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Metallomicellar Catalysis. Effects of Bridge-Connecting Ligands on the Hydrolysis of PNPP Catalyzed by Zn(II), Co(II), and Ni(II) Complexes of Ethoxyl-diamine Ligands in Micellar Solution Jiang Fubin, Jiang Bingying, Yu Xiaoqi, and Zeng Xiancheng* Department of Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China Received February 4, 2002. In Final Form: June 5, 2002 The syntheses of three ligands are reported: N,N,N′,N′-tetra(2-hydroxyethyl)-1,3-propylene-diamine (1), N,N,N′,N′-tetra(2-hydroxyethyl)-1,10-decadiamine (2), and N,N,N′,N′-tetra(2-hydroxy-ethyl)-1,4xylyldiamine (3). The catalytic hydrolysis of p-nitrophenyl picolinate (PNPP) by bivalent metal ions Zn(II), Ni(II), and Co(II) was studied kinetically in the buffered CTAB and Brij35 micellar solution at 25 °C and different pH values. The results indicate that 1:2 and 2:1 complexes of these ligands and metal ions in CATB and Brij35 micellar solution are the active species for the catalytic hydrolysis of PNPP, respectively. A ternary complex kinetic model for metallomicellar catalysis was employed to interpret the results to obtain relative kinetic and thermodynamic parameters. The comparison of these parameters indicates that the catalytic activity of different metal complexes catalyzed hydrolysis of PNPP depends on the rigidity of the bridge-connecting ligand and the microenvironment. The effect of the structure of the ligands on the hydrolytic reaction of PNPP has been discussed in detail.
Introduction Micellar catalytic hydrolysis of carboxyl acid esters has been extensively investigated in the past 2 decades.1 As far as the mimetic model of catalytic function is concerned, the investigations of Cu2+, Zn2+, Ni2+, and Co2+ complexes as hydrolytic metalloenzymatic models have been reported a lot.2,3 They show good catalytic properties in the hydrolysis of p-nitrophenyl picolinate (PNPP). And in these hydrolytic processes, the transition metal ions play a very important role;4-9 several research groups developed some binuclear metal complexes that show significantly higher catalytic activity as an artificial hydrolytic enzyme model.10-15 In previous studies on the catalytic hydrolysis of carboxyl acid esters, we have investigated quantitatively many kinds of metallomicelles composed of various bivalent transition metal complexes in different micellar * To whom correspondence may be addressed. E-mail: zengxc@ pridns.scu.edu.cn. (1) Fendler, J. H., Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Fujita, T.; Inaba, Y.; Ogino, K.; Tagaki, W. Bull. Chem. Soc. Jpn. 1988, 61, 1661. (3) Fornasier, R.; Scrimin, P.; Tecilla, P.; Tonellato, U., J. Am. Chem. Soc. 1989, 111, 224. (4) Sigman, D. S.; Jorgensen, C. T. J. Am. Chem. Soc. 1972, 94, 1724. (5) Breslow, R. Acc. Chem. Res. 1995, 28, 146. (6) Hampl, F.; Liska, F.; Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 1999, 15, 405. (7) Bunton, C. A.; Scrimin, P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 1996, 2, 419. (8) Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 2000, 16, 227. (9) Couderc, S.; Toullec, J. Langmuir 2001, 17, 3819. (10) Scrimin, P.; Tecilla, P.; Tonelllato, U. J. Org. Chem. 1991, 56, 161. (11) Scrimin, P.; Tecilla, P.; Tonelllato, U. J. Org. Chem. 1994, 59, 4194. (12) Scrimin, P.; Tecilla, P.; Tonelllato, U. J. Org. Chem. 1994, 59, 18. (13) Tagaki, W.; Ogino, K.; Tanaka, O.; Machiya, K. Bull. Chem. Soc. Jpn. 1991, 64, 74. (14) Ogino, K.; Kashihara, N.; Ueda, T.; Isaka, R. Bull. Chem. Soc. Jpn. 1992, 65, 373. (15) Tagaki, W.; Ogino, K.; Fujita, T.; Yosbida, T. Bull. Chem. Soc. Jpn. 1993, 66, 140.
