Remarkable selectivity of per-O-methylated tricationic 6(A),6(C),6(E)-tripyridinio-6(A),6(C),6(E)-trideoxy-alpha-cyclodextrin for basic anions over non-basic anions

The per-O-methylated tricationic 6(A),6(C),6(E)-tripyridinio-6(A),6(C),6(E) -trideoxy-alpha -cyclodextrin shows a high affinity for basic anions, espec ially highly charged phosphate and pyrophosphate anions, at near-neutral pH with association constants of 7 000 and 9 000 M-1, respectively, but it di d not bind non-basic anions, such as I-, ClO4-, SCN- or even doubly charged SO42-, in contrast to the non-methylated counterpart. H-1 NMR spectral stu dies verify a plausible structure, in which the bound anion is located more closely to the pyridinio meta and para positions rather than to the ortho positions, due probably to the greater MeO- vs. HO-group hydrophobicity, wh ich rejects deep intrusion of the anion into the positive cavity. Molecular mechanics calculations supported the above conclusions.