Synthesis of L-cystine modified cyclodextrin monomers and dimers with primary-side versus secondary-side and their molecular binding behaviours

A series of b-cyclodextrin (b-CD) derivatives modified by L-cystine, including 3,30-L-cystine-bridged bis(b-CD) (2), 3-Lcystine-b-CD (3), 6,60-L-cystine-bridged bis(b-CD) (4) and 6-L-cystine-b-CD (5), were synthesised inmoderate yields by the reaction of L-cystine with mono-[2-O-(p-tolysulfonyl)]-b-CD (2-O-Ts-b-CD) or mono-[6-O-(p-tolysulfonyl)]-b-CD (6-OTs-b-CD). Their binding manners and inclusion abilities towards some dye guests (ANS, TNS, AR, NR, EYand FL) were, respectively investigated by the methods of 2D NMR spectrometry and fluorescence spectrometry in aqueous solution (pH 7.2). The results obtained show that the stoichiometric 1:1 complexes formed by L-cystine modified b-CD monomers and dimers 2–5with dye guests give higher complex stability constants (KS) than those of native b-CD. In addition, the difference of inclusion complexation between primary-side and secondary-side modified/bridged b-CDs was compared in detail and discussed from the viewpoint of inclusion orientation, size/shape fit, cooperative binding and hindrance of substituent.