Investigation Of Polymer Nanocomplex Using AFM-Based Single Molecule Force Spectroscopy

Investigation on the interactions between different components in polymer complex and the molecular mechanism for the assembly and disassembly are essential for promoting the performance of the complex.Traditional characterization（Fourier Transform Infrared Spectrometer,Circular Dichroism,Molecular Dynamics）of these interactions within polymer complex,even though are successful in probing average interactions,it is quite difficult to derive the binding mode and quantify the stability of the complexes and describe the assembly/disassembly mechanism directly from those ensemble measurements.Atomic Force Microscopy（AFM）-based single molecule force spectroscopy（SMFS）showed up for the urgent demand for molecular level investigations.In this article,we investigated the molecular mechanism of the assembly/disassembly of polymer complex with AFM-based SMFS in a combination with CD,FTIR and XPS.We have developed a new method to monitor the driven force,assembly-reassembly process in real time and regulate the binding mode and binding strength.The main findings are as follows:1)By conducting SMFS on PLL/POM complex,sawtooth pattern can be observed in the low force region of F-E curves.In the presence of salt,our results show that PLL-POM interaction can be weakened by high ionic strength,which indicates that PLL binds to POM mainly via electrostatic interaction.The representative stretching-relaxation curves show that the PLL-AlMo₆ complexes can reform during relaxation.Experimental results show that the particle shape has greatly influenced the binding mode and strength between polymers and POM particles.However,charge density and particle size are not crucial for the unbinding strength.Our dynamic force spectroscopy studies indicate that the disk-like geometry facilitate the unbinding of PLL/POM complexes in shearing mode,while the unzipping mode becomes dominate in spherical PLL/POM system.These results prove that SMFS is quite suitable for investigating polymer-particle interactions in polymer nanocomposite.Furthermore,the established single-molecule method can be used to study many other polymer-nanoparticle interactions.2)We introduce a single molecule AFM method that can demonstrate the long-range cooperative unwrapping mechanism of ssDNA from chiral SWCNT.In our approach,long ssDNA was composited with chiral CNT to form helical wrapping.Our results show that wrapping-unwrapping of the complex is reversibly quasi-equilibrium in water driven byπ-πinteractions between DNA base and CNT.However,the unwrapping in phosphate buffer saline（PBS）is loading rate dependent,whereas the wrapping of ssDNA on CNT undergoes two distinct stages dominated byπ-πinteraction and hydrogen bond,respectively.Cations could stabilize the structure of ssDNA/CNT by blocking the electrostatic interactions between DNA phosphate backbones and stimulating the formation of hydrogen bond between DNA bases.Moreover,cyclic unbinding-rebinding of the complex has enabled us to interpret the binding process of the complex.Dynamic force spectroscopy analysis helps us to extract the kinetic parameters of binding-unbinding realities of the complex and draw the full free energy landscape of the rupture and re-folding of the complex.Overall,the helical structure of ssDNA/CNT complex will be tightened with the formation of hydrogen bond.3)We have investigated the unbinding-rebinding process of a tandom poly-rotaxane.The binding mode and binding strength have been also revealed.Through SMFS method,we are able to get the unbinding information of each rotaxane on the whole tandom chain and quantifying the strength of the host-guest interaction.The experiments in different solvent conditions shows that poly-rotaxane chains keep intact in solvent but unbind immediately once enough external force is applied.Consecutive unbinding-rebinding cycles reveals that after being unlocked,poly-rotaxane are able to travel back to its original structures once the chain is relaxed.The loading rate dependence tells us that unbinding force increase linearly with the increase of force loading rate,whereas the rebinding force decrease linearly with the increase of force loading rate.Further,we are able to get the kinetic parameters governs the dynamic motion of poly-rotaxane.With these kinetic parameters,we are able to draw the potential energy landscape.We also quantified the strength of axle-wheel desorption strength,which is helpful for rational design of molecular machine.4)We have investigated the motions of macrocycles on both poly[2]catenanes and poly[n]cateananes by conducting SMFS on poly[2]catenanes and poly[n]catenanes in different solvent conditions and extremely low pulling speed.We are successful in deriving the kinetic parameters govern the linear motion of poly[2]catenanes.We find that without IHB,the rings on poly[2]catenanes can move in a combination many motions.At low pulling speed,we capture some fluctuations in poly[2]catenane with confinement of H-bond.These hopping from unbinding to binding is a result of limited elongational and rocking motion.We have also compared the motion of poly[n]catenanes and poly[2]catenanes.Interestingly,the rings on poly[n]catenanes owns more freedom of motion due to the good flexibility of the chain.