Ligand-induced association of helical polypeptides at the air–water interface. Orientation of helix rods on water

Abstract

Helix bundle structure has been successfully constructed at the air–water interface using ion complexation. A quaternary ammonium-terminated poly(γ-benzyl-l-glutamate) (PBLG-N⁺) was prepared and used as the helical component. Bathophenanthroline disulfonate (BPS) was employed as a template to associate α-helices. The ion complexations of the ammonium group of PBLG-N⁺ with the sulfonate groups of BPS or the iron BPS complex (PBLG-N⁺ /BPS or PBLG-N⁺ /Fe-BPS) were carried out in solution. The surface pressure(π)–area(A) curve of PBLG-N⁺ /Fe-BPS gave a more expanded monolayer compared with those for PBLG-N⁺ and PBLG-N⁺ /BPS. The PBLG helical rod in the PBLG-N⁺ /Fe-BPS monolayer was calculated to tilt at an angle 〈θ〉=41° from the surface normal by means of FTIR spectroscopy. Subsequently, the complexations were performed at the air–water interface. When the iron BPS complex was added into the subphase, the monolayer of PBLG-N⁺ gave a condensed π–A isotherm compared with that on pure water, suggesting ion complexation between the monolayer and the iron BPS complex. The average tilt, 〈θ〉, of the helical axis in this monolayer was estimated to be 25°. To improve the orientation of the helical rods, the PBLG-N⁺ /BPS monolayer was spread on an aqueous solution containing BPS and Fe²⁺ . As a result, the helix rod was found to align with almost vertical orientation.