Specific binding of α-amino acid onto poly(L-glutamic acid) monolayers:: Effects of helicity and lateral helix distribution

This study examines the mixing behavior of the dialkyl poly(L-glutamic acid) amphiphile (2C(18)PLGA) with the dialkyl acetamide amphiphile (2C(18)Ac) at the dr-water interface and the relation between the secondary structure of the poly(L-glutamic acid) (PLGA) segment and the binding of the alpha -amino acid onto the mixed monolayers. It was found from surface pressure-area isotherms that the 2C18PLGA was miscible with 2C(18)Ac and formed a stable mixed monolayer at the air-water interface. In particular, the 2C(18)PLGA/2C(18)Ac mixed monolayer was most stabilized in the case of f(PLGA) = 0.3 (f(PLGA) is the mole fraction of 2C(18)PLGA), and the collapse pressure of this mixed monolayer became 15 mN/m higher than that of the pure 2C(18)PLGA monolayer. This value Of f(PLGA) = 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 2C(18)PLGA/2C(18)Ac mixed monolayers depended on f(PLGA) and could be controlled by adjusting the monolayer composition. The Langmuir type saturation behavior was observed for binding of D- or C-tryptophan to the mixed monolayers of 2C(18)PLGA and 2C(18)Ac. Its binding constant (K) was found to depend on the secondary structure and lateral packing density of PLGA segments. The maximum enantioselectivity (R = K-D/K-L) was Obtained in the case of f(PLGA) = 0.3 (K-D = 1690 M-1, K-L = 695 M-1), in which PLGA segments retained an alpha -helix structure and packed most closely.