solutions that catalyzed the hydrolysis of PNPP.16-21 However, among the complexes studied, the effect of the structure of bridge-connecting ligands on the catalytic hydrolysis of carboxyl acid esters is seldom reported. Moving along these lines, we have been continuing our efforts to explore the effect of the rigidity of the ligand on the metallomicelle catalyzing the hydrolysis of PNPP. To elucidate the effect, we studied the hydrolysis of PNPP catalyzed by the Zn2+, Co2+, and Ni2+ complexes of three kinds of ligands with different bridges in CTAB and Brij35 micellar solutions, respectively. In the reaction systems, the kinetically active species has been determined to be the 1:2 complex of the ligand to the metal ions in CTAB micelle, and the complex with opposite ratio of the ligand to the metal ions (2:1) has been found in Brij35 micelle, which was ascribed to the difference of the structure of the ligands and the microenvironment of the investigated system.10,15 The results indicated that the catalytic activity in these complexes formed from different ligands is related to the difference of the bridge-connecting ligands and also dissected in detail in this paper. Results and Discussion Apparent Rate Constants for Hydrolysis of PNPP at pH 7.00 and 25 °C. The ligands investigated in this paper were all soluble, so the kinetics studies were carried out in buffer and in buffered micellar solutions. The apparent rate constants (kobsd) of the reaction shown in Table 1 were obtained by monitoring the liberation of (16) Cheng, S.; Zeng, X.; Meng, Z.; Yu, X. J. Colloid Interface Sci. 2000, 224, 333. (17) Xiang, Y.; Zeng, X.; Cheng, S.; Li, Y.; Xie, J. J.Colloid Interface Sci. 2000, 235, 114. (18) Zeng, Z.; Cheng, S.; Yu, X.; Huang, Z. J. Dispersion Sci. Technol. 1999, 20, 1581. (19) Cheng, S.; Yu, X.; Zeng, X. J. Dispersion Sci. Technol. 1999, 20, 1821. (20) Cheng, S.; Zeng, X. J. Dispersion Sci. Technol. 2000, 21, 655. (21) Xiang, Y.; Zeng, X.; Cheng, S.; Li, Y.; Xie, J., J. Dispersion Sci. Technol. 2000, 21, 857.
10.1021/la020120f CCC: $22.00 © 2002 American Chemical Society Published on Web 08/08/2002
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Table 1. Apparent Rate Constants (103kobsd/s-1) for the Hydrolysis of PNPP in a Metallomicellar System at pH 7.00, 25 °Ca M2+ system 1 2 3 CATB 1 + CTAB 2 + CTAB 3 + CATB Brij35 1 + Brij35 2 + Brij35 3 + Brij35
0.0132 0.0226 0.0595 0.0457 0.0213 0.0134 0.0462 0.0142 0.0239 0.0337 0.0513 0.0402
Zn2+
Ni2+
Co2+
0.674 0.318 0.848 0.704 0.323 0.311 0.621 0.583 0.449 0.721 1.358 0.868
0.453 0.381 1.590 1.290 0.186 0.257 0.293 0.571 0.445 0.830 1.520 1.180
0.356 0.241 0.764 0.641 0.171 0.226 0.529 0.444 0.410 0.651 1.300 0.931
a Note. In 0.01 mol dm-3 Tris-HNO buffer [µ ) 0.1 (KNO )]. 3 3 [CATB] ) 0.01 mol dm-3, [Brij35] ) 0.001 mol dm-3, [PNPP] ) 5 × 10-5 mol dm-3, [ligand] ) [M] ) 1 × 10-3 mol dm-3 (correlation coefficients ) 0.98).
