Understanding Defect Structures and Host-guest Interactions in Metal-organic Frameworks
Time: Fri 2022-06-10 14.00
Location: U1, Brinellvägen 26, Stockholm
Subject area: Theoretical Chemistry and Biology
Doctoral student: Kang Zhengzhong , Teoretisk kemi och biologi
Opponent: Professor Lars Ojamäe, Linköping University
Supervisor: Universitetslektor Yaoquan Tu, Teoretisk kemi och biologi
Metal-organic frameworks (MOFs) represent a class of crystalline porous materials with prospective applications in molecular capture, separation, storage and catalysis. It has been discovered that there exist various structural defects in MOFs, which can have dramatic impact on their properties. However, the fundamental understanding of the structure and chemistry of the defects in MOFs is still very limited.
In this thesis, I focus on the studies of defect structures and host-guest interactions in MOFs by using theoretical chemistry methods. These studies cover the dynamic interplay of water and acetate molecules at the defect sites of linker missing MOFs, host-guest interactions of drug molecules loading in defective MOFs, preparation and triggered release of intrinsically coordinated CpG-MOF conjugate nanoparticles, and the defect-assisted single-strand DNA adsorption and folding on MOFs.
The main results of the studies are described below.
1. The possible binding structure of acetate and water molecules on defect sites in MOFs are explored. Acetate is coordinated to an unsaturated metal site in a monodentate way, which is accompanied by a water molecule coordinated to the neighboring metal site. The highly flexible acetate molecule performs a fast rotation and undergoes a slow kinetic exchange with a water molecule. Two models, one with insufficient water and the other with excess water, were built to study the role of dynamic hydrogen bonds in the exchange process.
2. The loading behavior of drug molecules in MOFs has been studied. The interactions between the drugs and MOFs are fundamentally different for the ideal and defective MOFs. The defects play a key role in the loading of the drugs with phosphate or phosphonate groups on MOFs. The host-guest interactions are dominated by the Coulombic attraction between the phosphate/phosphonate groups and the defect sites, which greatly enhances the loading capacity of the drugs. The conformations of the drugs at the defect sites have also been studied.
3. A new strategy for preparing DNA-MOF conjugates is provided and the interactions between oligonucleotides and MOFs have been studied. Compared to the covalent modification of DNA-MOF nanoparticles, the unmodified oligonucleotides can be densely loaded on the MOFs via the intrinsic, multivalent coordination of the DNA backbone phosphate to the unsaturated zirconium sites on the MOFs. The loading state of CpG on the MOFs is the coexistence of nucleotides in the adsorption state and dangling state. The surface-bound DNA can be efficiently released by free phosphate ions in the acidic environment.
4. The loading and folding of ssDNA on MOFs have been systematically studied. ssDNA prefers to be on the surface of the MOFs rather than inside a cage due to size limitation. The defect-assisted surface adsorption is mainly contributed by the electrostatic interactions between the ssDNA and MOFs. The binding of the ssDNA to the MOFs is dominated by the multiple point anchoring of the phosphate groups of the ssDNA on the MOF clusters. Water layers with complex hydrogen-bond network function as a gate, preventing the DNA from approaching the MOFs before adsorption and inhibiting the DNA from leaving the MOFs after anchoring. Unlike the circular folding structure in the solution, the adsorbed ssDNA display a slender conformation or duplex like structure.