Behaviors Of Biomacromolecules In Aqueous Solutions

Biomacromolecules play an important role in the field of life sciences.Study on their behaviors in aquesou solutions can help to understand their functions.In this thesis,we have investigated the thermodynamic and dynamic behaviors and properties of DNA and proteins in aqueous solutions by using a combination of analytical ultracentrifugation(AUC)via sedimentation velocity(SV)and microcalorimetry(microDSC).The main contents and results are as follows:(1)We have investigated the conversion from single-stranded DNA(ssDNA)to double-stranded DNA(dsDNA)using SV.For solutions containing monovalent cations(LiCl,NaCl,KCl,CsCl),the conversion of ssDNA to dsDNA first increases with ionic strength and then levels off at I= 0.01 M.All the four of the monovalent cations behave similarly in promoting the formation of dsDNA.A degree of ion-specificity in dsDNA formation still exists since the cations have difference influenc in the equilibrium constant for the conversion of ssDNA to dsDNA.In the case of divalent cations(e.g.MgCl2),the conversion from ssDNA to dsDNA also increases with increasing ionic strength.On the other hand,the plateau starts at a much lower ionic strength(I= 5.0 × 10-4 M)because of higher electrostatic screening efficiency of Mg2+ cations and their superior ability to link DNA chains.We also studied the effect of monovalent and divalent cations on the dynamic behavior of ssDNA without complementary chains.(2)We have investigated the pH-induced conformational change of individual i-motif DNA chains by sedimentation velocity from analytical ultracentrifugation.In the pH range of 6-7,the hydrodynamic radius(Rh)of DNA chains suddenly increases as pH increases.It undergoes an individual DNA conformational change from an i-motif to a random coil rather than aggregation since the molar mass is constant in all solutions with different pH values.Relatively stable dimers are formed as pH sharply decreases from 7.5 to 4.5 with DNA concentrations higher than 1.0?M,Moreover,the weight percentage of the dimers increases with the initial DNA concentration.(3)We have studied the denaturation of lysozyme with a combination of SV and microDSC.Our studies reveal that the degree of denaturation irreversibility increases as heating times increases since the protein spatial structure is destroyed.The denaturation is not in equilibrium during the heating,so the heating rate has influence on the denaturation temperature(Td)and enthalpy change(?H).The value of Td and?H in equilibrium can be obtaioned by extrapolation of heating rate to zero.In a dilute solution,no aggregation but unfolding happens in the denaturation.However,lysozyme molecules readily form trimers or other oligomers,when the concentration is above a critical value(?15.0 mg/mL).Lysozyme molecules unfold into stretched chains at pH>6.0.The further formed aggregates make the refolding of lysozyme impossible.