p-nitrophenol from the substrate under the conditions of excess metal ion and ligand over the substrate at pH 7.00 and 25 °C. Table 1 indicates that the uncatalyzed rate constant in buffer is k0 ) 1.32 × 10-5 s-1 at pH 7.00 and 25 °C. The surfactants CTAB and Brij35 in themselves showed rather little rate enhancement effect on hydrolysis of PNPP. However, the apparent rate enhancement was observed when these three kinds of ligands catalyzed the hydrolytic reaction, and the kobsd followed the order ligand 2 > 3 > 1, which may be contributed to the different structure of the ligands. In CTAB micellar solution, both the ligands and the complexes, which formed by metal ions (Zn2+, Co2+, and Ni2+) and the ligands, showed little inhibition on the hydrolysis of PNPP when compared with the rate enhancement by the ligands or metal ions only. This may be ascribed to the electrostatic interaction between the ionic headgroups of cationic surfactant and the hydroxyl of the ligands, which is involved in the unfavorable factor for rate enhancement. However, two different results were observed in Brij35 micellar solution. In the absence of metal ions, ligands also showed relatively little inhibition on the hydrolysis of PNPP, and this may be caused by the long chain of polyoxylethene of Brij35 micelle preventing the molecules of the reactants from colliding frequently,22 and thus, the hydrolysis of PNPP was inhibited. On the other hand, the obvious rate enhancement is observed in the system of the ligands and different metal ions in the Brij35 micellar solutions, and that depends on the ligands and metal ions. The results listed in Table 1 indicated that something complicated must be formed in the investigated systems. Stoichiometry of Metal Complexes in Reaction Systems. A convenient method to determine the chelating stoichiometry of metal complexes is the kinetic version of Job plots shown in Figures 1 and 2, in which the apparent rate constants (kobsd) are plotted as a function of the mole fraction (r) of a ligand or metal ion, keeping their total concentration being constant.23 Figure 1 indicates that for three different ligands investigated in CTAB micellar solutions, r values corresponding to the maximum kobsd are all at about 0.35, suggesting that the 2:1 complexes (metal/ligand) are the active species. From Figure 2, the rate maxima of three plots are all seen at about r ) 0.67, indicating that the 1:2 complexes (metal/ligand) are the (22) Chapman, W.; Breslow, R. J. Am. Chem. Soc. 1995, 117, 5462. (23) Zeng, X.; Zhang, Y.; Yu, X.; Tian, A. Langmuir 1999, 15, 1621.
Figure 1. Job plots for the ligands 1 (b), 2 (9), and 3 (2), and Zn2+ ion complexations as measured by the rates of hydrolysis of PNPP at 25 °C, pH 7.00 in 0.01 mol dm-3 CTAB; [L] + [Zn2+] ) 5 × 10-3 mol dm-3, [PNPP] ) 5 × 10-5 mol dm-3.
active species in Brij35 micellar solution. Similar results were obtained for Co2+ and Ni2+ complexes; i.e., the complexes of 2:1 (meta/ligand) were observed in CTAB micelle while the complexes of 1:2 metal/ligand were in Brij35 micelle. The different chelating ratios of metal ions and the ligands depend on the microenvironment such as solutions, micelles, reversed micelles, etc.24 The Ternary Complex Kinetic Model for Metallomicellar Catalysis. Generally, the rates of the hydrolysis of PNPP in a metallomicelle are dependent on both metal ion and ligand concentrations. In the metallomicellar system, much equilibrium exists between ligand (L), metal ion (M), and substrate (S). On the basis of the phase-separation model of micelle,25 metallomicellecatalyzed reaction can be supposed to take place in the bulk phase and the metallomicellar phase simultaneously to afford the products (P).
KM ) KT )
[MmLn] [M]m[L]n [MmLnS]
[MmLn][S]
k0′ ) k0 + kM[M] + kL[L]
(2)
(3) (4)
where KM is the association constant between m metal ions and n ligands, KT is the association constant between a binary complex (MmLn) and a substrate, kN′ and k0′ are the apparent first-order rate constants for product formation in the metallomicellar phase and in the bulk phase, respectively, k0 is the rate constant due to the buffer, kL and kM are the second-order rate constants due to the ligand and metal ion alone, [L], [M], and [S] are the concentrations of ligand, metal ion, and substrate in the bulk phase, respectively, [MmLn] is the concentration of m metal ions or n ligands in the metallomicellar phase, and [MmLnS] is the concentration of substrate in the (24) Fejita, T.; Inaba, Y.; Dgino, K.; Tagaki, W. Bull. Chem. Soc. Jpn. 1988, 61, 1661. (25) Menger, F. M.; Portney, C. E. J. Am. Chem. Soc. 1967, 89, 4698.
