH 7 involve a 3:1
III
rather than a 1:1 Co (tn)2–diphosphate complex [8].
−
3
buffer (20 mmol dm ) was used to maintain the pH
5]. To increase the solubility of NA, 10% CH3CN–
In the present paper we focus on the hydrolysis of
a simple carboxylic monoester 4-nitrophenyl acetate
[
H2O solution was used. For the initial rate determina-
tion, the following typical procedure was employed:
After NA and the complexes in 10% CH3CN solution
at the appropriate pH were mixed, the UV absorption
increase was recorded immediately (the reference ex-
periment did not contain the catalyst). The increase in
concentration of nitrophenolate was measured every 5–
60 s. The initial slope (<5% conversion) of a plot of
the measured absorbance versus time was determined
(correlation coefficient 0.99). All the experiments were
run in triplicate and the tabulated data represent the av-
erage of these experiments.
(
1
NA) catalyzed by the zinc(II) complexes of a tripod,
,3,5-tri(2,5-diazahexyl)benzene (L) in different stoi-
chiometries. Our interest was to understand functions
of polymetal ions in biological hydrolysis process.
EXPERIMENTAL SECTION
Materials
All reagents and solvents used were of analytical grade.
Acetonitrile (CH3CN) was distilled over calcium hy-
dride and stored in a dark bottle. 4-Nitrophenyl ac-
etate (NA) was recrystallized from dry diethyl ether.
RESULTS AND DISCUSSION
4
-Nitrophenol (pKa = 7.1), MES (2-(N-morpholino)-
enthanesulfonic acid, pKa = 6.39), MOPS (3-(morpho-
Active Nucleophile in 1:1 or 3:1 Zn/L
Systems
lino)-proanesulfonic acid, pKa = 7.2), HEPES (N-
ꢁ
1
7
-hydroxyethylpiperazine-N -2-ethanesulfonic, pKa =
ꢁ
.5), EPPS (N-2-hydroxyethylpiperazine-N -3-pro-
For tripod L, the use of benzene as spacer aims: (i)
to increase the hydrophobicity around zinc(II), (ii) to
widen the size of semicavity formed by tripod, and (iii)
to help binding of aromatic substrates by ꢀ–ꢀ stacking
interaction.
panesulfonic acid, pKa = 8.0), TAPS (N-tris(hydroxy-
methyl)methylamino)propamesulfonic acid, pKa =
8
.4), and CHES (2-(cyclo-hexylamino)-ethanesulfonic
acid, pKa = 9.3) were obtained from Aldrich.

PROMOTED HYDROLYSIS OF 4-NITROPHENYL ACETATE BY ZINC(II)-TRIPOD
43
The stepwise protonation constants of L are 10.98,
0.42, 9.52, 8.66, 6.49, and 5.89 respectively [10].
Potentiometric titrations indicate that the complexes
formed by Zn(II) with the tripod are performed at both
:1 and 3:1 metal–ligand molar ratios. In the 3:1 metal–
ligand ratio the stability constants are 47.27, 39.65,
8.89, and 21.42 for species ZnLH4, Zn2LH2, Zn3L,
produced in solution at 400 nm. The use of buffer for
the analysis was required since the pH of the solu-
tion would cause changes in the concentration of the
1
−
NP . To determine the rate constant one should know
−
1
the absorption coefficient of NP , which varies con-
siderably with pH values of the solution. In solution
p-nitrophenol hydrolysis as follow:
2
and Zn3LH−1 [10], whereas, the stability constants of
+
−
ZnLHm (m = 4–0) complexes in the 1:1 ratio are 40.92,
HNP = H + NP
(3)
(4)
3
2.10, 27.42, 19.18, and 9.75 (this paper), respectively.
ꢁ
−
+
K = [NP ][H ]/[HNP]
a
The increased stability of the former is contributed to
the polynuclear coordination, as shown in Scheme 1.
In Zn3L each branch of L coordinates with metal
ions using two secondary nitrogen atoms, while in ZnL
tripod L uses six nitrogen atoms or five nitrogen atoms
togetherwithawatermoleculetobindZn(II). However,
the Zn(II) bound water in M3L(310) is easy to deproto-
nate and give the monohydroxide M3LH−1 (31-1). The
pKa values corresponding to Eqs. (1) and (2), are 9.43
and 7.47 for ZnLH and Zn3L, respectively.
