Conservation law for peptide insertion into lipid membranes

 

M.T. Lee and  F.Y. Chen*

Department of Physics, National Central University, Chung-li Taiwan 32054

 

 The peptide insertion into lipid membranes was studied by oriented circular dichroism (OCD). The peptide used in this study was alamethicin, a neutral anti-microbial peptide. Two membranes, pure DPhPC and the mixture of DPhPC/DPhPE in molar ratio 5/1, were respectively used for testing. The OCD was conducted in 100% humidity vapor to measure the molar fraction f of peptides inserting into the fully hydrated membrane as a function of peptide-to-lipid molar ratio, P/L. The result shows that there is a threshold (P/L)* such that (P/L)f/n + (P/L) (1-f)= (P/L)* for P/L > (P/L)*, otherwise f=0, all peptides are adsorbed on membrane surface. Here n >1 is a parameter, like (P/L)* depending on lipids. Because of the fact that peptides such as alamethicin when inserting into the membrane are aggregated to form pores, the quantity (P/L)f/n can be regarded as the number of pores, each in average consisting of n peptides. Also shown in our measurement, the non-inserting peptides, (P/L) (1-f), are all adsorbed on the membrane surface. Thus, the equation of f suggests a conservation law for peptide insertion that the total number of pores and surface-adsorbed peptides must be a constant equal to (P/L)*. The relation between this conservation law and the elastic theory of the membrane was discussed

 

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