The Phase Transition Behaviors Of Macromolecules In Solutions Investigated By Microcalorimetry

The thesis deals with the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in dilute and semidilute aqueous solutions as well as in in aqueous solutions with polyethylene glycol(PEG) as the crowding agent,association and dissolution of linear poly(N-n-propylacrylamide)(PNNPAM) chains in water,the thermal denaturation and re-denaturation behaviors of lysozyme in aqueous solutions and aqueous solutions with polyethylene glycol(PEG) as the crowding agent.Such behaviors of the polymers have been investigated by use of ultra sensitive differential scanning calorimetry(US-DSC) and pressure perturbation calorimetry(PPC).The results are as follows.1.Aggregation and dissolution of PNIPAM in water were investigated using an US-DSC and a PPC.US-DSC reveals that both the aggregation and dissolution of PNIPAM chains are greatly dependent on the scanning rate,indicating that the processes are kinetically controlled.The hysteresis in the dissolution process was found to have a nonequilibrium nature,which is thought to be related to the additional hydrogen bondings formed in the collapsed state of PNIPAM chains.A birnodal appearing in the cooling process at a slow scanning rate indicates the dissolution involves two different processes,i.e.,the disruption of additional hydrogen bondings and the dissolution of the collapsed chains.PPC reveals that the solvent accessible surface area of PNIPAM chains in the cooling process is smaller than that in the heating process,which further indicates the dissolution of the PNIPAM aggregates involves such two processes.2.The concentration(C) effect on association and dissolution of linear PNIPAM chains in dilute and semidilute aqueous solutions was investigated by using US-DSC. When C is below the overlap concentration(C^*),both the phase-transition temperature(T_p) and enthalpy change(△H) in the heating process decrease as the solution becomes more dilute,while T_p and△H become independent of C in the semidilute regime(C＞C^*).By extrapolating concentration to zero,we have obtained the phase transition temperature(T_s) and enthalpy change(△H_s) of the single chain folding.△H_s is higher than that for a phase transition involving intrachain collapse and interchain aggregation,indicating that a single chain folding can not be taken to be a macroscopic phase transition.3.Collapse and aggregation of PNIPAM chains in aqueous solutions with PEG as the crowding agent have been investigated by using US-DSC.For any PEG with a certain molecular weight,the transition temperature of PNIPAM decreases with PEG concentration(C_PEG) due to the complexation between PEG chains and water molecules.A hysteresis has been observed in one heating-and-cooling cycle.As C_pEG increases,short PEG chains lead the hysteresis to enlarge,whereas longer PEG chains result in a decrease in the hysteresis.The facts indicate that longer PEG chains suppress the interchain aggregation of PNIPAM chains.When C_PEG is above the overlap concentration(C^*),the hysteresis is almost independent of PEG concentration, suggesting that PEG chains form a transient network which locates PNIPAM chains in a number of pores.The widening of the transition with increasing C_PEG at C_PEG＞C^* indicates that the network is inhomogeneous.The fact that△H decreases with C_PEG further indicates that PEG chains reduce the interchain aggregation of PNIPAM chains.The same△H in heating and cooling processes demonstrates that PNIPAM chains form small-scale aggregates in the presence of PEG.4.The concentration(C) and heating/cooling effects on association and dissolution of linear PNNPAM chains in water were investigated by using US-DSC. In the heating process,PNNPAM chains undergo a LCST transition at～24℃.△H increases with increasing heating rate and concentration.The highest T_p in the cooling process is close to that in the heating process,indicating no additional hydrogen bondings.The multiple-transition observed in the cooling process indicates that the dissolution of aggregates undergoes multiple hydration processes.5.The effects of lysozyme concentration,ion strength,pH,etc.on the thermal denaturation and re-denaturation behaviors of lysozyme were investigated by using US-DSC.The results reveal that T_p increases with the increasing lysozyme concentration.A new endothermic peak located at lower temperature is observed in the re-denaturation process,which indicates that the increasing concentration induces the association of lysozyme in the heating process.T_p of both the thermal denaturation and re-denaturation processes increase with the inceasing heating rate,suggesting the association behaviors exist in the thermal denaturation process.A slower heating rate promotes the association.As the ion strength increases,T_p in the first heating times decreases and the endothermic peak becomes wide,which is caused by the disruption of the hydration around lysozyme.The excessive amounts of NaCl in solution act as the denaturant in the thermal denaturation of lysozyme.The activity of lysozyme disappears in the second heating times when C_NaCl＞0.6 mol/L.T_p increases with the increasing pH values.Tp increases from 51.9 to 70.09℃as pH increases from 2.0 to 3.0,which is caused by the decreasing content ofα-helix with increasing pH.6.The thermal denaturation and re-denaturation of lysozyme solutions with PEG as the crowding agent have been investigated by using US-DSC.Ethyl glycol(EG) in lysozyme solution increases T_p.When M_w,PEG＜1000 g/mol,Tp has a small decreases with increasing molecular weight of PEG and the enderthermic peak weaks,which indicates that the shorter PEG chains are easy to induce the association of lysozyme. T_p increases with increasing molecular weight of PEG when M_w,PEG＞1000 g/mol.T_p is independent of the change of C_PEG when 6000＞M_w,PEG＞2000 g/mol.On the other hand,△H decreases with increasing C _PEG,which is caused the restriction of the hydration of lysozyme by adding PEG.More PEG chains in solution decreases the hydration of lysozyme,△H thus decreases.The number of cooperative unit(n) decreases with increasing C_PEG,which further confirms that PEG can promote the association of lysozyme.