Theoretical Study On The Folding Thermodynamics And Dynamics Of Single Chain Polymers-force Field Effect

The study of polymer single chain folding can not only reveal the physical behavior of polymer chain in dilute solution,but also help to understand the universal mechanism of biological macromolecular folding.In solution,due to the interaction of hydrogen bond,hydrophobicity and van der Waals force,the polymer chain usually presents collapse structure.In order to clarify how polymer chains fold,some mean field models have been used to study the folding behavior of polymer chains at different temperatures or solvent properties.In these models,the equivalent attraction between chain units is usually simplified as Lennard Jones(LJ)potential.However,how the spatial range of attraction affects the static and dynamic properties of polymer chain folding is still unclear.In addition,in the folding process of polymer chain,the chain units need to cross the desolvation barrier and combine closely,resulting in the range of attraction and the shape of interaction potential different from the general LJ potential.In order to clarify how the range and shape of the equivalent interaction potential affect the thermodynamic and dynamic properties of polymer chain folding,the following two aspects are mainly carried out in this paper:(1)The effect of desolvation barrier on the folding thermodynamics and kinetics of semiflexible polymer chains.We have studied the difference of folding behavior between the general LJ potential and desolvation barrier potential(db).By drawing the phase diagrams with temperature T and bending stiffness ke as order parameters,we find that there are significant differences in the phase diagrams of polymer chain folding transition obtained by the two models:With the increase of chain stiffness,in LJ model,the low temperature steady-state structure changes from the compact structure(F)to the two ring structure(T2)through the disk structure(D)and the three ring structure(T3)respectively;In the db model,the low temperature steady-state structure changes from the compact structure(F)to the two ring structure(T2)through the bundle structure(R)and the three ring structure(T3)respectively.Furthermore,it is found that the difference of low temperature steady-state structure between the two models is due to the different range of interaction potential.In terms of kinetics,although the folding paths of the flexible homopolymer chains obtained by the two models are similar,the db model has a longer folding time than the LJ model,indicating that the desolvation barrier causes the surface of the folding energy landscape to be rougher.(2)The influence of the spatial range of attraction on the folding structure and folding cooperation of polymer chains.For semiflexible polymer chains,by drawing phase diagrams with the order parameters of interaction range λ and temperature T,we find that with the decrease of interaction range,the folding process of semiflexible polymer chains becomes simpler and the transition temperature of coil to globule decreases gradually.Although the low temperature steady-state conformations of the long range and short range interaction models are annular structures,the number of loops of the annular structure decreases with the decrease of the interaction range.Further studies show that the attractive range can change the shape of the folding energy landscape of semiflexible polymer chain.In addition,we study the folding cooperation of flexible homopolymer chains under short range attractive potential(λ=1.05).Although the transition between the globule and the compact structure is cooperative in thermodynamics,there is a serious rollover in the kinetic chevron plot,which indicates that the polymer chain is not cooperative in kinetics.We find that the dynamical non cooperative folding is mainly caused by the ground state shift effect,Hammond effect and in chain friction effect.