Kinetics of the accelerator ligand effect for the complex formation of Cd(II) ion with a cationic water-soluble porphyrin

Inorganica Chimica Acta 360 (2007) 3287–3295 www.elsevier.com/locate/ica

Kinetics of the accelerator ligand effect for the complex formation of Cd(II) ion with a cationic water-soluble porphyrin Kunio Kawamura *, Shukuro Igarashi 1, Takao Yotsuyanagi

2

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University Aoba, Sendai 980-8579, Japan Received 13 September 2006; received in revised form 22 March 2007; accepted 31 March 2007 Available online 11 April 2007

Abstract A kinetic study of the acceleration activities of inorganic and organic ligands for the incorporation of a Cd(II) ion into 5,10,15,20tetrakis(4-N-methylpyridyl)porphine (TMPyP) has been performed. The acceleration activities of the inorganic ions decreased in the order I > NO2 > Br > Cl > SO4 2 > OH . The logarithmic values of the rate constants of the Cd-TMPyP formation were proportional to the values of the nucleophilic constant. This fact suggests that the acceleration of the Cd(II) incorporation into TMPyP is mainly due to the enhancement of the water exchange rate in the inner coordination sphere of the Cd(II)–accelerator complex. Furthermore, the acceleration effects of organic ligands increased with the hydrophobicities of the accelerator ligands. In addition, accelerators possessing negative charges, which are capable of interacting with the positive charges of the N-methylpyridyl groups of TMPyP, significantly enhanced the incorporation of Cd(II) into TMPyP. The rate constant of the metal ion exchange reaction of Cd(II) with PbTMPyP in the presence of bathophenanthroline sulfonic acid was 1 400 000-fold greater than the reaction of Cd(II) with TMPyP in the absence of an accelerator. The acceleration effect of organic ligands was due to the enhancement of the hydrophobic interaction and the electrostatic interaction between the Cd(II)–accelerator complex and Pb-TMPyP in the outer coordination sphere.  2007 Elsevier B.V. All rights reserved. Keywords: Porphyrinoids; Kinetics; Amino acids; Cadmium; Ligand effect

1. Introduction For the last several decades, mechanistic investigations of metal–porphyrin formation have been extensively performed to study metal–porphyrin formation in vivo and in petroleum [1–7]. Typically, the incorporation of metal ions into porphyrins is 109 times slower than the complex formation of metal ions with noncyclic multidentate ligands [6]. This is considered to be due to the slow distortion of porphyrin rings. The formation of metal–porphyrin has also * Corresponding author. Present address: Department of Applied Chemistry, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan. Fax: +81 72 274 9284. E-mail address: [email protected] (K. Kawamura). 1 Present address: Department of Materials Science, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316, Ibaraki 316-8511, Japan. 2 Present address: Miyagi National College of Technology, Medeshima Shiote Nodayama 48, Natori, Miyagi 981-1239, Japan.

0020-1693/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.03.054

been investigated from the viewpoint of the analytical applications of water-soluble porphyrins. The slow incorporation of metal ions into porphyrin rings is a disadvantage to the application of porphyrin compounds as highly sensitive chromogenic reagents for metal ions using the Soret band [8–10]. Kinetic analyses of the formation of metal–porphyrin complexes and the acceleration effect using several different additives are important not only understanding metal–porphyrin formation in vivo but also for facilitating the analytical applications of porphyrins. First, heavy metal ions such as Hg(II) and Pb(II) were identified as positive catalysts for the incorporation of several metal ions into porphyrin rings, where a heavy-metal ion causes the distortion of a porphyrin ring by forming a sitting–atop complex and then accelerates the incorporation of metal ions into the porphyrin ring [11–14]. Second, certain organic ligands were found to exhibit catalytic activities for the incorporation of metal ions into the porphyrin ring

