The Study Of Non-covalent Interactions By Single-molecule Force Spectroscopy

As a series of weak interactions between molecules,non-covalent interactions play a vital role in nature.Without non-covalent interactions,the formation of most molecular structures and the birth of life would never occur.Due to the important effects of non-covalent interactions on nature phenomena and life activities,the non-covalent interactions have been widely investigated for decades.However,since non-covalent interactions are rather weak,most relative researches remain on the stage of qualitative analysis;even the nature and origin of these interactions remain unclear.In order to deepen the understanding of non-covalent interactions,it is necessary to study their strength,kinetics and influencing factors at the single-molecule level.In this thesis,several non-covalent interactions have been investigated by using singlemolecule atomic force microscope(AFM).Firstly,we propose a simple method to measure the van der Waals(vdW)forces at the single-molecule level.With this method,the vdW forces between a single polymer repeating unit and a solid surface are successfully detected.Secondly,we systematically investigate the intramolecular hydrogen bond(H-bond)of poly(N-isopropyl acrylamide)(PNIPAM).As a result,the strength and influencing factors of intramolecular Hbonds are obtained.Finally,we systematically investigate the intramolecular ?-? interactions in polystyrene(PS).Accordingly,the strength and stability of intramolecular ?-? interactions are obtained.Besides,the effects of hydrophobic effect on ?-? interactions are also revealed.On the basis of these investigation,the main conclusions in this thesis can be summarized as follows:(1)We propose a method to carry out single-molecule experiments by AFM in vacuo.With this method,the disturbance from the environment are eliminated almost thoroughly,which makes the detection of non-covalent interactions at the single-molecule level possible.(2)We propose a simple method based on single-molecule AFM to detect the vdW forces at the single-molecule level.With this method,the vdW forces between a single polymer repeating unit and a solid surface are successfully obtained.By using different target molecules and substrates,we find that the vdW forces(21-54 pN)depend on both the property of solid surface and the polymer repeating unit size.For a solid surface with higher Hamaker constant,the vdW potential energy between molecule and surface is also higher,hence the vdW forces between molecule and surface are also stronger;for a polymer repeating unit with larger size,since vdW forces are additive,the vdW forces are also stronger.Moreover,we find that the vdW forces will decay largely in a liquid environment,whose values are several pN or even lower.These phenomena suggest that the molecular vdW forces strongly depend on the local micro-environment.These experimental results are well supported by theoretical calculations.This method will simplify the measurement of vdW forces at the single-molecule level,which may also provide method for the detection of other non-covalent interactions.(3)We systematically investigate the intramolecular H-bond of a macromolecule,PNIPAM.Accordingly,the strength of H-bond at the single-molecule level is successfully obtained.Firstly,we find that PNIPAM maintains its inherent elasticity of its backbone in a typical H-bond breaker,dimethyl sulfoxide(DMSO).However,in high vacuum,the formation of intramolecular H-bonds among side chains of PNIPAM can strongly affect the single-chain elasticity of PNIPAM.In nonane,the intramolecular H-bonds will be weakened to some extent,which still have neglectable effects on the single-chain elasticity of PNIPAM.Theoretical,the strength of intramolecular H-bonds of PNIPAM in high vacuum and nonane are calculated to be ~0.9 and 0.4 kcal/mol,respectively.Due to the restriction of backbone,however,these values are much lower than that in small molecule analogues.Besides,we also study the influencing factors of intramolecular H-bond.We find that the size of solvent molecule and dielectric constant have little effect on the intramolecular H-bond.These results will provide mechanism insights to the intramolecular H-bond,which can be applied to the precise design of supramolecular material based on H-bonds in the future.(4)We systematically investigate the ?-? interactions in PS.Accordingly,the strength of ?-? interactions and the relation between ?-? interactions/hydrophobic effect are revealed at the single-molecule level.Firstly,we obtain the single-chain elasticity of PS in the ?-solvent,cyclopentane,where the ?-? interactions are eliminated thoroughly.However,in high vacuum,the ?-? stacking among PS groups can be formed,which will strongly affect the single-chain elasticity of PS.These ?-? stacks are well maintained even if the chain undergoes large conformational changes,suggesting an adaptation of stacking configuration in a macromolecule.Upon further stretching,a transition of the phenyl groups from E-type to A-type stacking can be observed.Besides,in contrast to cyclopentane,we find that the ?-? stacks in PS haven't be eliminated in water but are similar to those in high vacuum.This result indicates that hydrophobic effect is the driving force of ?-? stacking in PS.Under the effects of hydrophobic interactions,the water molecules will keep away from the PS chain,leading to an air/liquid interface around the chain.Therefore,the situation of ?-? interactions in water are similar to that in high vacuum.This coordination between hydrophobic effects and ?-? interactions reveals an internal connection between non-covalent interactions.These results will provide mechanism insights for non-covalent interactions,which may cast new light on construction of nanomaterials.(5)We propose a new theoretical model,the TSQM-FRC model,to describe the polymer single-chain elasticity.With this method,the polymer single-chain elasticity under the effects of non-covalent interactions among side groups can be well fitted.Besides,many kinetics parameters including strength of these non-covalent interactions are obtained.The TSQMFRC model will become a powerful tool for quantifying the intramolecular interactions in the future.