According to Beer’s law,
T
−
A = εobsb[HNP] = εNPb[NP ]
(5)
where εobs is the observed extinction coefficient of
HNP, εNP is the extinction coefficient of the NP an-
ion, b is the cell length, and A is the absorption of the
samples. From Eqs. (4) and (5), Eq. (6) is obtained:
−
ꢁ
+
εobs = εNP Ka/(K + [H ])
(6)
a
pK1 = log β111 − log β110
pK2 = log β310 − log β31-1
(1)
(2)
+
With the plot of 1/εobs versus [H ], absorption coef-
ficient εNP and K of 4-nitrophenolate at 400 nm in
.020 mol dm Good’s buffers were obtained with
ꢁ
a
−
3
0
−
1
3
−1
−8
The reason for the facilitation of the deprotonated
Zn(II)-water in complex Zn3L at neutral pH is at-
tributed to the formation of hydrogen-bound between
the Zn(II)-water of different metal centers. It is ob-
served that the low pKa of the coordinated water
molecule in Zn3L–H2O is almost the same value re-
the values of 17900 mol dm cm and 7.66 × 10
−
3
mol dm , respectively, which is consistent with the
reported value [12].
General Presentation of the Reaction
Rate
II
portedforthehydrolyticZn -enzymes[4]. Ontheother
hand, in the 1:1 Zn(II)/L system, the mono-protonated
Zn(II) complexe can release a proton to give neutral
complex at basic solution, which is also a fairly good
nucleophile.
Under our experimental conditions, the hydrolysis rate
increases linearly with the increased concentration of
[NA]. Thus, the hydrolysis is first-order in [NA]. The
rate v can be written as follow:
−
Molar Absorption Coefficient of NP
v = dcHNP/dt = dA/εbdt = kobs[NA]
where kobs is the observed rate constant.
(7)
The kinetic study of NA hydrolysis was carried out by
monitoring the amount of p-nitrophenolate ion (NP )
−
Scheme 1 The structure of complexes Zn3LH−1 and ZnL.

4
4
GUO ET AL.
The hydrolysis of NA is a combination of three fac-
Table I The k
Complexes
Values in the Absence of Metal
obs
a
tors: (1) kcom, the catalysis by metal complexes, (2) kOH,
−
the catalysis by OH , and (3) k0, the effect of solution,
−7 −1
s )
pH
kobs (10
so v should be represented by Eq. (8):
6
6
7
7
7
7
8
8
8
9
9
.54
.95
.23
.42
.61
.90
.19
.52
.84
.17
.58
0.86
1.53
2.82
3.83
5.71
12.08
21.42
47.03
86.32
187
446
T
v = kobs[NA] = (kcom[Zn–L]
−
+
kOH[OH ] + k0)[NA] (8)
where [Zn–L]T is the total concentration of metal
complexes.
As discussed earlier, there are two types of nucle-
ophile in the 3:1 system: Zn3L and Zn3LH−1, with the
equilibrium constant Ka . The concentration of Zn3L
2
and Zn3LH−1 can be represented by Eq. (9),
10.03
1330
ꢀ
ꢁ
ꢂ
a
◦
−3
−3
T
+
2
+
25.0 ± 0.1 C; I = 0.1 mol dm KNO
CN.
3
; 0.02 mol dm Good’s
[
Zn3L] = [Zn–L] [H ] Ka + [H ]
buffer; 10%CH
3
ꢀꢁ
ꢂ
T
2
2
+
[
Zn3LH−1] = [Zn–L] Ka
Ka + [H ]
(9)
According to kinetics theory of enzymatic catalysis,
Eq. (10) is then obtained.
and least squares fitting by plotting of reaction rate v
versus [NA] gives kobs (Table I). With the plot of kobs
−
−1
T
versus [OH ], as shown in Fig. 1, kOH = 12.41 mol
kcom[Zn–L] = kA[Zn3L] + kB[Zn3LH−1]
(10)
3
−1
−6 −1
dm s and k0 = 3.67 × 10
s [13] are obtained,
with R = 0.998. The small value of k0 is probably due
where kA and kB stand for the second-order hydrol-
ysis rates of NA catalyzed by Zn3L and Zn3LH−1,
respectively.
to the solvolysis of NA (i.e., water attack on the ester).
