Fluorometric Studies on Inclusion Complexation of L/D-Tryptophan by β-Cyclodextrin 6-O-Pyridinecarboxylates

Novel β-cyclodextrin (β-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 β-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 spectrofluorome try in aqueous buffer solution (pH = 7.20) at 25.0°C. All of the modifications dramatically enhanced the original K for β-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 β-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.