Development And Application Of Coarse-grained Force Field For Amphiphilic Molecules

Soft matter features strong response to a weak outside stimulus,which endows it with extensive value of application.Hence,it becomes a research hotspot.Amphiphilic molecule,as an important soft material,can spontaneously form ordered structures in the solvent and has been applied widely.Lipid molecule,as a small amphiphilic molecule,is an integral part of the biological membrane and can be utilized as carriers for genes and drugs.Star block copolymer,as an amphiphilic macromolecule,has big differences with traditional linear copolymers in terms of the physical and chemical properties,and possesses the property of spontaneously forming more structures.With the constant development of computer technology,the simulation method has become a bridge between theory and experiment.In this paper,we use computer simulations to study dynamic self-assembled processes of amphiphilic molecules,to investigate stability of self-assembled aggregates and to research transportation behavior of amphoteric macromolecules under non-equilibrium conditions.Our work has great value in guiding the control of the self-assembly theory of amphiphilic molecules,as well as the application of these materials.The content of this research and its main conclusions are shown as follows:1.An implicit solvent coarse-grained(CG)lipid model using three beads to represent the basical molecular structure of two-tailed lipid is developed.In our model,the non-bonded interaction employs a variant MIE potential and the bonded interaction utilizes a Harmonic potential form.The CG force field parameters are achieved by matching the structural and mechanical properties of dipalmitoylphosphatidylcholine(DPPC)bilayers.Our model can commendably reproduce the formation of lipid bilayers with a random initial state and the process of spontaneous vesiculation of lipid bilayers from a disk-like structure.2.We apply the Martini force filed to study the micellization behaviors of small mixed droplets.By varying DPPC/HPC composition and the size of lipid droplets,various micelles such as spherical and nonspherical(oblate or prolate)vesicles,disk-like micelles,double or single ring-like and worm-like micelles are observed.By using the implicit solvent coarse-grained(CG)lipid model mentioned above,we study the effect of three different initial states(sphere,cylinder and ring)on the self-assembly.In the self-assembling processes,we can observe the metastable prolate and oblate unilamellar vesicle,spherical unilamellar vesicle,multilamellar vesicle and “life buoy” vesicle.The prolate and oblate vesicles are not stable,and they tend to become spherical vesicles ultimately.The multilamellar vesicles are also not stable,and they are likely to form spherical unilamellar vesicles after a long ripening process.The results imply that the present CG model can effectively simulate the formation and evolution of mesoscale complex vesicles.3.We study the wrapping process of lipid vesicles on polymer chains in soft confinement.The dependences of the properties of polymer chains on the wrapping behavior are studied including rigidity,degree of polymerization and the number.The results show that the rigidity and the number of polymer chains have the same effect on the wrapping behavior.The stronger the rigidity of the polymer chains are(or the more number of polymer chains there are),the deformation of vesicles and the extension of polymer chains are easier to occur.The polymer chains form a beam-like structure.When the polymerization degree of polymer chains is low and the number of polymer chains is sufficient,the vesicle deforms into a long cylindrical structure induced by the extension of polymer chains.When the polymerization degree of polymer chains is high,the vesicle deforms into an oblate shape and the polymer chain forms a coil structure.4.A coarse-grained model of ABC star block copolymer is developed.Applying the solvation free energy of a polymer chain,the simulation parameters match the properties of the nature systems.By using this model,we study the influences of the annealing rate and the length of hydrophilic chain segments on the self-assembly of ABC star block copolymers.The slower the annealing rate we set,the easier order multicompartment micelles to form.Whether the hydrophilic chain is short or long,ABC star block copolymers prefer to occur in-situ micellization,instead of forming order structures.The results show that this model can be used to study the self-assembly of ABC star block copolymers.Using a Helfrich's membrane model combined with the strong segregation theory of block copolymers,we analyze the stability of this stripe prolate vesicle.5.The net mobility of an amphoteric macromolecule with complete charge in a ratchet slit under AC electric field with explicit solvents is studied.The sequence of the amphoteric macromolecule,the width of the ratchet slit and the external electric field can affect the net mobility.The results show that three types of ratchet effect,i.e.the entropic effect,conformation-inversion effect and the solvent dissipative effect,contribute to the net mobility.The proportion of three types of ratchet effect determines the direction of net mobility under different conditions.Under the low-frequency electric field,the center of mass velocity of amphoteric macromolecule composes a linear relationship with the frequency of electric field.