Specific Binding of α-Amino Acid onto Poly(l-glutamic acid) Monolayers: Effects of Helicity and Lateral Helix Distribution

Abstract

This study examines the mixing behavior of the dialkyl poly(l-glutamic acid) amphiphile (2C18PLGA) with the dialkyl acetamide amphiphile (2C18Ac) at the air−water interface and the relation between the secondary structure of the poly(l-glutamic acid) (PLGA) segment and the binding of the α-amino acid onto the mixed monolayers. It was found from surface pressure−area isotherms that the 2C18PLGA was miscible with 2C18Ac and formed a stable mixed monolayer at the air−water interface. In particular, the 2C18PLGA/2C18Ac mixed monolayer was most stabilized in the case of fPLGA = 0.3 (fPLGA is the mole fraction of 2C18PLGA), and the collapse pressure of this mixed monolayer became 15 mN/m higher than that of the pure 2C18PLGA monolayer. This value of fPLGA = 0.3 agrees well with the theoretical composition when both alkyl chains and PLGA helices are packed hexagonally. Circular dichroism and Fourier transform infrared spectroscopies showed that the secondary structure of PLGA segments in 2C18PLGA/2C18Ac mixed monolayers depended on fPLGA and could be controlled by adjusting the monolayer composition. The Langmuir type saturation behavior was observed for binding of d- or l-tryptophan to the mixed monolayers of 2C18PLGA and 2C18Ac. Its binding constant (K) was found to depend on the secondary structure and lateral packing density of PLGA segments. The maximum enantioselectivity (R = KD/KL) was obtained in the case of fPLGA = 0.3 (KD = 1690 M-1, KL= 695 M-1), in which PLGA segments retained an α-helix structure and packed most closely.