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For the particular case in which n ) 1 and m ) 1, eq 12 may be written as
(
)
1 1 1 1 ) +1 + kobsd - k0′ (kN′ - k0′)KT KM[M]T [L]T 1 1 + (13) kN′ - k0′ KT(kN′ - k0′)[M]T For the particular case in which n ) 1 and m ) 2, eq 12 may be written as
(
)
1 1 1 1 ) +1 + kobsd - k0′ (kN′ - k0′)KT K [M] 2 [L] T M T 4 1 + (14) kN′ - k0′ KT(kN′ - k0′)[M]T Figure 2. Job plots for the ligands 1 (b), 2 (9), and 3 (2), and Zn2+ ion complexations as measured by the rates of hydrolysis of PNPP at 25 °C, pH 7.00 in 0.001 mol dm-3 Brij35; [L] + [Zn2+] ) 5 × 10-3 mol dm-3, [PNPP] ) 5 × 10-5 mol dm-3.
metallomicellar phase.
[S] ) [S]T - [MmLnS]
(5)
[M] ) [M]T - m[MmLn]
(6)
[L] ) [L]T - n[MmLn]
(7)
where [L]T, [M]T, and [S]T are the total concentrations of ligand, metal ion, and substrate, respectively. According to the rate law, the rate equation of reaction 1 can be written as
r ) kobsd[S]T ) kN′[MmLnS] + k0′[S]
(8)
For the particular case in which n ) 2 and m ) 1, eq 12 may be written as
(
Equations 10, 12, 13, 14, and 15 are all referred to as the ternary complex kinetic equations for metallomicellar catalysis, which is similar to that found in the previous reports.23 But it can be seen that the physical train of thought of the derived process is clearer. To obtain the values of kN′, KM, and KT, the consecutive graphic method should be used. When m ) 2 and n ) 1 in the CTAB micelle solution, from eq 14, the plots of 1/(kobsd - k0′) versus 1/[L]T should give straight lines. The intercept, I, and slope, Q, of the straight line are respectively expressed as
k0′ + (kN′[MmLnS]/[S])
(9)
1 + ([MmLnS]/[S])
Inserting eq 3 into eq 9 and rearranging give
1 1 1 + ) kobsd - k0′ KT(kN′ - k0′)[MmLn] kN′ - k0′
(10)
Inserting eqs 6 and 7 into eq 2 and neglecting the highorder terms of [MmLn], we have
Q)
2
m
1 + n KM[M]T [L]T
n-1
2
n
+ m KM[L]T [M]T
m-1
(16)
1 1 + (kN′ - k0′)KT KTKM(kN′ - k0′)[M]T2
(17)
According to eqs 16 and 17, the plots of I versus 1/[M]T and Q versus 1/[M]T2 would allow estimations of kN′, KM, and KT. When m ) 1 and n ) 2 in the Brij35 micelle solution, from eq 15, the plots of 1/(kobsd - k0′) versus 1/[M]T should give straight lines. The intercept, I, and slope, Q, of the straight line are respectively expressed as
(11)
Inserting eq 11 into eq 10 and rearranging give rise to
1 1 ) + kobsd - k0′ K (k ′ - k ′)K [M] m[L] n T N 0 M T T m2 1 n2 + + KT(kN′ - k0′)[L]T KT(kN′ - k0′)[M]T kN′ - k0′ (12)
4 1 + kN′ - k0′ KT(kN′ - k0′)[L]T
(18)
1 1 + (kN′ - k0′)KT KTKM(kN′ - k0′)[L]T2
(19)
I)
[MmLn] ) KM[M]Tm[L]Tn
4 1 + kN′ - k0′ KT(kN′ - k0′)[M]T
I)
Combination of eqs 5 and 8 and rearrangement give
kobsd )
)
1 1 1 1 ) +1 + kobsd - k0′ (kN′ - k0′)KT K [L] 2 [M] T M T 4 1 + (15) kN′ - k0′ KT(kN′ - k0′)[L]T
Q)
According to eqs 18 and 19, the plots of I versus 1/[L]T and Q versus 1/[L]T2 would also allow estimations of kN′, KM, and KT. Discussion The hydrolysis of PNPP was investigated at room temperature and in near neutral solutions because the nature enzyme showed their extraordinary catalytic activities under very mild conditions. As for reaction
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Table 2. Apparent Rate Constants (106kobsd/s-1) for the Hydrolysis of PNPP in EDTA System at pH 7.00, 25 °Ca M2+ system EDTA E+1 E+2 E+3 E + CATB E + CTAB + 1 E + CTAB + 2 E + CATB + 3 E + Brij35 E + Brij35 + 1 E + Brij35 + 2 E + Brij35 + 3
4.44 7.21 5.77 0.77 0.30 2.12 1.97 0.13 1.28 2.60 2.85
Zn2+
Ni2+
Co2+
2.33 2.37 5.47 3.78 0.52 0.72 2.23 0.38
0.79 7.91 5.21 3.33 0.69 0.74 4.34 1.35 0.08 0.81 7.80 2.87
0.61 2.74 4.45 5.74 0.04 1.11 3.94 1.99
0.28 0.96 1.25
0.03 1.61 1.60
a Note. In 0.01 mol dm-3 Tris-HNO buffer [µ ) 0.1 (KNO )]. 3 3 [CATB] ) 0.01 mol dm-3, [Brij35] ) 0.001 mol dm-3, [PNPP] ) 5 -5 -3 2+ -3 × 10 mol dm , [EDTA] ) [ligand] ) [M ] ) 1 × 10 mol dm-3 (correlation coefficients ) 0.98).