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[15–19]. Third, some reducing agents act as accelerators for the incorporation of noble metal ions into porphyrins [20,21]. These additives have been successfully applied to trace-metal analysis using porphyrin compounds. Fundamental studies of metal–complex formation have been extensively conducted for several decades [22–24]. However, the details of the roles of the moieties of organic ligands in the metal–complex formation, such as hydrophobic groups, have not been sufficiently clarified [25,26]. On the other hand, enzymes generally possess greater acceleration activities as compared with nonenzymatic catalysts; it is surprising that enzymes generally catalyze biochemical reactions approximately 105–1017-fold [27]. For instance, it is true in enzymatic reactions that the role of a hydrophobic pocket in an enzyme is important in determining the net activity of the enzyme as well as the role of the active center of the enzyme. As compared to regular multidentate ligands, porphyrins may be regarded as relatively large hydrophobic compounds, although they are much smaller than real enzymes. In addition, metal–porphyrin compounds are indeed important as catalytic sites for certain enzymes. Thus, metal–porphyrin formation can be regarded as a simple model to understand the correlation between the active centers of enzymes and their hydrophobic properties. We have investigated several acceleration methods for metal-ion incorporation reactions into water-soluble porphyrins in order to facilitate the development of trace-metal analysis using porphyrin compounds [17–19]. From our investigations on organic accelerator ligands, it has been deduced that the accelerators facilitate the association in the outer coordination sphere between porphyrin and a metal–accelerator complex prior to the formation of metal–nitrogen bonding in the porphyrin ring. However, it is important to investigate the ability of these ligands to perturb the water exchange rate in the inner coordination sphere of a metal ion during the incorporation of the metal ion into porphyrin. In the present study, we conducted systematic kinetic investigations of the acceleration effects of inorganic and organic ligands using both 5,10,15,20-tetrakis(4-N-methylpyridyl)porphine (TMPyP) and Pb-TMPyP. It was found that the Cd-TMPyP formation using Pb-TMPyP was accelerated up to 7700-fold in the presence of accelerator ligands [18]. The contributions of accelerators to the enhancement of the water exchange rate of a Cd(II) ion in the inner coordination sphere and the enhancement of the association of a Cd(II)–accelerator ligand (CdL1) with porphyrin in the outer coordination sphere are discussed. Based on the kinetic analysis, the roles of each acceleration step in the acceleration effect of the organic and inorganic ligands are discussed in comparison to the acceleration effects of enzymes.

tophenyl)porphine (TPPS) [4], 5,10,15,20-tetrakis(4-carboxyphenyl)porphine (TCPP) [29] and 5,10,15,20-tetrakis(5sulfothienyl)porphine (TSTP) [30] were synthesized by the methods in literatures. Coporporphyrin III (COPRO) was obtained by the hydrolysis of tetramethylester of Coporporphyrin III (Nihon Sekiyu). A 1 · 104 M porphyrin solution was standardized with a standard cadmium(II) solution by photometric titration. The Pb(II) and Cd(II) solutions were standardized by titration with EDTA. All other reagents were used of analytical grade. Abbreviations of reagents are summarized as follows: Gly: glycine, Ala: L-alanine, Val: L-valine, Leu: L-leucine, Nor: L-norleucine, Phe: L-phenylalanine, Glu: L-glutamic acid, Ser: L-serine, Trp: L-tryptophan, His: L-histidine, Met: L-methionine, Tyr: L-tyrosine, Cys: L-cysteine, Orn: ornithine, Sar: b-alanine, BPY: 2,2 0 -bipyridyl, PHEN: 1, 10-phenanthroline, TPTZ: 2,4,6-tris(2-pyridyl)-1,3,5-triazine, OX: 8-quinolinol, MOX: 2-methyl-8-quinolinol, SOX: 5-sulfo-8-quinolinol, BPHS: bathophenanthroline sulfonic acid, ACAC: acetylacetone. A Union RA-401 stopped-flow spectrophotometer was used with a data analysis system of SORD M233. A Hitachi-Horiba F-7ss II pH meter was used for the pH measurements. 2.2. Kinetic measurements The Cd(II) ion incorporation reactions were investigated for both the Cd(II) ion exchange reaction with Pb-TMPyP and the direct Cd(II) ion incorporation into TMPyP. A solution containing Cd(II) ions and an accelerator ligand was prepared as a slightly acidic solution. In addition, a 1 mL 104 M Pb-TMPyP or TMPyP solution was prepared, where the Pb-TMPyP solution was prepared from a solution containing 1.25 · 104 M Pb(II) and 1.0 · 104 M TMPyP. The ionic strength (l) was adjusted to 0.05. The pH of the mixture was adjusted to 9–11, and it was then diluted with distilled water to 25 mL. By this procedure, Pb-TMPyP forms quantitatively. The solution containing Cd(II) and the solution containing Pb-TMPyP were placed in the reservoirs of the stopped-flow spectrophotometer and the absorbance change at 476 nm was monitored. The pH values of solutions containing equal volumes of both the Cd(II) solution and the Pb-TMPyP solution were measured. The apparent rate constants (kapp) were determined from Guggenheim plots using a built-in computer program of the stopped-flow spectrophotometer system. Kinetic measurements of the reactions of Cd(II) with TMPyP and those of Cd(II) with other water-soluble porphyrins were performed following the same procedure. 3. Results

2. Experimental 3.1. Acceleration behavior of accelerator ligands 2.1. Reagents and apparatus TMPyP was synthesized and purified according to the previous methods [16,28]. 5,10,15,20-Tetrakis(4-sulfona-

