Chiral recognition of helical metal complexes by modified cyclodextrins

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 similar to2 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)32+, Co(phen)3 2+, and Zn(phen)32+ 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.