Background: poly (pyridine) bivalent platinum complexes have various spectral properties and excellent luminescent properties. They are important metal complexes (chem. Rev., 2015, 115, 7589). The complex can form ordered aggregates through noncovalent metal and π - π stacking in both solid and solution conditions. The formation of metal metal interaction is usually accompanied by the appearance of Near-infrared Luminescent Properties of the system. Molecular structure, liquid crystals, nanostructures and supramolecular gels based on poly (pyridyl) two valent platinum complexes have been reported through metal metal interactions. Two valent platinum supramolecular gels exhibit abundant photophysical properties, but few other properties have been reported.
Recently, in the field of supramolecular assembly, a class of interesting supramolecular systems have emerged, which have complex dynamic paths (nature, 2012, 481, 492). The intermediate state and the termination state of these supramolecular systems often have different properties, which means that people can obtain properties that cannot be obtained by thermodynamic design through the control of system dynamics, which is of great significance for the development of new materials. On the other hand, there are also "fuel driven" supramolecular systems (Science, 2015, 349, 1075), some special chemical reagents are coupled with the supramolecular system, which drives the property transformation of the supramolecular system through the chemical reaction between the chemical reagents and the supramolecular unit. The properties of the supramolecular system are no longer controlled by the thermodynamic factors, but are completely controlled by the "fuel" concentration and the chemical reaction dynamics, which provides a new way for the behavior regulation of the supramolecular system and the design of soft material Train of thought.
The study found that the researchers at University of Hong Kong's Ren Yong research group discovered that in the preparation of tripyridine acetyridine (Figure 1) and double chain DNA embedded compounds, it was discovered that the yellow platinum complex 1 solution added to DNA aqueous solution first appeared in brown gel like substances. With the stirring, the gelatinous material gradually disappeared and finally formed a yellow clear solution. The spectroscopic study of the gel formation and spontaneous dissociation process showed that (Fig. 2), the platinum complex was formed from the initial monomer state through the gel aggregation state to the final DNA embedded state, and there was no chemical reaction in the whole process. The supramolecular transition of platinum complexes has attracted researchers' interest in gel behavior.
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Figure 1. Chemical structure of platinum complex
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Fig. 2. absorption spectra (a) and emission (b) spectra of tripyridine alkyne platinum 1 from the initial monomer state through the gel aggregation state to the final DNA embedded state.
By adjusting the feed concentration and DNA/ platinum ratio, the hydrogel was obtained in water for a long time when the molar ratio of DNA base pair to platinum complex 1:1. A series of structural characterization experiments showed (Fig. 3) that the hydrogel is a supramolecular hydrogel with DNA as a continuous phase, and platinum complexes are stacked into six dimensional columnar phases through metal metal and pion interactions. The platinum columnar phase has multiple positive charges, and the DNA chain acts as a non covalent crosslinking agent through electrostatic attraction.
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Fig. 3. transmission electron microscopy, selected area electron diffraction and powder X- ray diffraction results of DNA supramolecular hydrogels with non covalent crosslinking of tripyridine alkyne platinum
In the presence of excess DNA, the embedding effect of DNA and tripyridine alkyne platinum is stronger than that of metal metal and pion piling. The competition results in the dissociation of DNA supramolecular hydrogels to form a clear yellow solution of DNA- platinum embedded compound (Fig. 4). Changing the structure of tripyridine alkyne platinum and introducing a more hydrophobic platinum complex 2 as a non covalent crosslinking agent can improve the stability of DNA supramolecular hydrogels in excess DNA solution, thus regulating the kinetics of hydrogel dissociation.
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Figure 4. Supramolecular behavior of tripyridine platinum acetylation and DNA system. In the presence of excess DNA, platinum complex was first released through metal metal and pion stacking instead of covalently cross-linked double stranded DNA. In the presence of excess DNA, DNA supermolecule hydrogel dissociated from the competition between metal and metal and PI pion due to the embedding of DNA and platinum complexes.
When the researchers replaced tripyridine alkyne platinum into two benzimidazole pyridinium chloride (Fig. 1), two benzimidazole pyridinium chloride could also form a non covalent crosslinking agent of DNA supramolecular hydrogels through stacking aggregation, and formed strong metal metal and palladium stacking interactions at the crosslinking point, excessive DNA. The aqueous solution is not enough to destroy the crosslinking point. The DNA supramolecular hydrogels, which are non covalently cross-linked by two benzimidazole pyridinium chloride, have high stability.
Interestingly, two benzimidazole pyridinium chloride, a non covalent crosslinked DNA supramolecular hydrogel, was soaked in glutathione aqueous solution. It was found that with the prolongation of immersion time, DNA was detected in the soaking solution. The free platinum complex dissociated from hydrogel is in fact a ligand exchange between the thiols of glutathione and chlorine on the two benzimidazole pyridinium chloride platinum complex, giving the platinum complex a great hydrophilicity and reducing the planarity of the platinum complex. It greatly weakens the gold metal and palladium accumulation between platinum complexes and causes the dissociation of DNA supramolecular hydrogels (Fig. 5). This way of controlling metal metal interaction by chemical reaction is rarely reported in the supramolecular system of platinum, which provides a new idea for the study of this kind of system. Glutathione is often used as an irritant reagent in redox response system. In this work, researchers use the combination of bivalent platinum and glutathione sulfhydryl soft acid and base to give the supramolecular system response performance, reflecting the distinct coordination chemistry characteristics.
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Figure 5. The supramolecular system of dibenzimidazolidine platinum chloride and DNA. Two benzimidazole pyridinium chloride is stabilized by DNA - supramolecular hydrogels through metal metal and palladium stacking interactions and non covalent crosslinking of DNA. The hydrogel was dissociated by ligand exchange reaction in glutathione aqueous solution.
The study concluded that the two valence platinum non covalent crosslinking agent for DNA supramolecular hydrogels was developed by accidental discovery in experiments and by the metal metal and pion stacking interactions of platinum complexes. The obtained DNA supramolecular hydrogel has excellent luminescent properties and good processing properties. The supramolecular behavior and properties of DNA supramolecular hydrogels can be well regulated by adjusting the structure of two valent platinum non covalent crosslinking agents, and introducing DNA embedding and ligand exchange reactions. Due to the use of binary supramolecular system, the supramolecular units needed can be selected flexibly, and a series of supramolecular materials with rich properties can be obtained through a small amount of chemical synthesis by means of modular preparation. This work provides a new research idea for the design of supramolecular materials, especially the metal containing supramolecular systems.
Platinum (II) non covalent crosslinkers for supramolecular DNA hydraulics
Kaka Zhang (post doctoral, Department of chemistry, University of Hong Kong, now researcher, Shanghai Institute of organic chemistry, Chinese Academy of Sciences) and Vivian Wing Wah yam * (ren Yonghua, Professor, Department of chemistry, University of Hong Kong, academician, Chinese Academy of Sciences)
Chem. Sci., 2020
http://dx.doi.org/10.1039/C9SC05910E
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