Enantioselective recognition of amino acids by beta-cyclodextrin 6-O-monophosphates

Mono-(6-O-diphenoxyphosphoryl)-beta-cyclodextrin 1 and mono-(6-O-ethoxyhydroxyphosphoryl)-beta-cyclodextrin 2 have been synthesized by a convenient method in 40 and 35% yields, respectively The stability constant (K-s) and Gibbs free energy change (-Delta G degrees) for the 1:1 inclusion complexation of the two beta-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 degrees C. The results obtained indicate that the modified beta-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 beta-cyclodextrins 1 and 2 are discussed from the viewpoints of the size/ shape-fit relationship and the multipoint recognition mechanism, The binding constant (K-s) and Gibbs free energy change (-Delta G degrees) for the modified beta-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.