+
Thus, with the plots of kcom versus [H ], the values
Catalyzed by Complexes
of kA and kB and Ka2 can be obtained according to
For the conditions used, linearity in plots of ln(A/εb)
versus time is observed, with appearance of a short in-
duction period before the linear region and a fall-off
in slope at longer times. Thus, we found it satisfactory
Eq. (11a).
ꢁ
2ꢂꢀꢁ
ꢁ
2ꢂ
+
+
kcom = kA[H ] + kB Ka
[H ] + Ka
ꢀ
ꢂ
−
+
2
with the kinetics using the “initial rates” of NP pro-
=
kA + (kB − kA) 1 + [H ]/Ka
(11a)
duction [5]. The “initial rate” v is the slope of the line
(
Eq. 8), referring to the period immediately following
As regards 1:1 ZnL–H2O acting as a catalyst, since the
complex ZnLH has no catalytic efficiency,
the short induction period.
With the plots of v versus [NA], the pH-dependent
observed first-order rate constants kobs are obtained
over the 6.5–8.2 and 8.2–10 pH range for the 3:1 and
1ꢀꢁ
1ꢂ
+
kcom = k1 Ka
[H ] + Ka
(11b)
1
:1 complexes, respectively. For example, the kobs of
a plot of (kcom) 1 versus [H ] will give one a line ac-
cording to Eq. (11b), and then the second-order rate
constant k1 can be calculated.
−
+
hydrolysis of NA catalyzed by Zn3L and Zn3LH−1 are
shown in Table II.
On the basis of kobs at different pH and complex con-
centration listed in Table II, the kobs shows first-order
dependence with the total concentration of zinc(II)
complexes and increase linearly with the increase of
complex concentration (Fig. 2). Regression analysis
of the kobs versus concentration of complex allows
kcom to be obtained. In Table III the kcom values are
summarized.
−
Catalyzed by OH
As shown in Table I, in the absence of complexes hy-
drolysisdoestakeplace. ThisindicatesthatOH and/or
solvent can catalyze NA hydrolysis. In the absence of
zinc(II) complex, Eq. (8) can be written in the form of
Eq. (12):
−
The pH–dependent profiles of the rate constant kcom
(
Fig. 3) display sigmoidal curves for the catalytic hy-
−
v = kobs[NA] = (kOH[OH ] + k0)[NA]
(12)
drolysis NA by neutral Zn3L–H2O and monohydroxide

PROMOTED HYDROLYSIS OF 4-NITROPHENYL ACETATE BY ZINC(II)-TRIPOD
45
−
−
Figure 1 The plot of kobs vs [OH ] in the absence of metal complexes. The insertion is the plot of kobs vs [OH ] in the pH
range 6.5–8.2.
Zn3L-OH. Such pH profiles were observed in a number
of phosphate or carboxyl ester hydrolyses promoted by
Zn(II), Co(II), and Cu(II) complexes [5–7,14,15], in-
dicating the involvement of metal-hydroxide species.
Higher rates were observed for the catalysis by com-
plexes in the 3:1 ratio than by the 1:1 complexes. More-
over, the deprotonation constants pKa were obtained in
both cases. The pKa values resulted from the calcula-
tion are in good agreement with those determined by
pH titrations, that is 7.44 (calcd.) versus 7.47 (titra-
tion) for Zn3/L system and 10.04 (calcd) versus 9.43
(titration) for Zn/L system respectively.
+
Analysis of kcom versus [H ] (Eq. (11a)) gives the val-
−
1
3
ues of the second-order rates kA = 0.0082 mol dm
−
1
−1
3
−1
s
and kB = 0.342 mol dm s for NA hydrolysis
catalyzed by Zn3L and Zn3LH−1, respectively. At the
2
same time, pKa = 7.44 and R = 0.995.