3.1.1. Influence of organic ligands First, the influence of organic ligands was investigated for both the metal ion exchange reaction of Cd(II) with

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Pb-TMPyP and the complex formation of Cd(II) with TMPyP. The reaction rate of the metal ion exchange reaction of Cd(II) with Pb-TMPyP was 115 times greater than that of the complexation of Cd(II) with TMPyP. The apparent rate constants (kapp) were obtained in the presence of several organic ligands. The logarithmic values of kapp were plotted as a function of the concentration of the accelerators. The relationships between kapp and the concentration of the accelerators ([L]T) are shown in Figs. 1a,b, 2, and Fig. S1a – S1e in the Supplementary material. A typical acceleration behavior for the case of Gly is shown in Fig. 1b. The graphs of log kapp versus log [L]T have a bell shape, where the values of log kapp initially increase with the concentration of the accelerator and drop again at higher concentrations. Amino acids accelerated the Cd(II) ion exchange reaction with Pb-TMPyP 16–318-fold at the maximum acceleration range, as compared to the reaction without accelerators (Fig. 1a and Fig. S1a, S1b in the Supplementary material). Trp exhibited the largest acceleration effect among the examined amino acids. The acceleration

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Fig. 2. Relationships between log kapp vs. log [L]T for the reaction of Cd(II) with Pb-TMPyP or TMPyP in the presence of organic accelerators possessing negative or positive charges. [Pb-TMPyP or TMPyP]T = 1.85 · 106 M, [Cd(II)]T = 4.16 · 105 M, pH 9.80, l = 0.05, 20 C. The line drawn through the experimental points were fitted by the calculation described in the text. Open circles: Pb-TMPyP system with BPHS, closed circles: TMPyP system with BPHS, open squares: Pb-TMPyP system with TPTZ, closed squares: TMPYP with TPTZ, open triangles: Pb-TMPyP with PHEN (see also Fig. S1e in Supplementary material).

effect is significantly dependent on the type of amino acid. Acceleration effects were also observed for other organic ligands such as nucleotide bases and pyridine derivatives for the reaction of Cd(II) with Pb-TMPyP (Fig. S1c). However, the acceleration effects of amino acids and other accelerator ligands were not as effective for the direct complexation of Cd(II) with TMPyP (Fig. S1d), where the acceleration effects were normally lower than 10-fold. On the other hand, accelerators possessing sulfonic acid groups, such as cysteic acid, SOX, and BPHS, exhibited extremely strong acceleration effects (Fig. 2 and Fig. S1e in the Supplementary material). BPHS accelerated the metal ion exchange reaction of Cd(II) with Pb-TMPyP 1280-fold and the complexation of Cd(II) with TMPyP 25-fold. The acceleration effects with these ligands were much greater than those of the corresponding ligands possessing no sulfonic groups. These effects are probably due to the influence of the negative charges of the sulfonic groups of the accelerators, where the association between Cd(II) with Pb-TMPyP or TMPyP in the outer coordination sphere would be enhanced. The acceleration with TPTZ was also observed for the reactions of Cd(II) with both Pb-TMPyP and TMPyP. While TPTZ exhibited the largest p–p stacking association with TPPS [17], the acceleration for the reaction with Pb-TMPyP and TMPyP was not as efficient as in the case of BPHS. Fig. 1. Relationships between log kapp vs. log [L]T for the reaction of Cd(II) with Pb-TMPyP in the presence of amino acids. [Pb-TMPyP or TMPyP]T = 1.85 · 106 M, [Cd(II)]T = 4.16 · 105 M, pH 9.80, l (ionic strength) = 0.05, 20 C. (a) Pb-TMPyP system. open circles: Gly, closed circles: His, open squares: Met, closed squares: Leu, open triangles: Nor. (see also Fig. S1a–S1d in Supplementary material). (b) The reaction of Cd(II) with Pb-TMPyP or TMPyP in the presence of Gly and the molar fractions of Cd(II)–glycine complex as a function of Gly concentration. Open circles: Pb-TMPyP system with Gly, closed circles: TMPyP system with Gly. The line drawn through the experimental points were fitted by the calculation described in the text.

