Exploring The Interactions Between Peptides And Lipid Membranes On The Nanometer Scale

There are a large number of peptides that are termed as cytolytic peptides which can damage cytomembrane and lead to cell death consequently.The cytolytic peptides can be devided into two classes,amyloidogenic peptides(AMYs)and antimicrobial peptides(AMPs).The AMYs are highly related to neurodegenerative disease such as Alzheimer's disease,Parkinson's disease,type ? diabetes,and so on.The function of AMPs is to eliminate exogenous bacterials and protect the host cells from infection.Although both of AMYs and AMPs display cytotoxicity through interfering the structures and functions of the specific target cells,they do not share common sequences and structures.Several disruption models have already been developed to explain the working mechanisms of AMYs and AMPs.However,the relationship between the working mechanisms of AMYs and AMPs is still not clear.Hence,in this thesis,the interactions between lipid bilayers and some model peptides with distinguishing properties were explored on the nanometer scale,trying to further understand the working mechanism of AMYs and AMPs.In this thesis,various techniques including Atomic force microscopy(AFM),fluorescence resonance energy transfer(FRET),DPH fluorescence anisotropy and Laurdan's generalized polarization were used to investigate the interactions between peptides and lipid bilayers.Three peptides,Pep11,QQ11,and P11-2,which share a similar backbone,were synthesized and employed as model molecules.AFM data showed clear changes in the morphology and the mechanical properties of lipid bilayers after interaction with these peptides.The FRET results displayed the impacts of the peptides on the structures of lipid bilayers.DPH fluoresecence anisotropy and Laurdan's generalized polarization data revealed the peptide-influenced regions in the lipid bilayers.The results showed that Pep11 peptide could insert and generalize defects on lipid bilayers.At high concentration,the hydrophobic QQ11 could aggregate on the lipid bilayers,influence the structure of the hydrophilic head of lipid molecule,and csuse defects on lipid bilayers.At low concentration,QQ11 would act like surfactant and make lipid molecules detach from the substrate.P11-2,which has a grand average of hydropathicity(GRAVY)value between Pep11 and QQ11,could also insert into lipid bilayers and form defects on it.In addition,P11-2 could interact with the hydrophobic region of lipid bilayers and interfere their structures.Cellular cytotoxicity assay indicated P11-2 has stronger cytotoxicity than Pep11 and QQ11.With these results,we have made a schematic drawing indicating how these peptides interacted differently with the lipid membrane because of the modification of their sequences.These results also indicated that multiple models are needed to fully understand the interaction between peptides and lipid bilayers.In addition,the roles of ionic strength in the interactions between peptides and lipid bilayers were also explored.The experiment data showed that ionic strength heavily influenced the interaction between a peptide termed GAV-9 and lipid bilayers,which may be attributed to that the conformation of GAV-9 changes in solutions with different ionic strength.