For the 1:1 Zn(II)–L complex catalytic hydrolysis
reaction, according to Eq. (11b), the plot of (kcom)
Mechanisms
−
1
+
−1
1
−1
−1
versus [H ] gives pKa = 9.40, k1 = 0.021 mol
Based on the results mentioned above, referring to
the catalytic mechanism of carbonic anhydrase [16],
the catalytic mechanism for the 1:1 Zn(II)/L sys-
tems is proposed as shown in Scheme 2. The hy-
drolysis process involves the initial interactions of
NA with the coordinated water and then the dissoci-
ation of p-nitrophenolate and the binding of acetate
by nucleophiles. Consequently, the acetate:nucleophile
3
−1
dm s , and R = 0.998. It is observed that kcom
+
increases linearly with the increased [H ], which in-
dicates that the catalytic efficiency increases with the
increase of pH (Fig. 4). At the same time, the kcom in-
creases with the increase of the concentration of Zn–L
complex, implying that it is the Zn(II)-bound water that
acts as nucleophile.
Table II kobs (10 5
−
−1
s
) Values of NA Hydrolysis Catalyzed by Zn(II) Complexes in 3:1 Metal–Ligand Ratioa
pH
1
03 [Zn3–L]
6.54
6.95
7.23
7.42
7.61
7.90
8.19
0
1
1
2
.5
.0
.5
.0
0.53
0.93
1.48
2.14
2.45
3.27
4.44
5.59
6.35
10.41
13.60
15.52
10.08
17.93
25.80
34.83
16.67
26.28
38.55
52.37
22.91
34.08
52.14
71.31
30.09
43.27
66.54
83.15
a
–L] = 0.5, 1.0, 1.5, and 2.0 × 10−3 M; 25.0 ± 0.1 C, I = 0.1 mol dm KNO
◦
−3
−3
[
Zn
3
3
; 0.02 mol dm Good’s Buffer; 10%CH
3
CN.

4
6
GUO ET AL.
Figure 3 Plots of kcom vs pH for the catalytic hydrolysis of
NA by 3Zn–L complexes.
Figure 2 Plots of kobs vs [ZnL]T (3:1 Zn–L). The pH from
bottom to up is 6.54, 6.95, 7.23, 7.42, 7.61, 7.90, and 8.19
respectively.
the carboxyl oxygen from which an electron is partially
withdrawn by the metal ion. If it occurs, as shown in
Scheme 4, there will exist a pre-equilibrium step [17]
where the metal ion binds to carboxyl oxygen before
decompose into acetate and Zn(II)L–H2O complex in
aqueous solution and a catalytic cycle is completed.
On the other hand, the catalytic mechanism for the
−
the attack of OH (water) on the carboxyl carbon atom.
3
:1 system shown in Scheme 3 is similar to that of
As a consequence, the kobs will not increases linearly
with the increase of the concentration of metal com-
plexes (Table II), but experience a hyperbola kobs =
kcom[ML]/(1/K + [ML]). This hypothesis is in con-
trast to the results, as shown in Fig. 2. However, the
formation of H-bond between the polynuclear centers,
attributed to the fast hydrolysis of NA catalyzed by
the 1:1 system. The main difference is that in com-
plexes Zn3L and Zn3LH−1 the unsaturated coordina-
tion core of zinc(II) facilitates the double-functional
attack. It is reasonable for the carboxyl oxygen to form
H-bond with the proton of H2O coordinated in the sec-
ond zinc(II) ion, which decreases the negative electron
density on carboxyl oxygen and then weakens the re-
pulsion between the hydroxide and carboxyl oxygen.
3
:1 Zn(II) complexes (Scheme 3), is preferable [15].
−
Since the nucleophile Zn(II)-OH (H2O) and the H-
bond acceptor (H -OH) are not at the same metal cen-
−
For the Zn(II)-water(OH ) nucleophile:OCCH3-NP,
+
hydrogen bonding should constrain the acetate tightly
ter, the formation of H-bond and the nucleophilic attack
can occur simultaneously. Thus, the pre-equilibrium
step does not exist necessarily and the kobs changes
and thus facilitate the expulsion of the leaving group
−
(
NP ). The role of the H-bond in the hydrolysis of
ester is also found in the rapid catalytic hydrolysis of
ꢁ
ꢁ
2
,3 -cAMP by the Cu(II) complex of 2,9-diamino-o-
phenanthroline [15] that the amino group had hydrogen
bonds with the OH group of coordinated water.