system we studied, at low pH (pH < 7.00), the complexes formed by ligands and metal ions inhibited the hydrolysis of PNPP notably, which may be attributed to their pKa values, that is to say, when pH value of solutions is lower than the pKa values of the complexes, the hydroxyl group of the complexes cannot be deprotonated and so cannot exert nucleophilic attack on carboxyl carbon of PNPP. In the high pH region (pH > 8.50), no first-order rate constants were obtained since precipitation occurs, in reaction systems. To, identify the intermediates of hydrolytic reactions, the effects of EDTA as quencher on reaction systems mentioned in this paper were investigated in detail at pH 7.00, 25 °C. In the absence and in the presence of metal ions, the apparent pseudo-first-order rate constants (kobsd′) were determined in the same method by adding aliquots of EDTA into reaction systems containing different ligands, and these values are listed in Table 2. Compared with Table 1, it is evident that EDTA apparently inhibited the hydrolytic reactions of PNPP both in nonmicellar and in micellar solutions, even the spontaneous hydrolysis of PNPP in buffer solution. This is may be ascribed to the following reasons: (1) As we know, EDTA is a kind of effective chelator. And in reaction systems, metal ions were mainly coordinated to EDTA strongly despite the existence of ligands investigated in this paper; because of this, metal ions could not form complexes with ligands, not to mention the polarization of hydroxyl of the ligands by metal ion and the formation of nuclephilic groups, therefore, in the presence of EDTA in the reaction systems, the hydrolysis of PNPP was certainly inhibited. (2) The carboxylic groups of EDTA have high negative charge density, so they can form hydrogen bonds with hydroxyl of ligands, and this made the hydroxyl of ligands difficult to deprotonate to form nuclephiles, and then the hydrolysis of PNPP was not accelerated.
Figure 3. The structure of complex in CTAB micellar solution.
From the above experiment and discussion, we may say that in the reaction system, the intermediates of the complexes formed by metal ions and ligands played a rather important role in accelerating the hydrolyses of PNPP. From Table 3, in both CTAB and Brij35 micellar solutions, it can be seen that the different metal complexes with the same ligand promoted the hydrolysis of PNPP differently, and even their catalytic activities follow the same sequence as Zn2+ < Co2+ < Ni2+, which is consistent with the order of the polarization of metal ion.26 So, the difference of the activity in metal complex catalyzed hydrolysis of PNPP in CTAB may be caused by the difference of the polarization of metal ion. The stronger the polarization of metal ion, the higher the activity in metal complex catalyzed hydrolysis of PNPP in micellar solution. On the other hand, it is worth noting that, for the same metal ion, the catalytic activity in the complex promoted hydrolysis of PNPP depends on the structure of the ligand and the microenvironment of the system. In CTAB micellar solution, the results listed in Table 3 show that the activity in different metal complex with the same metal ion follows the order ligand 3 > 1 > 2. From the analysis of the structure of the ligand-forming complex, it is indicated that the stronger the rigidity of the ligand, the higher the catalytic activity of the complex promoted hydrolysis of PNPP. Among the three ligands, ligand 2 has the most flexible bridge, so the complex formed by ligand 2 shows the lowest catalytic activity on hydrolysis of PNPP. This may be contributed to the active species of 2:1 ratios of metal/ligand (Figure 3), in CTAB micellar solution, and the bridge structure of the ligand. The stronger the rigidity of the bridge of the ligand, the