Enantioselective recognition of amino acids by β-cyclodextrin 6-O-monophosphates 1

Mono-(6-O-diphenoxyphosphoryl)-β-cyclodextrin 1 and mono-(6-O-ethoxyhydroxyphosphoryl)-β-cyclodextrin 2 have been synthesized by a convenient method in 40 and 35% yields, respectively. The stability constant (Ks) and Gibbs free energy change (-ΔG°) for the 1:1 inclusion complexation of the two β-cyclodextrin 6-O-monophosphates with some selected L/D-amino acids have been examined by means of differential UV spectrometry in buffered aqueous solution (pH = 7.20) at 25 °C. The results obtained indicate that the modified β-cyclodextrin compound 1 is favourable for complexation with D-amino acids except for alanine, giving fairly good enantioselectivity of up to 3.6 for D/L-serine, with a molecular selectivity of up to 12.9 for D-leucine/D-alanine. The molecular recognition ability and enantioselectivity for amino acids of the modified β-cyclodextrins 1 and 2 are discussed from the viewpoints of the size/shape-fit relationship and the multipoint recognition mechanism. The binding constant (Ks) and Gibbs free energy change (-ΔG°) for the modified β-cyclodextrins (host) inclusion complexation with L/D-amino acids (guest) may more explicitly be understood in terms of the induced-fit interaction and the complementary geometrical relationship between the host and the guest.