Molecular recognition studies on supramolecular systems. 29. Anilino- and m-toluidino-beta-cyclodextrins: Structural and conformational analyses and molecular recognition of aliphatic alcohols

Mono (6-anilino-6-deoxy)-beta -cyclodextrin (1) and mono[6-(m-toluidino)-6- deoxy]-beta -cyclodextrin (2) were synthesized and characterized. Circular dichroism and fluorescence spectral studies and fluorescence lifetime measu rements have been performed to elucidate the conformations of 1 and 2 in aq ueous buffer solution. 1-D and 2-D NMR spectra of 2 have been measured in D 2O to deduce its structure and detailed conformation in solution. From the circular dichroism, fluorescence, and NMR spectroscopic studies, it was rev ealed that the substituents appended to 1 and 2 penetrate into the cyclodex trin cavity forming a stable self-inclusion complex in aqueous solution, an d also that the short linkage between the m-toluidino and cyclodextrin moie ties makes the cyclodextrin ring of 2 deformed to some extent. The complex stability constants (K-S) of 1 and 2 for a series of aliphatic alcohols hav e been determined by using spectropolarimetric titrations in aqueous phosph ate buffer solution (pH 7.20) at 25 degreesC to elucidate the role of intro duced substituents and the weak interactions involved in inclusion complexa tion by the modified cyclodextrins. The results obtained indicate that the van der Waals and hydrophobic interactions mainly contribute to the formati on of complexes between the cyclodextrins and aliphatic alcohols, and the i nclusion complexation process involves the induced-fit mechanism. Modified beta -cyclodextrin 2 can recognize not only the size, shape, and hydrophobi city of the guest molecules, but also chiral guests, affording a moderate e nantioselectivity of 1.55 for (+)/(-)-borneol.