Conformation of poly(l-glutamic acid) in bipolar amphiphiles with alkyl ammonium cations of different hydrophobicity

Abstract

The conformation of the sodium salt of poly(l-glutamic acid) (P(Glu)) in solutions of the cationic bipolar amphiphile 1, 20-isosanediylbis(alkylammonium chloride) (C20(RA)2) with different alkyl head groups as a function of amphiphile concentration was investigated using circular dichroism (CD). RA included methylammonium (MA), ethylammonium (EA), propylammonium (PA), butylammonium (BA), and pentylammonium (PeA) cationic groups. The CD spectrum of each C20(RA)2 had a double minima corresponding to the α-helix of P(Glu), which was replaced by CD spectra with a single minimum at wavelengths larger than 222 nm as concentration increased. These changes in the CD spectrum were ascribed to the conformational change from random coil to α-helix and to aggregates of helices. In C20(EA)2 solutions, a step-like change in the CD intensity was observed at 222 nm as a function of the ratio of C20(RA)2 to P(Glu). At the step, the CD spectrum of the complete α-helix was observed. At 10–35 °C, an α-helix was induced in P(Glu) in the order: C20(EA)2>C20(MA)2>C20(PA)2>C20(BA)2>C20(PeA)2. This order was ascribed to the best fit of ethylammonium to the P(Glu) side chain.