Chirality of
metal complexes M(phen)3(n+) (M = Ru(II), Rh(III), Fe(II), Co(II), and Zn(II), and phen = 1,10-
phenanthroline) is recognized by heptakis(6-carboxymethylthio-6-deoxy)-beta-cyclodextrin heptaanion (per-CO2(-)-beta-CD) and
hexakis(2,3,6-tri-O-methyl)-alpha-cyclodextrin (TMe-alpha-CD) in D2O. The binding constant (K) for the Delta-
Ru(phen)3(2+) complex of per-CO2(-)-beta-CD (K = 1250 M(-1)) in 0.067 M
phosphate buffer at pD 7.0 is approximately 2 times larger than that for the Lambda-isomer (590 M(-1)). Definite effects of inorganic
salts on stability of the complexes indicate a large contribution of Coulomb interactions to complexation. The fact that hydrophilic
Ru(bpy)3(2+) (bpy =
2,2'-bipyridine) does not form a complex with per-CO2(-)-beta-CD suggests the importance of inclusion of the guest molecule into the host cavity for forming a stable ion-association complex. The positive entropy change for complexation of
Ru(phen)3(2+) with per-CO2(-)-beta-CD shows that
dehydration from both the host and the guest occurs upon complexation. Similar results were obtained with trivalent Rh(phen)3(3+)
cation. Pfeiffer effects were observed in complexation of racemic Fe(phen)3(2+),
Co(phen)3(2+), and Zn(phen)3(2+) with per-CO2(-)-beta-CD with enriched Delta-isomers. Native
cyclodextrins such as alpha-, beta-, and
gamma-cyclodextrins as well as
heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin do not interact with
Ru(bpy)3(2+). However,
hexakis(2,3,6-tri-O-methyl)-alpha-cyclodextrin (TMe-alpha-CD) interacts with
Ru(phen)3(2+) and
Ru(bpy)3(2+) and discriminates between the enantiomers of these
metal complexes. The K values for the Delta- and Lambda-
Ru(phen)3(2+)
ions are 54 and 108 M(-1), respectively. Complexation of the Delta- and Lambda-isomers of
Ru(phen)3(2+) with TMe-alpha-CD is accompanied by negative entropy changes, suggesting that cationic
Ru(phen)3(2+) is shallowly included into the cavity of the neutral host through van der Waals interactions. The Delta-enantiomer, having a right-handed helix configuration, fits the primary
OH group side of per-CO2(-)-beta-CD (SCH2CO2(-) side) well, while the Lambda-enantiomer, having a left-handed helix configuration, is preferably bound to the secondary
OH group side of TMe-alpha-CD. The asymmetrically twisted shape of a host cavity seems to be the origin of chiral recognition by
cyclodextrin.