farther the distance between the two metal ions, and the smaller the mutual influence of the two metal ions forming complex, which caused the higher catalytic activity, but this is different from the synergic effect of the two metal ions. The synergic effect of the two metal ions is that the two metal ions act on only a substrate, but the two metal ions forming the complex investigated in this paper act on two substrates. However, in Brij35 micellar solution, the results showed that the activity in different metal complexes with the same metal ion follows the order 2 > 1 > 3, which is contrasted with the case in CTAB micellar solution. This may be due to the active species of 1:2 ratios of metal/ligand in Brij35 micellar solution. As illustrated in Figure 4, the stronger the rigidity of the bridge of the ligand, the farther the distance between the hydroxyl
Table 3. kN′, KM, and KT of the Hydrolysis of PNPP in Metallomicellar System at pH 7.00, 25 °Ca system
L
104kN′ (s-1)
10-6KM (mol dm-3)
10-3KT (mol dm-3)
Zn + C
1 2 3 1 2 3 1 2 3
9.17 8.71 9.43 21.70 12.30 38.80 10.60 7.83 20.20
18.90 31.00 9.17 8.76 10.90 6.93 2.32 4.56 1.26
17.30 9.87 25.90 10.30 8.42 31.90 27.40 10.90 32.90
Ni + C Co + C
a
system
L
104kN′ (s-1)
10-6KM (mol dm-3)
10-3KT (mol dm-3)
Zn + B
1 2 3 1 2 3 1 2 3
8.44 8.75 6.16 31.20 69.00 7.54 14.30 33.00 6.71
4.14 3.93 26.80 51.80 39.40 72.10 316.00 170.00 512.00
1.67 5.15 1.32 1.14 1.28 0.97 1.03 1.67 0.89
Ni + B Co + B
Note: C represents the CTAB micelle system. B represents the Brij35 micelle system (correlation coefficients g0.98).
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Figure 4. The structure of complex in Brij35 micellar solution.
group generating the nucleophile and the substrate coordinated to metal ion, which is unfavorable for the catalytic hydrolysis of the substrate. That is to say, the stronger the rigidity of the bridge of the ligand, the lower the activity in the complex promoted hydrolysis of PNPP. In contrast, the more flexible bridge of the ligand makes it easier that the hydroxyl group is close to the substrate coordinated to metal ion to cause the faster catalytic hydrolysis of PNPP. From Table 3, in addition, it also can be seen that the binding constants (KM and KT) are closely related to the hydrolytic rates. In CTAB and Brij35 micellar solutions, as to any metal ion among the three, the sequence of the binding constants (KM) of binary complexes (MmLn) is opposite to that of the binding constants (KT) of ternary complexes and the catalytic rates (kN′). This is easy to understand: the bigger the binding constants of the binary complexes, the stronger the association of metal ion and the ligand. In other words, if KM is bigger, the binary complex is more stable, and then the binary complex cannot associate the substrate strongly, so the value of KT will be smaller than that of complexes with lower KM, and this results in the weaker catalytic hydrolysis and smaller value of hydrolytic rate (kN′). pH-Rate Profile. From the results obtained, it was obviously shown that kN′ is pH-dependent. In other words, kN′ is related to the acid dissociation constant (Ka) of the hydroxyl group of the ternary complex in the reaction system
where TH is the undissociated complex, T- is the dissociated complex anion assumed to be the active species in metallomicellar phase, and kN is the first-order rate constant which is pH-independent. Then, we have
[MmLnS] ) [TH] + [T-]
(21)
and
Ka )
[T-][H+] [TH]
(22)
By inserting eq 21 into eq 22, we obtain
[T-] )
Ka[MmLnS] [H+] + Ka
(26) Wang, K. Microelement of Life Science; Beijing, 1991.
(23)
Figure 5. pH-rate profile for release of p-introphenol from PNPP in the Zn2+ ion and CTAB micelle system at 25 °C: ligand 1 (b), 2 (9), and 3 (2) (correlation coefficients ) 0.98).
