Metal-promoted hydrolysis of bis(p-nitrophenyl)phosphate by trivalent manganese complexes with Schiff base ligands in Gemini micellar solution

https://doi.org/10.1016/j.colsurfa.2007.07.018Get rights and content

Abstract

Two crowned Schiff base manganese(III) complexes (MnL2Cl, L = L1, L2) were used to catalyze the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) in Gemini surfactants [bis(hexadecyldimethylammonium)hexane bromide (abbreviation 16-6-16, 2Br)] solution. In comparison with the rate of BNPP spontaneous hydrolysis, a six orders of magnitude in rate enhancement was observed in MnL21Cl/16-6-16 and MnL22Cl/16-6-16 systems. Owing to the different structure, MnL22Cl shows better activity than MnL21Cl. Especially, the effects of three different micelles, i.e., 16-6-16 micelles, CTAB (hexadecyltrimethylammonium bromide) micelles, LSS (n-lauroooylsarcosine sodium) micelles, on the hydrolysis of BNPP catalyzed by MnL22Cl were evaluated at pH7.50. Experimental observations clearly demonstrate that Gemini 16-6-16 micelle is the best one for BNPP hydrolysis among three kinds of micellar media.

Graphical abstract

Comparable studies of micellar effects on the hydrolysis of bis(p-nitrophenyl)phosphate catalyzed by two Schiff base Mn(III) complexes have been carried out in various micelles. An excess 106 folds rate enhancement has been observed in Gemini 16-6-16 micelles. Kinetic advantages (ca. 4.7–8.5 folds) in Gemini 16-2-16 micelles have, respectively, been given in comparison with in CTAB and LSS micelles under the same conditions.

Introduction

The hydrolytic cleavage of Psingle bondO linkage of phosphate esters is highly important not only in nature but also in biochemistry and chemistry fields [1]. Generally, metalloenzymes, which bear high efficiency and selectivity, are often used to catalyze the hydrolysis reaction. In the last two decades, many research groups had focused on the study of the hydrolysis of phosphate esters and obtained some satisfied results [2]. In our previous works, a series of modified crowned Schiff base complexes [3] had been employed to promote the hydrolysis of phosphate esters.

Micelle-catalytic system, which acts as a type of efficient catalytic system, has widely attracted researchers’ attention [4]. Since many reactants are not water-soluble, micelles formed from surfactants have generally been served as a reaction medium [5]. In the micelle-catalyzed hydrolysis of phosphate esters, some conventional surfactants are mostly used as hosts [6]. And yet, Gemini surfactants with some unique structures and properties that are different from those conventional surfactants [7], have seldom been applied in the hydrolysis reaction of phosphate esters [8]. Here we report that the hydrolytic reactivity of bis(p-nitrophenyl)phosphate (BNPP, a DNA model substrate) induced by two synthesized Schiff base Mn(III) complexes (MnL2Cl, see Fig. 1) in a Gemini micellar media formed by a kind of symmetric dicationic surfactant, i.e., bis(hexadecyldimethylammonium)hexane bromide (abbreviation 16-6-16, 2Br). The obtained results indicate that both MnL21Cl/16-6-16 and MnL22Cl/16-6-16 systems can handily accelerate the hydrolysis of BNPP with a six orders of magnitude rate enhancement.

To verify if Gemini surfactant micelles exhibit certain advantages in promoting BNPP hydrolysis reaction compared to conventional surfactants micelles, we have also studied the reaction activity of a selected catalyst (MnL22Cl) toward the hydrolysis of BNPP in other two solutions of micellar hexadecyltrimethylammonium bromide (CTAB) and n-lauroylsarcosine sodium (LSS) under a comparable conditions, respectively. Moreover, the effects of substituents on the reactivities of the two Schiff base Mn(III) complexes were discussed in the present paper.

Section snippets

Instruments and materials

Each pH of reaction solutions, which covered the range 6.50–9.00, was controlled with Tris-TrisH+ buffer. Values of pH were detected by a Radiometer PHM-26 pH meter (Made in China). Kinetic measurements of BNPP hydrolysis and the spectral scans of complexes were carried out spectrophotometically on a GBC 916 UV–vis spectrophotometer (Made in Australia) equipped with a thermostatic cell holder at 25 ± 0.1 °C. Ionic strength of the hydrolytic system was maintained constant (I = 0.1 mol dm−3 KCl) with

Pseudo-first-order rate constants of the BNPP hydrolysis at 25 °C

Primarily, kinetic experiments were performed in a series of buffers with various pHs at a fixed concentration of [BNPP] (2.00 × 10−4 mol dm−3). It is found that pH–kobs profiles for both MnL21Cl/16-6-16 and MnL22Cl/16-6-16 promoted hydrolysis of BNPP give two bell-shaped curves as shown in Fig. 2. This fact is similar to the characters of natural enzymes [10]. Interestingly, the rate maximums of BNPP hydrolysis catalyzed by both Schiff base Mn(III) complexes (MnL2Cl) are reached at pH  7.50 in the

Conclusions

In summary, we report the reactivity of BNPP hydrolysis promoted by two crowned Schiff base manganese(III) complexes in Gemini 16-6-16, CTAB and LSS micellar solutions, respectively. Bell-shaped curves of pH-rate provide an evidence that the single-deprotonated form [MnL2(OH2)(OH)] is the efficient catalyst. The experimental results reveal that the MnL2Cl/16-6-16 can remarkably accelerate the hydrolysis of BNPP by a six orders of magnitude in rate enhancement at pH 7.00. On the contrary, the

Acknowledgements

We greatly thank the financial support from China National Natural Science Foundation (Nos.: 20072025, 20173038) and Key Project of China Sichuan Education Office (No.: 2005D007). Especially, we also thank Organic Chemistry Institute of Sichuan University (PR China) for providing us the two Schiff base complexes.

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