3.1.2. Influence of inorganic ligands On the other hand, unexpectedly, inorganic ligands exhibited relatively strong acceleration activities for the reaction of Cd(II) ions with Pb-TMPyP (Fig. 3 and Fig. S2 in the Supplementary material). This unexpected finding was also observed for the reaction of Cd(II) with TMPyP. For instance, iodide ions accelerated the Cd(II) ion exchange reaction with Pb-TMPyP 547-fold, and that

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where ki (i = 0–4) indicate the rate constants of Cd(OH)i species with Pb-TMPyP and kj (j = 0–n) indicate those of the CdLj complex (L: accelerator) with Pb-TMPyP. The formation of a mixed–ligand complex was negligible in these cases. Using the stability constants for the formation of Cd(OH)i and CdLj species [31–33], the rate are given in Eqs. (3) and (4):  X d½Pb-TMPyP X j ¼ k i bOH;i ½OHi þ  k j bL;j ½L ½Cd2þ  dt  ½Pb-TMPyP ð3Þ Fig. 3. Relationships between log kapp vs. log [L]T for the reaction of Cd(II) with Pb-TMPyP in the presence of inorganic accelerators. [PbTMPyP]T = 1.85 · 106 M, [Cd(II)]T = 4.16 · 105 M, pH 9.80, l = 0.05, 20 C. The line drawn through the experimental points were fitted by the calculation described in the text. Accelerators: open circles: I, closed circles: NO2  , open squares: Br, closed squares: Cl, open triangles: SO4 2 (see also Fig. S2 in Supplementary material).

with TMPyP 45-fold. In these cases, the difference of the acceleration effect between the reaction of Cd(II) ion with Pb-TMPyP and that with TMPyP was greater than that for organic ligands. 3.1.3. Influence of the type of porphyrins The acceleration effect of amino acids and certain organic ligands was briefly investigated using other types of water-soluble porphyrins. The apparent rate constants are summarized in Table S1. This result shows that the acceleration effect of amino acids is dependent on the type of porphyrin, while Trp seems to be the strongest accelerator for all types of porphyrins. 3.2. Calculation of rate constants for active species The effect of accelerators was investigated on the basis of the complex formation of Cd(II) with accelerator ligands. The molar fractions of Cd(II) complexes in the presence of an accelerator were calculated as a function of the accelerator concentration (Fig. 1b). This result indicated that the acceleration effect is due to the formation of complexes of Cd(II) with the accelerator ligands, where a 1:1 and/or 1:2 complex of Cd(II) with the accelerator ligands are active chemical species. The rate constants of Pb(II) with individual Cd(II)–accelerator species were calculated from the relationship between the apparent rate constants by the following method: the Pb-TMPyP formation is given in Eq. (1), and the reaction rate is expressed in Eq. (2): Cd2þ þ Pb-TMPyP ! Cd-TMPyP þ Pb2þ 

d½Pb-TMPyP dt X  X ¼ k i ½CdðOHÞi  þ k j ½CdLj  ½Pb-TMPyP

ð1Þ

ð2Þ

where b indicate the stability constants for these species. Besides, the mass balance of cadmium ions is expressed in Eq. (4): X  X ½Cd2þ T ¼ bOH;i ½OHi þ bL;j ½Lj ½Cd2þ  ð4Þ Thus, the rate of the disappearance of Pb-TMPyP is expressed in Eq. (5): P  P i j k i bOH;i ½OH þ k j bL;j ½L d½Pb-TMPyP  ¼ P  P i j dt b ½OH þ b ½L OH;i

L;j

2þ

 ½Cd T ½Pb-TMPyP

ð5Þ

Besides, the mass balance of the ligand is expressed in Eq. (6): P jbL;j ½Lj ½LT ¼ ½L þ P ½Cd2þ T ð6Þ j bL;j ½L The apparent rate constant are given in Eq. (7): P  P i j k i bOH;i ½OH þ k j bL;j ½L  k app ¼ P P bOH;i ½OHi þ bL;j ½Lj

ð7Þ

The rate constants were obtained by the nonlinear leastsquare fitting of the values of kapp obtained from the experiments with those calculated from Eq. (7). The calculated rate constants are summarized in Tables 1 and 2. The experimental data were fitted using this model (see the line through the experimental plots in Figs. 1a, 2,3, Fig. S1a– S1b). For some cases, the values of k2 were obtained. These Table 1 Logarithmic values of the rate constants for the formation of Cd-TMPyP from Pb-TMPyP and TMPyP Ligands

Pb-TMPyP log k1

TMPyP log k1

Cl Br I NO2  SO4 2 OH H2O

4.79 ± 0.06 5.20 ± 0.10 6.29 ± 0.21 5.93 ± 0.22 4.35 ± 0.11 4.30 ± 0.14 3.61 ± 0.14

3.62 ± 0.05 4.16 ± 0.08 5.00 ± 0.28 4.83 ± 0.13

20 C.

3.43 ± 0.14 1.55 ± 0.14 (1.60 [48])

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Table 2 Logarithmic values of the rate constants for the formation of Cd-TMPyP from Pb-TMPyP and TMPyP

