Single Molecule Mechanochemistry And Soft-mechanochemistry

Regarding the activation of chemical reactions,traditional chemistry can be divided into thermochemistry,photochemistry and electrochemistry.Besides,the branch of chemistry where chemical conditions are induced by mechanical force is called mechanochemistry.Recently,with the burgeoning field of polymer mechanochemistry,many mechanophores have been designed and synthesized for various applications,including stress sensing,self-healing and catalysis.This approach often requires high energy input and high-intensity forces to induce covalent reactions.Mechanochemistry processes also play a great role in nature.Especially in cell biology,mechanical forces can be transduced into biochemical responses through mechanochemistry processes,which take place at much weaker forces than required to affect covalent bonds.In contrast to conventional mechanochemistry,this noncovalent mechanochemistry can be called soft-mechanochemistry.However,there are plenty mysteries to discover and many questions to answer in the infancy of mechanochemistry and soft-mechanochemisty.In this doctoral thesis,with atomic force spectroscopy based single molecule force spectroscopy study in different aspects,we would like to provide further fundamental insights into mechanochemistry and soft-mechanochemisty in order to solve the basic problems of mechanochemistry and prompt the development of mechanochemisty.This doctoral thesis mainly contains the following contents:（1）Single molecule study of cis-to-trans isomerization of carbon-carbon double bonds in polymersCarbon-carbon double bonds（C=C）are ubiquitous in natural and syntheticpolymers as basic chemical bonds.In most studies,they are thought to be mechanically inert.Based on a single molecule force spectroscopy study using atomic force microscopy,we directly observe the cis-to-trans isomerization of C=C bonds at room temperature by applying a tensile force～1.7 nN.The reaction proceeds through a radical intermediate state,as confirmed by both free radical quenching experiments and quantum chemical modeling.We propose that because the pulling direction is not parallel to C=C double bonds in the polymer,stretching the polymer not only provides tension to lower the transition barrier but also provides torsion to facilitate the rotation of cis C=C bonds.This explains the apparently low transition force for such thermally"forbidden" reaction.（2）Single molecule study of maleimide-thiol bondMaleimide-thiol adducts have been widely employed as covalent cross-linking.The mechanical stability of maleimide-thiol bond was still complexing for the thiol exchange process and the hydrolysis of succinimide ring.Here,we study the mechanical response of maleimide-thiol bond with single molecule force spectroscopy.We found that the aqueous rupture force of maleimide-thiol bond was larger than that in anhydrous solution.We designed pre-ring-open experiments and pre-stretch experiments to study the mechanochemistry pathway of maleimide-thiol bond,we found that the hydrolysis rate of maleimide-thiol bond can be accelerated by mechanical force,leading to a more stable linkage.In this way,the half life of ring-open reaction was shortened from 100 hours to less than 1 second,showing a powerful prospect of mechanochemistry.（3）Single molecule study of ligand-receptor interactionCell-adhesion molecules（CAMs）often exist as homodimers under physiological conditions.However,the molecular mechanism underlying this natural design is still unknown.Here,we present a theoretical model to understand the rupture behavior of cell-adhesion bonds formed by multiple binding ligands with a single receptor.We found that the adhesion strength could be greatly enhanced in comparison with the monomer case through a ligand rebinding and exchange mechanism.We also confirmed this prediction by measuring dimeric cRGD unbinding from integrin（α_vβ₃）using atomic force microscopy based single molecule force spectroscopy.Our finding addresses the mechanism of increasing the binding strength of cell-adhesion bonds through dimerization at the single-molecule level,representing a key step toward the understanding of complicated cell-adhesion behaviors.（4）Single molecule study of ligand-membrane interactionT-cell receptor-CD3 complex can initiate antigen-specific immune responses based on various dynamic processes of CD3 cytoplasmic domains binding to membrane,but the underlying structural basis remains elusive.Here we developed mechanochemistry approaches to study the conformational dynamics of CD3 cytoplasmic domain.At the single molecule level,we found that CD3 cytoplasmic domain could have multiple conformational states with different openness of three functional motifs,which have heterogeneous lipid-binding properties.Lipid-dependent conformational dynamics thus provide structural basis for the versatile signaling property of TCR.