Morphological Variation of Monolayers of the Poly(L-glutamic acid) Segment-Carrying Polyion Complex Induced with Helix-Helix Interactions

Abstract

A novel triblock polyion complex (1), composed of a poly(L-glutamic acid) (PLGA) segment and two poly(N,N-dimethyl-N,N-dioctadecylammonium styrenesulfonate) (PDOAS) segments, has been successfully prepared and the monolayer characteristics of 1 on the air-water interface have been studied by measuring the surface pressure/area (π/A) isotherms and by fluorescence microscopy. The π/A isotherm of 1 shows a drastic pH dependence corresponding to the conformational (helix-random coil) transition of the PLGA segment ; at an acidic pH (3.0), the monolayer has a well-defined liquid-expanded (LE) to liquid-condensed (LC) phase transition, while in higher pH region (5.8-9.0) the monolayer gives an LC phase alone and always larger molecular area compared with that observed at lower pH. Fluorescence microscopic images of the monolayer at pH 9.0 show formation of small nuclei but these nuclei do not significantly grow upon further compression, similar to the morphology observed for the PLGA segment-free PDOAS. The monolayer at pH 3.0 provides a well-grown crystalline domain, suggesting existence of a strong interaction among the rodlike, α-helical polypeptide segments due to the helix macrodipole.