Dimerisation and complexation of 6-(4′-t-butylphenylamino)naphthalene-2-sulphonate by β-cyclodextrin and linked β-cyclodextrin dimers

This study shows that stereochemical factors largely determine the extent to which 6-(4′-t-butylphenylamino)-naphthalene-2-sulphonate, BNS^- and its dimer, (BNS^-)₂, are complexed by β-cyclodextrin, βCD, and a range of linked βCD dimers. Fluorescence and ¹H NMR studies, respectively, show that BNS^- and (BNS^-)₂ form host-guest complexes with βCD of the stoichiometry βCD.BNS^- (10^-4K₁=4.67dm³ mol^-1) and βCD.BNS₂^2- (10^-2K₂¹=2.31 dm³ mol^-1), where the complexation constant K₁=[βCD.BNS^-]/([βCD][BNS^-]) and K₂¹= [βCD. (BNS^-)₂]/([βCD.BNS^-][BNS^-]) in aqueous phosphate buffer at pH 7.0, I = 0.10 mol dm³ at 298.2 K. (The dimerisation of BNS^- is characterised by 10^-2K_d = 2.65 dm³ mol^-1.) For N,N-bis((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)succinamide, 33βCD₂su, N-((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)-N'-(6^A-deoxy-6^A-β-cyclodextrin)urea, 36βCD₂su, N,N-bis(6^A-deoxy-6^A-β-cyclodextrin)succinamide, 66βCD₂su, N-((2^AS,3^AS)-3^A-deoxy-3^A-β-cyclodextrin)-N'-(6^A-deoxy-6^A-β-cyclodextrin)urea, 36βCD₂ur, and N,N-bis(6^A-deoxy-6^A-β-cyclodextrin)urea, 66βCD₂ur, the analogous 10^-4K₁=11.0, 101, 330, 29.6 and 435 dm³ mol^-1 and 10^-2K₂¹=2.56, 2.31, 2.59, 1.82 and 1.72 dm³ mol^-1, respectively. A similar variation occurs in K₁ derived by UV-vis methods. The factors causing the variations in K₁ and K₂ are discussed in conjunction with ¹H ROESY NMR and molecular modelling studies.