A 1H NMR titration study on the binding constants for D- and l-tryptophan inclusion complexes with 6-O-α-D-glucosyl-β-cyclodextrin. Formation of 1:1 and 2:1 (host:guest) complexes

Abstract

A 1H NMR titration study revealed that 6-O-α-D-glucosyl-β-cyclodextrin (G1-β-CD) forms 1:1 and 2:1 (host:guest) inclusion complexes with D- and l-tryptophan in alkaline D2O solutions (pD 11.0). The binding constants (K1’s) for the 1:1 complexes of D-isomer at 298K (59mol−1dm3) were virtually equal to that of L-isomer (54mol−1dm3). On the other hand, the K2 values for 2:1 complexes of D-isomer (42mol−1dm3) were larger than that of L-counterpart (12mol−1dm3). These facts suggest that the first CD molecule includes the indole ring moiety of tryptophan, followed by inclusion with the second CD molecule in the vicinity of chiral center, α-carbon of the guest, to result in the difference in K2’s for two enantiomers. Two-dimensional NMR measurement (Rotating-frame nuclear Overhauser Effect SpectroscopY, ROESY) supported this interpretation.