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

Helix bundle structure has been successfully constructed at the air-water interface using ion complexation. A quaternary ammonium-terminated poly(gamma-benzyl-L-glutamate) (PBLG-N+) was prepared and used as the helical component. Bathophenanthroline disulfonate (BPS) was employed as a template to associate a-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(pi)-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 [theta]=41 degrees 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 pi-A isotherm compared with that on pure water, suggesting ion complexation between the monolayer and the iron BPS complex. The average tilt, [theta], of the helical axis in this monolayer was estimated to be 25 degrees. 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.