Discrimination between N- and C-Termini of Polypeptides by a Two-Dimensional Array of Helical Poly(l-glutamic acid) Rods on Gold Surfaces

Abstract

A novel poly(l-glutamic acid) (PLGA) amphiphile, PLGA−SS, which has a disulfide bond at the N-terminus, formed spontaneously an adsorbed monolayer on gold substrates from aqueous solutions with various pHs. The adsorption rates of PLGA−SS were characterized by electrochemistry using a model of a microelectrode array. When the monolayer was prepared at a lower pH, at which the PLGA segment takes the α-helix structure, a densely packed monolayer was formed, and in contrast, the preparation at higher pH was found to provide a less packed monolayer. Subsequently, the interaction of this monolayer with guest PLGAs, PLGA−Fc−N and −C, which have a ferrocenyl group at the N- and C-termini, respectively, was examined by means of the quartz crystal microbalance (QCM) technique. The frequency shift of QCM revealed that a stoichiometric interaction could be caused between the PLGA−SS monolayer and both guest PLGAs. The electron-transfer experiment and angle-dependent X-ray photoelectron spectroscopy were also applied to elucidate such a specific interaction. As a result, the host helix monolayer was found to capture the guest helix PLGAs through an antiparallel, side-by-side helix−macrodipole interaction.