Host-guest complexation of aromatic carboxylic acids and their conjugate bases by 6(A)-(omega-aminoalkylamino)-6(A)-deoxy-beta-cyclo-dextrins in aqueous solution

A potentiometric titration study of the complexation of the guests benzoic acid, 4-methylbenzoic acid and (RS)-2-phenylpropanoic acid and their conjug ate bases by host 6(A)-(omega-aminoalkylamino)-6(A)-deoxy-beta-cyclodextrin s, where the omega-aminoalkylamino groups are -NH(CH2)(n)NH2 (n = 2, 3, 4 a nd 6), is reported. Of the 64 host-guest complexes whose formation is stati stically possible over the pH range 2.0-12.0 studied, 35 were detected. The ir stability constants range from 140 +/- 35 dm(3) mol(-1) for the [beta CD NH(CH2)(2)NH3. 4-methylbenzoate] complex to 1760 +/- 150 dm(3) mol(-1) for the [beta CDNH(CH2)(6)NH2.(S)-2-phenylpropanoate](-) complex at 298.2 K and I = 0.10 mol dm(-3) (NaClO4). The charge and hydrophobicity of both host a nd guest appear to be significant factors,in the variation of host-guest co mplex stability. Qualitative structural information on the host-guest compl exes obtained from 600 MHz H-1 NMR ROESY spectroscopy is generally consiste nt with the structures generated by molecular modelling.