−
When Zn3L–H2O or Zn3L-OH attack the carboxyl
group, it is not reasonable for the zinc(II) ion to bind to
Table III kcom (M 1
−
−1
s ) for 3:1 or 1:1 Zn(II)
Complexes Catalyzes the Hydrolysis of NA
pH
Zn–L (3:1)
pH
Zn–L (1:1)
6
6
7
7
7
7
8
.54
0.011
0.0211
0.0628
0.162
0.235
0.327
0.334
8.19
8.52
8.84
9.17
9.58
10.03
0.0012
0.0023
0.0041
0.008
0.0125
0.024
.95
.23
.42
.61
.90
.19
Figure 4 Plots of kcom vs pH for the catalytic hydrolysis of
NA by Zn–L complexes.

PROMOTED HYDROLYSIS OF 4-NITROPHENYL ACETATE BY ZINC(II)-TRIPOD
47
Scheme 2 The proposed mechanism of the hydrolysis of NA catalyzed by ZnL–H2O.
linearlywiththetotalconcentrationofmetalcomplexes
Eq. (8)). Moreover, one should keep in mind that it
(
is just the H-bond formed between two Zn(II)-bound
water that facilitates the deprotonation of the Zn(II)–
H2O with a lower pKa (7.44) compared with that of
ZnL–H2O.
Scheme 4 The proposed pre-equilibrium (K) step before
the attack of OH .
−
The enhancement of hydrolysis rate observed for
solutions containing 1:1 Zn(II) to L ratio was not sig-
nificant. With regard to ZnL, the almost saturated co-
ordination core occupied by amino nitrogens leads to
steric difficulty in catalyzing by the fashion as that
of Zn3L and Zn3LH−1. In contrast, it prefers to at-
tack the carboxyl group directly by the water molecule
−
formation of the stronger nucleophile Zn3L-OH ; (2)
the less steric effect than that in the facial coordinated
:1 system; and (3) the cooperation of the polynuclear
1
centers by the formation of H-bond, which neutralizes
the negative charge of acetate and stabilize the nucle-
ophile:acetate group.
Compared with other model complexes, the second-
order rate constant of the hydrolysis of NA, especially
for catalyst Zn3LH−1 (0.342 mol dm s ), is ob-
viously larger than the corresponding value of 0.047
mol dm s for the [12]aneN3Zn(II)OH complex
5]. This study shows that Zn(II)-OH complexes of
tripod can effectively catalyze the hydrolysis of NA.
These examples, together with those demonstrating the
importance of multinuclear metal complexation for di-
and triphosphates, are relevant to the roles of metal
ions in biological phosphoryl transfer, as in the case of
Escherichia coli alkaline phosphatase, where the ac-
tive site of the enzyme contains a triplet of metal ions
(
Scheme 3), whose nucleophilic ability is actually
−
weaker than that of Zn3L-OH [18]. Furthermore, a
possible increase in coordination number of zinc may
radically change the conformation of the complex thus
making it difficult for Zn(II)-OH2 to interact with the
substrate.
−
1
3
−1
−
1
3
−1
−
A combination of factors, then, can account for the
enhanced hydrolysis catalyzed by the 3:1 metal com-
plexes rather than the 1:1 complexes: (1) the lower pKa
of zinc(II) coordinated water in Zn3L facilitating the
−
[
[
19].
In summery, the results provide evidence for the
importance of a reactive 3:1 zinc to tripod complex
Zn3L and Zn3LH−1) by comparison of rates and by
(
establishing the mechanism of the 1:1 catalyst, ZnL.
The highly efficient catalysis of tripod complexes re-
ported in this paper may provide a clue for the design
of the hydrolytic enzyme through the combination of
different kinds of donor groups, such as the organic
mixture of hard and soft bases. One can vary the co-
ordination number, or can change the microenviron-
ment around the metal center in a homogeneous fash-
ion to achieve faster reaction rate. For example, the
environment around coordinated water molecules can
be changed from hydrophilic to hydrophobic. Work in
this area is underway.
Scheme 3 The proposed mechanism of the hydrolysis of
NA catalyzed by Zn3L and Zn3LH−1.

4
8
GUO ET AL.
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