Color change indicators for molecules using methyl red modified cyclodextrins

beta-Cyclodextrin derivatives, MRCD and p-MRCD, which have a 4-(dimethylami no)azobenzene moiety with a carbonyl substituent at 2' and 4' positions, re spectively, have been prepared as color change indicators for detecting org anic compounds, In a 10% ethylene glycol solution, MRCD and p-MRCD form int ramolecular self-complexes in which the pendant dye moiety is included in t he cyclodextrin cavity with an orientation parallel and perpendicular to th e cyclodextrin axisis, respectively, When guest molecules are added to the acidic solutions of MRCD (pH 1.60) and p-MRCD (pH 2.40), they exhibit color changes from yellow to red for MRCD and from orange to red for p-MRCD. The se color changes, which arise from the structural change of the dye moietie s from the azo form to the azonium one, are caused when MRCD and p-MRCD und ergo a conformational change in which the dye moieties inserted in the cycl odextrin cavities are excluded to outside of the cavities upon guest accomm odation. The extent of the guest-induced color changes of MRCD and p-MRCD d epend on the shape, size, number, and position of the functional group of g uest molecules. Selectivities between MRCD and p-MRCD in guest detection ar e roughly parallel and reflected in the host-guest binding constants. Among guest molecules examined, ursodeoxycholic acid and chenodeoxycholic acid w ere detected by MRCD and p-MRCD with high sensitivities. 1-Adamanetanecarbo xylic acid and (-)borneol were also detected with high sensitivities. In ne utral conditions, however, the selectivity in guest detection of p-MRCD is different from that in acidic conditions as shown by the fact that, for exa mple, 1-adamantanol and 2-adamantanol were detected by p-MRCD with larger s ensitivities than 1-adamanetanecarboxylic acid. The result indicates that t he ionic nature of the guest molecules is an important factor for detection of the guest molecules. All these results demonstrate that MRCD and p-MRCD can be used as color change indicators for detecting various organic compo unds in aqueous solution.