The Interaction Of Polyethylene Glycol, Cetyl Alcohol And BSA With The Phospholipid Membrane At The Gas-liquid Interface

The large surface area of the alveoli provides an important place for air exchange between air and blood.Nature has minimized this energy drain by coating the lung air-liquid interface with a thin film of lipids and proteins,collectively called pulmonary surfactant(PS).The thin film not only reduces the surface tension and thus the energy consumption,but also makes the spherical alveoli stable in the process of breathing.The absence or inhibition of PS can lead to a sharp increase in the surface tension of the alveoli and cause a series of serious problems.Acute respiratory distress syndrome(ARDS)is a serious respiratory disease,although its pathogenesis is not clear,but it is generally believed that it is related to the leakage of inhibitor,e.g.serum protein,into the alveolar,which cause a competitive adsorption of PS and thus inhibit the adsorption of PS to the alveolar air-liquid interface.Therefore,the interaction mechanism between serum protein and PS at the air-liquid interface and how to effectively prevent the inhibition of serum proteins on the PS have become an important scientific issue.The treatment of neonatal respiratory distress syndrome(NRDS)with exogenous PS preparation has been achieved a great success,but the animal derived PS preparationis expensive,and also the existence of viruses pollution risk and immune rejection,batch difference and other shortcomings,therefore,it is still important to develop synthetic pulmonary surfactant.Hexadecanol(HD),which can promote the adsorption of DPPC at the interface and and then spreading into a monolayer,often used as an additive of synthetic PS preparation.However,the interactions between HD and other pulmonary surfactant molecules are unknown,which has become an urgent problem to be solved.In this paper,DPPC,DOPG and their binary mixture were used as model membrane to simulate pulmonary surfactant monolayers,and bovine serum albumin(BSA)was used to model the inhibitory protein.The interaction between BSA and DPPC,DOPG molecules at the air-liquid interface was studied from the viewpoint of thermodynamics and mechanics by means of Langmuir-Blodgett(LB)film analysis technique.In addition,combine AFM and LB techniques,the intermolecular interactions and phase behavior as well as microstructure of DPPG/HD and DPPE/HD binary mixed monomolecular films were studied.In the first chapter,the research background and experimental means were summarized.The theory and method of data analysis were reviewed in the following chapter.In the third chapter,the effects of dosage as well as the subphase on the phase behavior of DPPC,DOPC,POPC,DOPG and DPPG films at the air-water interface were studied,the results show that:(1)The phase behavior of DOPC,POPC and DPPG with different spreading dosage is respectively same;but for DPPC and DOPG films,their respective phase behavior with different spreading mole numbers are different with each other in their high surface pressure.Further analysis of the isothermal curves of DPPC and DOPG using mass balance equation shows that with the increase of the surface pressure,the desorption of DPPC and DOPG molecules from the interface into the subphase increased,especially in the range of high surface pressure as well as phase transition the desorption is more significant.(2)NaCl has a significant effect on the phase behavior of POPC,DOPG and DPPG monolayers,which is mainly reflected in that the NaCl in the subphase makes the film more expansion,and this effect is particularly significant for DPPG monolayers.The effects of NaCl on the phase behavior of POPC,DOPG and DPPG monolayers were significantly reduced as the appearance of CaCl2 in the subphase.The subphase has no obvious effect on the phase behavior of DPPC and DOPC films.In the fourth chapter,the interactions between DPPC and DOPG at the air-liquid interface has been studied firstly.Then,the inhibitory effects of bovine serum protein(BSA)on the phospholipid molecules has been evaluated using the DPPC/DOPG binary membrane as a model membrane.The results show that:(1)The addition of DOPG to the DPPC monolayer increases the fluidity of the film,which makes the monolayer in liquid-expand phase through the compression.With the increase of the content of DOPG,the free energies involved in creating the air-liquid interface increase.The excess Gibbs free energy of the mixed monolayer is negative,which shows that the interaction between DPPC and DOPG in the interface is attractive.The two components of DPPC and DOPG can form a stable monolayer at the interface.(2)The phase behavior of DPPC,DOPG and DPPC/DOPG monolayers at the air-liquid interface were significantly changed,which mainly reflects in the changes of average molecular area and compression modulus,as well as the greatly decrease of maximum surface pressure,due to the competitive adsorption of Bovine Serum Albumin(BSA)to the interface.In the fifth chapter,the reverse effects of hydrophilic polymer PEG on the inhibition induced by BSA was studied.The results show that:(1)The PEG,which was added in the subphase,can prevent the decrease of surface pressure induced by the BSA.PEG can compel the BSA into the subphase and mean while restore the adsorption of phospholipid molecules to the interface.The BSA in the subphase can induce energy barrier to prevent the adsorption of phospholipid molecules to the air-liquid interface,but this can be reversed by PEG by generating an entropic depletion attraction between the surfactant aggregates and interface and reducing the energy barrier to adsorption imposed by the albumin.When the mass fraction of PEG(MW.10K)in the subphase is 1.5%,it can effectively reverse the inhibition of BSA and restore the lipids'normal rate of adsorption to the interface,as well as short the time to reach the stable state of the surface pressure.(2)There is nonlinear behavior in the response curve of the surface pressure to the harmonic area variation.The harmonics appeared in the response curve,and the phase angle are delayed with the amplitude of ?.Lastly,we studied the interaction between HD and the two lipids of DPPG and DPPE.Research shows that:HD and the given lipid formed a non-ideal monolayer at the air-liquid interface.There is not only repulsive interaction but also attraction interaction between HD and lipid(DPPG or DPPE).With different mixing ratio and surface pressure the interaction intensity is different,then showing different effects.