Chiral recognition of an anionic tetrahelicene by native cyclodextrins. Enantioselectivity dominated by location of a hydrophilic group of the guest in a cyclodextrin cavity

Chiral recognition of 1,12-dimethylbenzo[c]phenanthrene-5,8-dicarboxylic acid (1) by native beta- (beta-CD) and gamma-cyclodextrins (gamma-CD) has been studied by means of H-1 NMR spectroscopy. The binding constant (K) for complexation of (M)-1 with beta-CD (18700 +/- 1700 dm(3) mol(-1)) is much larger than that for (P)-1 (2200 +/- 100 dm(3) mol(-1)), Delta Delta G being 5.2 kJ mol(-1). gamma-CD forms less stable inclusion complexes (K = 3100 +/- 100 and 690 +/- 20 dm(3) mol(-1) for (M)-1 and (P)-1, respectively). The 2D ROESY spectra indicate that both CO2- groups of 1 are placed near the rim of the secondary OH group side of beta-CD though the (P)-1 molecule penetrates into the host cavity somewhat more deeply than the (M)-1 molecule. The deeper penetration of the (P)-l molecule seems to be an enthalpically unfavourable but entropically favourable process because such a complexation needs dehydration from the CO2- group(s) of 1. The enantioselectivity of beta-CD toward 1 is dominated by the difference in the enthalpy changes due to the difference in the extent of penetration between the enantiomers of 1. The 2D NMR spectra clearly indicate that at least one CO2- group of 1 is located inside of the gamma-CD cavity resulting in extensive dehydration from the guest molecule. Such an endothermic process reduces the K value for the 1-gamma-CD complex. The difference in the structures of the complexes between the guest enantiomers might be ascribed to the chiral helix-structure of the CD taken upon complexation in water.