Solubilities of polypeptides with hydrophobic and negatively charged side-chains into cationic artificial membrane vesicles

Abstract

The influence of relatively low molecular weight polypeptides on the structure of membrane vesicles composed of distearyldimethylammoniumchloride (DSACl) was investigated by means of calorimetric, fluorescence spectroscopic and fluorescence polarization measurements, and correlated with the degree of hydrophobicity and/or dissociation of the polypeptide side chains. The polypeptides used were poly(γ-methyl L-glutamate) (PMLG,Mv=4400), copoly(methyl L-glutamate L-glutamic acid) containing 20 mol % of L-glutamic acid (80/20 MG/GA,Mv=4200) and poly(L-glutamic acid) (PLGA,Mv=9200). The hydrophobic polypeptide, PMLG, was readily incorporated into the DSACl membrane vesicles to form membrane-spanning helices, resulting in a decrease in the microviscosity of the hydrophobic region of the membrane phase. The partially charged hydrophobic polypeptide, 80/20 MG/GA, was almost insoluble into the membrane below the phase transition temperature of the DSACl vesicle,Tc 40.4 °C, however, the solubility of the copolymer into the membrane was drastically increased aboveTc. The negatively charged polypeptide, PLGA, can hardly penetrate through the membrane vesicle and formed crosslinking between the positively charged DSACl vesicles. It was also confirmed that aggregation or clustering of the hydrophobic PMLGα-helices progressed in the membrane phase belowTc.