Construction Of Bioactive Supramolecular Polymer And Its Self-Assembly

Nature has developed a self-assembly strategy to construct macromolecules into sophisticated and complex nanoobjects. The folding of proteins or enzymes into their compact three-dimensional structures is the most fundamental and universal example of biological self-assembly. Therefore, a variety of artificial macromolecules are actively explored to mimic the structures, functionalities or in concert found in nature. Recently, the concept of supramolecular polymers (SPs) enables facile preparation of those macromolecules capable of possessing uniform structure and incorporating functional subunit. Though noncovalent polymerization of specifically designed building blocks, SPs provide a new class of functional and responsive macromolecular materials. However, it is relatively rare to study the self-assembly behaviors of SPs, especially in biological application.Glutathione peroxidase (GPx, EC.1.11.1.9), an important selenium-containing enzyme, functions to protect various living organism from aerobic oxidative stresses by catalyzing the reduction of hydroperoxides (ROOHs), using glutathione (GSH) as a reducing substrate [6].The catalytic active center of GPx, selenocysteine, is in a depression on the protein surface, in which some charged and hydrophobic amino acid residues (Phe, Trp, Asp) form a hydrophobic cavity. The discovery of Ebselen (2-phenyl-1,2-benzoisoselenazol-3(2H)-one), which functions as an antioxidant, has inspired a worldwide interest in the design of GPx mimics. In our previous research, we have demonstrated that one promising tactic for mimicking GPx is the introduction of essential catalytic center, selenium/tellurium, as well as the hydrophobic catalytic microenvironment for substrate binding. Based on the understanding of GPx, we report the self-assembly of the tellurium containing SPs with strongπ-πstacking moieties, thus bringing about the formation of catalytic entities in solution.Quinacridone (QA) is a developed technical pigment and recent evidence suggests that QA can be used as building blocks for functional supramolecular architectures duo to its strongπ-πinteraction and hydrophobic property. Herein we design a type of A-B alternating amphiphilic supramolecular polymer, via host-guest complexation, using ditopic adamantane guest from quinacridone derivative (QAAD) as hydrophobic block A and a cyclodextrin dimer (6-TediCD) as hydrophilic block B. We used the host–guest pair ofβ-cyclodextrin (β-CD) and adamantine (AD) because they are widely used in constructing SPs due to their high stability constant for complexation up to 1×105 M-1. We anticipated that the introduction of a strongπ-πstacking moiety would play a vital role in self-assembly as well as in catalysis.First of all, we designed and synthesized a ditopic adamantane guest dimmer from quinacridone derivative. The adamantines were attached via a linker to each nitrogen atom of the symmetrical quinacridone. Theβ-CD host dimer molecule 6-TediCD was prepared according to the literature reported by Engman and co-workers. The structures of compounds were confirmed by 1H NMR spectroscopy, mass spectra and IR spectrum.We predict that there would be the formation of SPs in their common solvent like DMF through theβ-CD/AD inclusion interaction. To investigate the above possibility, size exclusion chromatography (SEC) experiments and the dynamic light scattering (DLS) were carried out to determination of supramolecular polymer based on hydrodynamic volume in solution. The SEC result indicated the presence of higher molecular weight species (Mn>4.3×105), and clearly demonstrated the formation of a SP structure.In water solution, this supramolecular polymer will spontaneously self-assemble into micelle, which was indicated by a remarkable red shift in emission spectra from 543nm to 567 nm. The morphology of micelle was investigated by scanning electron microscopy (SEM). A large number of spherical micelles were observed with the diameter of around 70 nm, which is well agreed with the data obtained from the DLS. To obtain detailed information about the micelle structure, we recorded the 1H NMR spectrum of the SPM in D2O. The 1H NMR signals of the QA group were totally absent after adding water. One reasonable explanation for the undetectable 1H NMR signals of QA group is that theπ-πstacking moieties are buried in the core of the micelle without water accessible. In contrast, the 1H NMR signals corresponding to adamantyl group was still well resolved, duo to the host-guest inclusion complex withβ-CD. Hence, the result of the 1H NMR experiment not only indicates that the AD groups are deeply included in the cavities ofβ-CD. As a result, we deduce that the segregation of the hydrophobic and hydrophilic parts of the molecule during the self assembly in water results in the QA groups residing in the inner part of the micelles and the CD groups at the outer part of the micelles, that is a hydrophobic core of quinacridone and a hydrophilic shell of 6-TediCDs.More interestingly, the resulting micelle (SPM) efficiently catalyzed the reduction of cumene hydroperoxide (CUOOH) using 3-carboxy-4-nitrobenzenethiol (ArSH) as a substrate, and thus can be an excellent GPx-like mimic. We have measured the catalytic activity of the tellurium-containing fluorescent micelle according to the method reported by Bill and Hilvert et al. using ArSH as a glutathione alternative. Reaction was initiated by the subsequent addition of ROOH and the absorbance at 410 nm (ε= 13600 M-1 cm-1, pH 7.0) was recorded for a few minutes to calculate the reaction rate. The initial rate of the background (nonenzymic) reaction between CUOOH and ArSH is very slow (ν0 = 0.49μM min-1). A slight enhancement in the rate is observed (ν0 = 0.012μM min-1) when PhSeSePh (462×10-6 M) is added. Under the identical conditions, the SPM exhibits a remarkable rate enhancement (ν0 = 13.1μM min-1). Assuming that the rate has a first-order dependence on the concentration of catalysts, this data suggests that the activity of SPM is at least 504000-fold more efficient than that of PhSeSePh. Moreover, typical saturation kinetics of the SPM catalysis for the peroxidase reaction is observed at the individual concentrations of ArSH and CUOOH, which indicates that this model is a real catalyst for peroxidase reaction.In this preliminary report, we have demonstrated the supramolecular polymer strategy is an efficient strategy for construction functional complex macromolecular structure by incorporation of bioactive andπ-πstacking building blocks. The self-assembly of supramolecular polymers can be successfully utilized for the formation of GPx mimic. We anticipate that this strategy would become a promising and versatile one for constructing macromolecules into structures of various shapes and functions found in nature.