4.2. Influence of the dissociation of water molecules in the inner coordination sphere

Ligands

For the case of inorganic ligands, the enhancement of the reaction rate decreased in the order I ; NO2  > Br > Cl > NO3  ; SO4 2 for the reactions of Cd(II) with Pb-TMPyP and with TMPyP, where the rate constants (k1) for the reactions of CdL1 with Pb-TMPyP and TMPyP were more than 400 times greater than the rate constants for the reactions of a Cd(II)–aqua complex with PbTMPyP and TMPyP. The difference in the acceleration effects of the inorganic ligands between the reactions of CdL1 with Pb-TMPyP and those with TMPyP is smaller than that for organic ligands, which will be discussed later (Tables 1 and 2). The CdL1-type complex, for the cases of inorganic accelerators, has a single positive charge except for the case of SO4 2 . Thus, the magnitude of the associate formation of CdL1 with Pb-TMPyP or with TMPyP in the outer coordination sphere would not be strongly affected by the type of the ligand since the contribution of the electrostatic interaction to the association in the outer coordination sphere is mainly determined by the number of charges of the Cd(II)–accelerator ligand and TMPyP [35]. This fact suggests that the acceleration effect is due to the enhancement of the dissociation of water molecules, as shown in Eq. (8c), where the accelerator ligand bound to Cd(II) reduces the strength of the bonding between Cd(II) and water molecules in the inner coordination sphere. If this assumption is correct, the acceleration effect could be correlated with the parameters that indicate the strength of the bonding between Cd(II) and the accelerator ligand CdL1. Two typical parameters, H and En, defined by Edward, were examined to determine the correlations between the parameters and the magnitude of k1 [36]. It was found that the values of log k1 were proportional to the values of En (Fig. 4). In addition, for some multidentate ligands, there was a trend that the magnitude of log kdiss increased with En [36,37]. The values of the water

Pb-TMPyP

TMPyP log k2

log k1 Gly Ala Val Leu Nor Phe Glu Ser Trp His Met Tyr Cys Orn Sar BPY PHEN OX MOX SOX BPHS

6.19 ± 0.10 6.00 ± 0.11 6.02 ± 0.14 6.28 ± 0.16 6.06 ± 0.12 6.44 ± 0.06 6.41 ± 0.12 5.75 ± 0.08 6.57 ± 0.07 5.82 ± 0.05 6.34 ± 0.07 6.99 ± 0.07 5.59 ± 0.11 5.63 ± 0.11 5.71 ± 0.14 6.24 ± 0.10 6.43 ± 0.09 6.16 ± 0.44 4.63 ± 0.22 6.87 ± 0.13 7.71 ± 0.14

3.66 ± 0.05

3.75 ± 0.10 3.60 ± 0.17

6.36 ± 0.07

3.82 ± 0.05 3.83 ± 0.05 3.73 ± 0.03

2.66 ± 0.11 4.01

5.59 ± 0.40

results indicate that the experimental values of kapp were well fitted with the values based on the present reaction model, in most of the present reaction systems. In some cases, the calculated lines deviated from the experimental values since the stability constants were only partially available for some ligands. 4. Discussion 4.1. General mechanism of metal–porphyrin formation In general, the mechanism and the rate constants of the formation of metal–porphyrin complexes are expressed in Eq. (8), and the overall rate constant for the formation of metal–porphyrin complexes is given in Eq. (9) [1–7]: K ass

Mnþ þ H2 por Mnþ    H2 por KD

Mnþ    H2 por Mnþ    H2 por M

nþ

 k diss

   H2 por ! M-por þ 2H

k form ¼ k diss K ass K D

þ

ð8aÞ ð8bÞ ð8cÞ ð9Þ

Here, kdiss is the dissociation rate constant of water molecules in the inner coordination sphere of a Cd(II)–accelerator ligand complex; Kass, the association constant in the outer coordination sphere of the Cd(II)–accelerator ligand complex with porphyrin; KD, the deformation constant of a porphyrin ring in assuming a conformation suitable for forming metal–nitrogen bonding in the porphyrin ring; and H2por*, the deformed porphyrin. This reaction model is consistent with the complex formation of metal ions with regular multidentate ligands (Eigen model) [34–39]. The acceleration effect is evaluated on the basis of the three steps.

Fig. 4. Relationships between log k1 vs. En value for the reaction of Cd(II) with Pb-TMPyP or TMPyP in the presence of inorganic ligands. Open circles: Pb-TMPyP systems, closed circles: TMPyP systems. Open squares show the rate constants of water exchange of CdL1 complexes.