Figure 6. pH-rate profile for release of p-introphenol from PNPP in the Zn2+ ion and Brij35 micelle system at 25 °C: ligand 1 (b), 2 (9), and 3 (2) (correlation coefficients ) 0.98).
The rate equation in the metallomicellar phase can be expressed as
r′ ) kN′[MmLnS] ) kN[T-]
(24)
From eqs 23 and 24 and rearranging, we have
1 1 1 ) + [H+] kN′ kN kNKa
(25)
On the basis of eq 25, the kN and Ka values can be afforded from the slope and the intercept of the plot 1/kN′ vs [H+]. The results are shown in Table 4 and Figure 5 and Figure 6. From Figures 5 and 6 and Table 4, it can be seen that the kN values of the fully dissociated hydroxyl group of complexes of the same metal ion are also related to the structure of the different ligands and follow the same sequence as that of corresponding kN′ values regardless of the different micellar solutions. Also, the real rate constant (kN) in metallomicellar phase is several times larger than kN′, not to mention k0, especially for Ni2+ complex formed by ligand 2 in the Brij35 micelle, for its real rate constant (kN) is up to 4.01 × 10-2 s-1, suggesting that the rate constant of hydrolysis of PNPP in Brij35 metallomicelle was more than 3000 times larger than that in buffer solution. This indicates that the metallomicellar system investigated in this paper, to a certain extent, mimics the reaction of enzymatic catalysis not only on microenvironment but also on active center. As far as pKa values are concerned, they are all lower than those of corresponding metal hydrates.26 This indicates that the metal ions can make the hydroxyl groups of the ligands dissociate to active species in the reaction system, and then the nucleophilic attack on the substrate takes place easily, which increases the rate of hydrolysis of PNPP.
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Table 4. kN and pKa of the Hydrolysis of PNPP in the Metallomicellar Systems with Zn2+, Ni2+, Co2+, and Ethoxyl-diamine Liganda 103kN/s-1
system 1 + CTAB + 2 + CTAB + Zn2+ 3 + CTAB + Zn2+ 1 + CTAB + Ni2+ 2 + CTAB + Ni2+ 3 + CTAB + Ni2+ 1 + CTAB + Co2+ 2 + CTAB + Co2+ 3 + CTAB + Co2+ Zn2+
a
2.93 1.97 4.17 11.12 13.31 8.17 12.05 1.23 11.79
pKa
system
7.34 7.10 7.51 7.49 7.78 7.10 7.85 6.75 7.62
1 + Brij35 + 2 + Brij35 + Zn2+ 3 + Brij35 + Zn2+ 1 + Brij35 + Ni2+ 2 + Brij35 + Ni2+ 3 + Brij35 + Ni2+ 1 + Brij35 + Co2+ 2 + Brij35 + Co2+ 3 + Brij35 + Co2+ Zn2+
103kN/s-1
pKa
2.27 4.65 2.04 16.24 40.10 2.04 2.31 8.15 1.98
7.22 7.63 7.36 7.63 7.51 7.28 6.84 7.11 7.22
Correlation coefficients g0.98.
