Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Phosphatases Formed from a Monoalkylated Dizinc(II) Complex, Cyclic Diimide Units, and Copper(II) in Two-Phase Solvent System

Abstract

Design and synthesis of enzyme mimic with programmed molecular interaction among several building blocks including metal complexes and metal chelators is of intellectual and practical significance. The preparation of artificial enzymes that mimic the natural enzymes such as hydrolases, phosphatases, etc. remains a great challenge in the field of supramolecular chemistry. Herein we report on the design and synthesis of asymmetric (nonsymmetric) supermolecules by the 2:2:2 self-assembly of an amphiphilic zinc(II)−cyclen complex containing a 2,2′-bipyridyl linker and one long alkyl chain (Zn 2 L 3 ), barbital analogues, and Cu 2+ as model compounds of an enzyme alkaline phosphatase that catalyzes the hydrolysis of phosphate monoesters such as mono(4nitrophenyl)phosphate at neutral pH in two-phase solvent system (H 2 O/CHCl 3 ) in pH 7.4 and 37 °C. Hydrolytic activity of these complexes was found to be catalytic, and their catalytic turnover numbers are 3−4. The mechanistic studies based on the UV/vis and emission spectra of the H 2 O and CHCl 3 phases of the reaction mixtures suggest that the hydrophilicity/ hydrophobicity balance of the supramolecular catalysts is an important factor for catalytic activity.

Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Complexes Formed by the Self-Assembly of a Hydrophobic Bis(Zn2+-cyclen) Complex, Copper, and Barbital Units That Are Functionalized with Amino Acids in a Two-Phase Solvent System

“…It should be noted that 16f demonstrated the highest MNP hydrolysis yield as shown in Figure 8b, and the catalytic turnover number (CTN) was more than 2 after 1 day. The CTN values for 9a , 10a , 16b , 16d – n , 17d , and 17f at [MNP] =1000 μ M, which are summarized in Figure 9, imply that the CTNs of 16b , 16e , 16f , 16g , 17d , and 17f are over 2, while these values are smaller than that of our previously reported 10a (~4) [54].…”

“…The distribution of 13 and 16 in the organic phase and the aqueous phase of the two-phase solvent system was determined from the UV/Vis absorption spectra of both layers, as described in our previous report [54]. For the in situ formation of 13 and 16 , 11d and 11f were selected as more hydrophobic barbital units, and 4a , 11i, and 11k were selected as more hydrophilic barbital units.…”

“…Since the addition of Cu 2+ quenched the emission from the bpy units, it was not possible to determine the distribution of 9a , 16d , 16f , 16i , and 16k in both layers. These behaviors of 6a , 13d , 13f , 13i , and 13k are different from 7a prepared from 3 (Zn 2 L 3 ) and 4a (Bar), which were distributed both in the aqueous and the organic layers [54].…”

“…We also reported on the formation of the hydrophobic supramolecular complexes 9a – c and the amphiphilic supramolecular complexes 10a – c (Scheme 1a) by the 2:2:2 assembly of a hydrophobic bis(Zn 2+ -cyclen) complex containing two long alkyl chains 2 (Zn 2 L 2 ) and a complex that contained one long alkyl chain 3 (Zn 2 L 3 ), respectively, with Bar 2− derivatives and Cu 2+ in a two-phase solvent system (CHCl 3 /H 2 O) [53,54]. As shown in Scheme 2, we expected that the hydrophobic or amphiphilic 2:2:2 complexes 9 or 10 would be formed mainly in the organic layer, and that product inhibition by HPO 4 2− could be avoided, since the hydrophilic HPO 4 2− would be released into the aqueous layer, thus permitting the Cu 2 ( μ -OH) 2 core to be regenerated.…”

“…The results showed that 9 and 10 accelerated the hydrolysis of MNP and one of most important findings was that the hydrolysis of MNP by 9 and 10 in the two-phase solvent system obeyed Michaelis–Menten kinetics, suggesting that these reaction systems consisting of a supramolecular complex in two-phase solvent system closely mimic the active sites of AP. However, catalytic activity was observed only in the case of 10 (the catalytic turnover numbers (CTN) were 3~4 [54]), but not in 9 [53]. In addition, the product inhibition of 9 and 10 by inorganic phosphate (HPO 4 2− ) was not competitive, although it is well known that HPO 4 2− inhibits natural AP in a competitive manner.…”