The Studies Of Noncovalent Interaction Based On Single Molecule Force Spectroscopy Method

Hydrophobic interaction,one kind of noncovalent interaction,plays an important role in biochemical processes,including protein folding,ligand-receptor binding and membrane formation.Calculation of hydrophobic free energy is a significant field in studying hydrophobic interaction.Lum,Chandler and Weeks proposed that hydrophobic free energy was dependent on the scale of hydrophobic solute(the LCW theory),which provided a new insight into the mechanism of hydrophobic hydration for us.Hydrophobic free energy scales with volume for hydration of small hydrophobic solute,while hydrophobic free energy scales with surface area for hydration of large hydrophobic solute.The hydration process of the former is dominated by the entropy of the system and the hydration process of the latter is dominated by the enthalpy of the system.1 nm is the crossover length which reflects the distinct feature of hydration mechanism at micro level.A lot of simulated and computational studies achieved great successes after the LCW theory,however,there yet was no experimental method to directly verify the LCW theory for the difficulties in designing the experiment.Fortunately,our experimental works achieved that.Cation-π interaction,another noncovalent interaction,also acts an important role in biochemical processes,including protein folding,molecular recognition,molecular adhesion as well as the selectivity of potassium channel.Researchers paid great attention to studying the binding energy between the cation and the π bond.They used electrostatic force and induction force to interpret the attractive force between the cation and the π bond which was widely accepted.However,the binding energy was mostly calculated by simulated and computational methods in gas phase because the binding of the cation and the π bond was not disturbed under the environment of gas phase.In fact,the cation-π interaction in the biochemical processes is under the environment of the aqueous solution.Complex environment of aqueous solution leads to a great difficulty in studying cation-π interaction.Application of single molecule force spectroscopy method makes many issues can be investigated directly by experiment,overcoming the difficulties of designing experiment.Our studies of the length-scale dependence of hydrophobic free energy,the hydrophobic and oleophobic strength of fluorinated polystyrene and polystyrene,and the cation-π interaction were just based on the single molecule force spectroscopy method.Here,the paper is mainly divided into following five chapters:In chapter one,we provided a systematic background introduction of the hydrophobic interaction,the hydrophobic and oleophobic phenomenon,and the cation-π interaction,which simultaneously classified the necessity of our works.Considering the studying object and method,we showed a simple introduction of polymer,atomic force microscopy,and single molecule force spectroscopy.In chapter two,we calculated the hydrophobic free energy of polystyrene(PS)nanosphere and showed the length-scale dependence of hydrophobic free energy.We synthesized PS with bifunctional groups by reversible addition-fragmentation chain transfer(RAFT)polymerization method.This method was used through the whole paper.PS is a typical hydrophobic polymer.Under the hydrophobic driving force,the PS chain modified on the substrate will collapsed into a compact nanosphere.We used atomic force microscope(AFM)to unfold such nanosphere.Under stretching,the PS nanosphere was extended to a long chain and the AFM recorded the curves of force spectroscopy.Initial radius of PS nanosphere was several nanometers and the final radius was zero.The transition of radius went across the crossover length of 1 nm predicted by the LCW theory.That was exactly what we needed.We conducted refolding experiment and different-speed unfolding experiment to demonstrate that the unfolding of nanosphere was a quasi-static process.Based on that,we used Lagrange multiplier method to solve the extreme point of hydrophobic free energy and derived the expression of hydrophobic free energy of PS nanosphere.Combining with the curves acquired in the experiments,we calculated the hydrophobic free energy of PS nanosphere.The hydrophobic free energy was linear with volume when the radius was smaller than 1 nm and the hydrophobic free energy was linear with surface area when the radius was larger than 1 nm.The transition scale of hydrophobic free energy did occur around 1 nm.In chapter three,we investigated the difference of hydrophobicity and oleophobicity between fluorinated PS(FPS)and PS.We used single molecule force spectroscopy method to study the hydrophobicity and oleophobicity at micro level.There was no difference of hydrophobicity between FPS and PS at micro level.The study of oleophobicity enlightened us that oleophobic force was likely to be in existence at micro level because the hydrophobic force at micro level corresponded with the hydrophobic phenomenon at macro level.The hydrophobic force and oleophobic force may be related to the entropic force.At macro level,we measured the contact angle to characterize the difference of hydrophobicity and oleophobicity.Because the introduction of fluorine reduced the surface free energy,the hydrophobicity and oleophobicity of FPS was larger than PS’.Such result was agreeable with Young’s equation.In chapter four,we studied the cation-π binding strength of Li+-Benzene,Na+-Benzene,K+-Benzene and NH4+-Benzene in aqueous solution.We designed the unfolding experiment of PPFS nanosphere and PS nanosphere,respectively.For PPFS,the hydrogen on the benzene ring was completely replaced by the fluorine.As a result,it can hardly bind with the cation in aqueous solution by the means of cation-πinteraction.However,the benzene ring in the PS can bind with the cation forming cation-π.We compared the difference of plateau force of unfolding PPFS and PS and found that the binding strength order was K+ > Na+ > NH4+ > Li+.Besides,we found that the plateau force of unfolding nanosphere was linear with the surface tension of salt solution.Our experiment can be conducted at high concentration of salt solution.The result did not show saturation of cation-π binding.In chapter five,we give a summary and an expectation.