Novel α-cyclodextrin (α-CD) derivatives, bearing a nicotinic or isonicotinic moiety, have been synthesized by a convenient method in 21 and 25% yields, respectively. The stability constants (K) and Gibbs free energy changes (-ΔG°) for the inclusion complexation of α-cyclodextrin 6-O-mono(3-pyridinecarboxylate) (1), 6-O-mono(4-pyridinecarboxylate) (2), and 6-O-monobenzoate (3) with L- and D-tryptophan have been determined by spectrofluorometry in aqueous buffer solution (pH = 7.20) at 25.0°C. All of the modifications dramatically enhanced the original K for α-CD by a factor of 30-280 and interestingly switched the original enantiomer preference for L- to D-tryptophan, thus affording the inverted enantio-selectivities of KL/KD = 2.5 for α-CD and KD/KL = 1.2-2.1 for the modified CDs 1-3. These results are discussed from the viewpoints of the size-fit and geometrical complementary relationship between the host and guest.
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