The NMR Investigation Of The Host-gest Interactions Based On Dendrimers

Polyamidoamine (PAMAM) dendrimers is a new class of artificial polymers with nano-scale size and symmetrical globular shape which are synthesized by Tomalia in1985. Compared to the traditional polymers, PAMAM dendrimer has several unique properties such as symmetrical, well-defined and hyperbranched structure, extremely low polydispersity, well-defined size, well solubility, multiple hollow relative non-polar cavities and number of peripheral groups. Due to these structural properties, PAMAM dendrimers have been widely applied in many fields include catalyst, drug and gene delivery, and the template of the nanopartical synthesis. These applications of PAMAM dendrimer are based on the non-covalent host-guest interactions between dendrimers and guest molecules. Nowadays, the study of the host-guest interaction based on PAMAM dendrimer is a hotspot in the polymeric chemistry and physics fields.In this dissertation, the host-guest interactions between PAMAM dendrimer and guest molecules are investigated by the nuclear magnetic response (NMR) techniques. The focuses of this dissertation are the binding kinetics between dendrimer and the guests, the binding sites, the surface binding/encapsulation transitions and the structure of the aggregates. The main research works and conclusions are organized as followings:(1) The detailed interaction mechanisms between PAMAM dendrimer and surfactants (sodium dodecyl sulfate, SDS) in aqueous solutions are investigated by a combination of1H-NMR, diffusion measurements (PFG), and NOE techniques. The diffusion studies suggested that different types of dendrimer-surfactant aggregates were formed by varying the surfactant concentrations in the dendrimer solution. A inclusion structure is formed at a low SDS concentration. With the addition of the SDS, the bilayer micelles of SDS are formed and binded at the surface of PAMAM dendrimer and the supermolecular aggregates are formed at last. The1H NMR analysis for the binding kinetics between dendrimer and the guests further proved the presence of fast exchange/slow exchange transitions in the dendrimer-surfactant aggregates. The formation of molecular inclusions in the dendrimer/SDS complex is concluded by the NOE measurements. The supramolecular structure of the aggregate was based on the hydrophobic interactions between the dendrimer scaffold and the surfactant aliphatic chain, as well as electrostatic/hydrogen-bond interactions between dendrimers and SDS monomers, bilayer or globular micelles.(2) The interaction between G5PAMAM dendrimer and the guano sine monophosphate (GMP) which is a basic unit of DNA and RNA, is investigated by NOE NMR techniques and the precise information of the binding sites between PAMAM dendrimer and GMP molecules is obtained. The NOE results indicated that a balance occurs between the surface binding of GMP on the dendrimer and the encapsulation in the cavities of dendrimer and this balance is controlled by the solution pH value. The GMP molecules are binding on the surface of dendrimers in the basic solutions, and the encapsulation of the GMP by the dendrimer cavities occurs at the neutral and mild acidic conditions. GMP molecules are in a free-state and do not bind with host molecules in the strong acidic solution. The binding/encapsulation transitions are depended on the surface and interior charge densities of the dendrimer and the charged state of the guest molecules.(3) The NMR techniques including1H NMR and1H-1H NOESY are used to characterize the host-guest interaction between fully acetylated PAMAM dendrimers and the congo red (CR) and indocyanine green (ICG) molecules bearing multiple. The cationic PAMAM dendrimers with amine groups at their surfaces are found to form precipitate with CR and ICG, but acetylated PAMAM dendrimers avoid the formation of cross-linking structures in aqueous solutions. NOESY studies revealed that the stable complexes are formed between acetylated dendrimers and CR/ICG at the mild acidic conditions and acetylated dendrimers show much stronger ability to encapsulate the guest molecules with multiple charges and large molecular weight than cationic ones. Also, UV-Vis-NIR studies suggested that acetylated dendrimers significantly improve the photo-stability of indocyanine green and prevent the formation of indocyanine green J-aggregates in aqueous solutions.These results show that NMR technique is a sensitive, high-efficiency and convenient method for the investigation of the intermolecular interaction between dendrimers and guests, and can provide a new physical-chemical view and a new idea for the host-guest system of the dendrimers. This research can also give some important theoretical supports for the drug delivery development based on dendrimers.