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dissociation rate constants (kdiss) for CdCl+ and CdBr+ are indeed found in the literature [38], and these values are also plotted in Fig. 4. The slope of the line of log k1 versus En coincides with the slope of the line of log kdiss versus En. This fact is consistent with the fact that the dissociation of water molecules in the inner coordination sphere of metal complexes can be enhanced in several metal–ligand complexes [36,37]. The fact that a correlation is observed between k1 with En rather than H is probably due to the fact that the Cd(II) ion is a somewhat soft ion. The values of En include the electrophilic nature as well as the softness of the ligands. This is reasonable since the bonding between Cd(II) and water molecules is weakened when a Cd(II)– aqua complex is partly coordinated with a strong electrophilic accelerator ligand. Conclusively, the acceleration effect of these inorganic ligands is due to the enhancement of the dissociation rate of water molecules in the inner coordination sphere of CdL1, as shown in Eq. (8c). 4.3. Influence of hydrophobicity of organic accelerators On the other hand, the acceleration behavior of organic ligands is discussed in order to determine whether the difference of the acceleration effect of amino acids is due to a mechanism similar to that in the case of inorganic ligands. The rate constants in the presence of organic ligands are in the range of 105–106 M1 s1 for the reactions with Pb-TMPyP and 103–105 M1 s1 for the reactions with TMPyP. These rate constants are significantly greater than that of the reaction of a Cd(II)–aqua complex with TMPyP (Table 1) [39]. First, we attempted to find correlations between k1 and several parameters, which would indicate the strength of the bonding between Cd(II) and the accelerator ligand. The relationships between the k1 values and pK a1 (a carboxyl group), pK a2 (a amino group), and the stability constants (b1) of Cd(II)–amino acid complexes were evaluated (Fig. S4a–S4c). These parameters would reflect the strength of the coordination bonding between Cd(II) and the accelerator ligand, which would affect the strength of the bonding between Cd(II) and water molecules in the inner coordination sphere of CdL1. However, there was no clear correlation between the values of k1 and the values of pK a1 , pK a2 , and b1. Thus, it was concluded that the influence of the organic ligands is fundamentally different from that of the inorganic ligands. In general, it is known that the magnitude of kdiss varies within a factor of 10 due to the coordination with organic ligands [40,41]. However, the acceleration by organic ligands was in the range of 103–104 times that without accelerators. This fact also supports the hypothesis that the acceleration effect of organic ligands is not due to the activation of the dissociation of water molecules in the inner coordination sphere. Thus, other physicochemical parameters of amino acids, that is, Grantham’s parameters [42], molecular weights (MW), and partition coefficients into 1-octanol (Poctanol) [43], which would indicate the properties of the different

Fig. 5. Relationships between log k1 values vs. several parameters concerning amino acids. (a) molecular weight of amino acids. (b) Partition coefficients of amino acids into n-octanol (see also Fig. S3 in Supplementary material).

moieties of the amino acids, were examined for the reactions of Cd(II) with both Pb-TMPyP and TMPyP (Figs. 5a,b and Fig. S4d–S4f). Grantham’s parameters did not show any strong correlations with the values of k1. However, positive correlations with the k1 values were observed for Grantham’s parameter v, MW, and the values of Poctanol. The partition coefficients reflect the hydrophobicity of amino acids, and the value of v and MW are also correlated with the hydrophobicity of amino acids. In particular, amino acids possessing aromatic groups exhibited strong acceleration effects, while Tyr exhibited a greater acceleration effect than that reflected by the line extrapolated from the plots of log k1 versus Poctanol; the case of Tyr is discussed in the subsequent section. These facts indicate that the acceleration effect of organic ligands is due to the enhancement of the hydrophobic interactions. This is consistent with the results of preliminary studies [18,44]. In addition, the acceleration by PHEN is greater than that by BPY. The fact that organic accelerators possessing large aromatic moieties exhibit greater acceleration effects than organic ligands without such aromatic moieties is consistent with the fact that the free-energy change in the association constants between a water-soluble porphyrin and pyridine derivatives is proportional to the aromatic-ring size of the pyridine derivatives [17,45,46]. The relationship between these parameters with the k1 values for the reac-

