Induced orientation of α-helical polypeptides in polyelectrolyte multilayers

Polyelectrolyte multilayers (PEMs) consisting of stiff polypeptides at texturized silicon substrates were studied. These studies are focused on anisotropic PEMs in contrast with common ones consisting of flexible polyelectrolytes. When stiff polycation poly(L-lysine hydrobromide) (PLL, M-w = 200000 g/mol) was used at pH = 11 or in the presence of NaClO4, a transition from random coil (R-PLL) to alpha-helix (alpha-PLL) was found. When stiff polyanion poly(L-glutamic acid) (PLG, M-w = 70000) was chosen, the alpha-helical conformation (alpha-PLG) was adopted at pH = 3. The conformation was checked by IR spectroscopy and circular dichroism. The layer-by-layer technique was used in order to get homogeneous thin films of the polypeptides. Texturized Si-ATR-substrates with parallel nanosized grooves 50-70 nm in width were used for orienting the a-helical polypeptides with contour length L > 70 nm. Two different types of PEMs were fabricated: (i) alpha-PLG/ poly(diallyldimethylammoniumchloride) (PDADMAC) and (ii) alpha-PLL/poly(vinyl sulfate) (PVS). The orientation was studied by dichroic ATR-FTIR spectroscopy using polarized light. An ATR-FTIR-dichroism concept was applied, which was established at low molecular liquid-crystalline systems. Via the dichroic ratio of the amide 1 and 11 band areas measured with parallel (A(p)) and with vertically polarized IR light (A(s)) an order parameter S could be determined, knowing the angle theta between the a-helical axis and the amide mode transition dipole moments. S is a measure for the uniaxial orientation of the multilayer embedded polypeptides within the nanogrooves of the silicon plate, where S = 0 means no and S = 1 means high orientation. S > 0.5 was found for both multilayer embedded alpha-PLL and alpha-PLG (i and ii), indicating a significant orientation with respect to the nanogroove direction. The influence of M-w, concentration, number of adsorption steps, and drying was investigated. PEMs exposing charged alpha-helical polypeptides in the outermost layer can be used to create biomimetic and biosensor surfaces.