Summary It is necessary for us to establish different kinetic models for different reaction systems in micellar solutions to understand the mechanism of artificial enzymatic mimicking catalysis more clearly. The kinetic model proposed in this paper is applicable to the cases in which metallomicelle formed by complexes of different chelating ratios catalyzed the hydrolysis of PNPP. Also some interesting results should be noticed: (1) For the same ligand, the complexes with different metal ions also show the different catalytic activity on hydrolysis of PNPP, the order follows as Zn2+ < Co2+ < Ni2+, which is in accordance with the sequence of the polariztion of metal ions. (2) The chelating ratios of the ligand and metal ion are not the same in different micellar solutions due to the microenvironment: in CTAB micellar solution, the complexes with the ratios of 2:1 (metal/ligand) were obtained; in contrast, the complexes with the ratios of 1:2 (metal/ ligand) were the kinetically active species in Brij35 micelle. (3) The apparent first-order rate constants (kN′) of the hydrolysis of PNPP depend on the structure of the bridgeconnecting ligands investigated in this paper. In CTAB micelle, the more rigidity the ligand, the larger the kN′, whereas in Brij35 micelle, the results are completely opposite. This case is also true for real rate constants (kN). In the systems studied in this paper, kN values are much larger than the apparent frist-order rate constants (k0) for the product formation in the buffer solution, in particular, when Ni2+ complexes with ligand 2 in the Brij35 micelle catalyzed the hydrolysis of PNPP, its kN is larger than k0 up to 3000 times. Experimental Section Materials. Zn(NO3)2‚6H2O, Ni(NO3)2‚6H2O, Co(NO3)2‚6H2O, CTAB, Brij35, EDTA, nitric acid, acetonitrile, and tri(hydroxymethyl)aminomethane were analytical-grade commercial products. CTAB was recrystallized before use, p-nitrophenyl picolinate (PNPP) was supplied by Organic Chemical Laboratory of Sichuan University.4,27 PNPP stock solution for kinetics was prepared in acetonitrile. To avoid the influence of chemical components of different buffers, Tris-TrisH+ buffer was used in all case and its pH was adjusted by adding analytically pure nitric acid in all runs. (27) Yu, X.; Cheng, S.; Xiang, Q.; Xiao, S.; Chen, M.; Zeng, X. J. Sichuan Univ. (Nat. Sci. Ed.) 1998, 35, 251.
Figure 7. The structure of ligands. Synthesis and Properties of Ligands. N,N,N′,N′-Tetra(2-hydroxyethyl)-1,3-propylenediamine (1). To a solution of diethanolamine (5.26 g, 0.05 mmol) and dried K2CO3 (14 g, 0.1 mol) in freshly distilled CH3CN (80 mL), 1,3-dibromopropane (2.53 mL, 0.024 mmol) was added. After the addition was complete, the solution was further stirred (2 h) at room temperature, then heated under reflux about 72 h. The slurry was subsequently filtered off, and the acetonitrile was evaporated under reduced pressure to give the crude product as oil, which was purified by column chromatography (SiO2, CHCl3/CH3OH/ NH3‚H2O 3:1:0.2). The yield of compound 1 is 53% as colorless oil. 1H NMR (400 MHz, CDCl3): δ 1.70 (m, 2H), 2.55 (m, 4H), 2.60 (m, 8H), 3.65 (t, 8H). IR (KBr pellet): 3335, 2950, 1654, 1458, 1036. MS: 251 (M+ + 1). N,N,N′,N′-Tetra(2-hydroxyethyl)-1,10-decadiamine (2). This compound was synthesized by the same procedure for compound 1, 1,10-Dibromodecane took the place of 1,3-dibromopropane in use. The yield of compound 2 is 40% as yellow solid, mp 25-28 °C. 1H NMR (400 MHz, CDCl3): δ 1.28 (m, 16H), 1.45 (t, 4H), 2.50 (t, 4H), 2.64 (t, 8H), 3.61 (t, 8H). IR(KBr pellet): 3322, 2914, 1470, 1088, 724. MS: 349 (M+ + 1). N,N,N′,N′-Tetra(2-hydroxyethyl)-1,4-xylyldiamine (3). This compound was synthesized by following the same procedure for compound 1, R,R′-Dibromo-p-xylene took the place of 1,3dibromopropane in use. The yield of compound 3 is 43% as yellow solid, mp 20-25 °C. 1H NMR (400 MHz, CDCl3): δ 2.66 (t, 8H), 3.26 (broad, 4H), 3.56 (t, 8H), 3.64 (s, 4H), 7.30 (s, 4H). IR (KBr pellet): 3332, 2872, 1662, 1448, 1038, 824. MS: 313 (M+ + 1). Kinetics. Kinetic measurements were made spectrophotometrically at 25 °C, employing a GBC 916 UV-vis spectrophotometer with a thermostatic cell compartment. Reactions were initiated by injecting 30 µL of a 0.005 mol dm-3 stock solution of PNPP into 3 mL of buffer solution containing the desired reagents. Kinetic data were obtained by observing the rate of appearance of p-nitrophenol at 400 nm and different pH values (pH 6.28.5). The apparent rate constants were obtained by fitting an equation (ln(At - A∞) - ln(A0 - A∞) ) -kobsdt) by a nonlinear least-squares treatment, and its average relative standard deviation is smaller than 1.5%.
Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 20173038). LA020120F