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tions of Cd(II) with TMPyP was not as clear as that for the reactions of Cd(II) with Pb-TMPyP. In addition, Trp was the most effective accelerator among the amino acids, for all types of water-soluble porphyrins (Table S1). However, the acceleration by hydrophobic amino acids seems to be weak in the case of the reaction of Cd(II) with Pb-TMPyP. This is contrary to the results of a preliminary study, where the acceleration behavior of amino acids for the reaction of Zn(II) with an anionic porphyrin was somewhat different from the relationship between the acceleration behavior and the types of accelerators in the present study [17,18,47,48]. Conclusively, the hydrophobic interaction for the complex formation of Cd(II) with other water-soluble porphyrins is important for the acceleration of the metal–complex formation with water-soluble porphyrins, while the magnitude of the contributions are dependent on the type of porphyrin. 4.4. Influence of charges of organic accelerators According to the present kinetic analysis, it was elucidated that the associate formation between Pb-TMPyP and Cd(II)–accelerator ligand (CdL1) in the outer coordination sphere is an important factor. If the associate formation in the outer coordination sphere can be enhanced not only by the hydrophobic interaction but also by the electrostatic interaction, the acceleration would be strongly enhanced for such an accelerator ligand (Fig. 6). Indeed, it has been observed that the incorporation reaction of different metal ions into TMPyP was notably accelerated with SOX [19]. In the present study, organic ligands possessing sulfonic groups exhibited considerably strong acceleration activities for the reactions of Cd(II) with both Pb-TMPyP and TMPyP. BPHS, SOX, and cysteic acid accelerated the formation of Cd-TMPyP more efficiently than the corresponding accelerator ligands possessing no sulfonic groups (Fig. 2 and Fig. S1e in the Supplementary material). The value of k1 for BPHS was 19 times greater than that for

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PHEN, and the value of k1 for SOX was 5.1 times greater than that for OX. Since BPHS possesses additional phenyl groups as compared to PHEN, the acceleration by BPHS would reflect the contribution of the phenyl groups. Although the acceleration by cysteic acid for the reaction of Cd(II) with Pb(II)-TMPyP was more than four times greater than that by Cys at 1 · 104 M, it was not possible to determine the value of k1 as the stability constants are not available. BPHS exhibited the highest acceleration effect among the ligands tested in the present study. This is reasonable since BPHS possesses large aromatic moieties and two positive charges. The acceleration by BPHS was also observed for the reaction of Cd(II) ions with TMPyP. The acceleration effect by organic ligands seems to be affected by the type of metal ions. For instance, it is known that the acceleration by SOX is dependent on the type of metal ions [19]. Based on this model, the fact that the acceleration by Tyr is much greater than the value determined by the extrapolated line of the relation between the values of k1 and the partition coefficients of amino acids can be explained by the following reasons (Fig. 5b). Tyr has negative charges at the phenol group due to deprotonation at the present reaction pH since the pKa value of the –OH group of Tyr is 10.43; this enhances the associate formation of Cd(II)–Tyr with Pb-TMPyP in the outer coordination sphere [33]. The reaction model is shown in Fig. S4 in the Supplementary material. In addition, the relatively large k1 for Glu and the relatively small k1 for Orn are due to the fact that Glu possess a negative charge and Orn is capable of possessing a positive charge at the side chains of the amino acids within the CdL1 complexes. In addition, a brief investigation of SOX for different types of water-soluble porphyrins shows that the role of sulfonic groups is not effective for anionic porphyrins (Table S1). In these reaction systems, the distance between the center Cd(II) and the negative charge of CdL1 is coincident with the distance between the center of the porphyrin ring and the positive charges of methylpyridinium in TMPyP (Fig. 6 and Fig. S4 in Supplementary material). Conclusively, the ligands possessing both large hydrophobic aromatic moieties and electrostatic charges effectively accelerate the metal ion incorporation into cationic water-soluble porphyrins. 4.5. Cooperative effect of the acceleration of organic ligands and the metal ion exchange reaction of Cd(II) with PbTMPyP

Fig. 6. Estimated configurations of the association of Cd-BPHS with PbTMPyP (see also Fig. S4 in Supplementary material).

The influence of the center Pb(II) in the acceleration by BPHS seems to be weaker than that for the acceleration by amino acids, where the ratio of the value of k1 for BPHS for the reaction of Cd(II) with Pb-TMPyP to that for the reaction of Cd(II) with TMPyP is 132, and that for glycine is 339. A similar trend is observed for other amino acids. This fact implies that amino acids enhance the acceleration effect of the center Pb(II) of Pb-TMPyP. It is known that

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Hg(II) and Pb(II) ions notably accelerate the formation of metal–porphyrin complexes; these ions enhance the distortion of the porphyrin ring enabling it to assume a conformation suitable for the formation of metal–nitrogen bonding in the porphyrin ring (sitting–atop complex mechanism) [46–49]. The enhancement of the reaction rate by the center Pb(II) in Pb-TMPyP in the present case is a moderate enhancement among the catalytic incorporations of metal ions into porphyrins. From the results, the fact that the enhancement by organic ligands of the reaction of Cd(II) with Pb-TMPyP is greater than that with TMPyP may be regarded as a cooperative effect of organic ligands and Pb(II) for the distortion of the porphyrin ring. In other words, this fact suggests that the distortion is amplified by accelerator ligands. The Cd-TMPyP formation reaction using both accelerator ligands and Pb-TMPyP are accelerated 1.45 · 106-fold with BPHS and 2.75 · 105-fold with Tyr, as compared to the rate constant of the Cd-TMPyP formation from a Cd(II)–aqua complex with TMPyP. It is surprising that the acceleration reaches 106-fold—a magnitude comparable to the accelerations by relatively weak enzymes. The acceleration of the formation of CdTMPyP from the reaction of Cd(II) with Pb-TMPyP in the presence of organic accelerators can be divided into a few acceleration steps. That is, the acceleration by the influence of the center Pb(II) of Pb-TMPyP instead of TMPyP, the enhancement of the formation of an outer coordination sphere complex by the hydrophobic interaction and the electrostatic interaction, and the enhancement of the distortion of porphyrin by accelerator ligands. The use of Pb-TMPyP instead of TMPyP results in a 115-fold acceleration. The reaction of Cd(II) with PbTMPyP is accelerated 427-fold by BPY, 2400-fold by Tyr, and 380-fold by Gly, as compared with the reactions in the absence of the organic ligand accelerators. The reaction of Cd(II) with TMPyP is accelerated 288-fold by BPY, 191-fold by His, 186-fold by Trp, and 129-fold by Gly, as compared with the reactions in the absence of the organic ligand accelerators. Thus, the contribution of hydrophobic interactions can be estimated to be in the range of 100–1000-fold. Moreover, the reaction of Cd(II) with Pb-TMPyP is accelerated 12 600-fold by BPHS, and the reaction of Cd(II) with TMPyP is accelerated 11 000-fold by BPHS. The ratios of the k1 values for the accelerators with and without sulfonic groups are 5.1 for (k1 for OX)/(k1 for SOX) and 19.1 for (k1 for BPHS)/ (k1 for PHEN). The enhancement in the latter case would involve the additional influence of two phenyl groups since BPHS possesses two additional phenyl groups as compared with PHEN. Thus, the approximate contribution of electrostatic interactions to the overall acceleration is estimated to be of the order of a few fold. On the other hand, the overall acceleration effect in the system of CdTMPyP formation using both accelerator ligands and Pb-TMPyP reaches 1.45 · 106-fold for BPHS and 2.75 · 105-fold for Tyr, as compared with the original

reaction rate for a Cd(II)–aqua complex with TMPyP. It is surprising that an acceleration of more than one-millionfold was obtained by the combination of the multiple acceleration steps. This large acceleration effect is comparable to the acceleration activities of relatively weak enzymes, since it is known that the acceleration by enzymes is in the range of 105–1017-fold. The proposed model reaction may imply that the enzymatic activities in vivo are dissected into the relatively small acceleration steps such as the acceleration by an active center, the enhancement by the electrostatic interaction, that by the hydrophobic interaction, and that by hydrogen bonding. 5. Conclusions A systematic investigation of the acceleration by inorganic and organic ligands of the incorporation reaction of Cd(II) into a water-soluble porphyrin TMPyP was performed. The accelerations by I ; NO2  ; Br ; Cl ; NO3  ; SO4 2 , and OH were mainly due to the enhancement of the water exchange rate of the Cd(II) complex with the anions in the inner coordination sphere. The acceleration effect was proportional to the nucleophilic constant defined by Edwards. However, the acceleration by organic ligands was mainly due to the enhancement of the associate formation of porphyrin and Cd(II)–accelerator complex in the outer coordination sphere. The enhancement of the hydrophobic interaction was observed for amino acids and some other organic ligands. The enhancement of the electrostatic interaction was observed for the acceleration of organic ligands possessing sulfonic groups, in which the associate formation was enhanced by the interaction between the positive charges of TMPyP and the negative charges of the accelerators. In addition, the enhancement by Pb(II) at the center of Pb-TMPyP can be amplified by the organic accelerators. It was surprising that the reaction of Cd(II) with Pb-TMPyP in the presence of BPHS was accelerated one-millionfold as compared with the native reaction of Cd(II) with TMPyP. The large acceleration was dissected into individual acceleration processes, that is, the influence of the center Pb(II) of Pb-TMPyP, the enhancement by the hydrophobic interaction, and the enhancement by the electrostatic interaction. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2007. 03.054. References [1] E.B. Fleischer, E.I. Choi, P. Hambright, A. Stone, Inorg. Chem. 3 (1964) 1284. [2] E.B. Fleisher, S. Jacobes, S. Mestichelli, J. Am. Chem. Soc. 90 (1968) 2527.

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