INCLUSION COMPLEXES AND GUEST-INDUCED COLOR CHANGES OF PH-INDICATOR-MODIFIED BETA-CYCLODEXTRINS

被引:83
作者
KUWABARA, T
NAKAMURA, A
UENO, A
TODA, F
机构
[1] Department of Bioengineering, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Midori-ku, Yokohama 227
关键词
D O I
10.1021/j100076a011
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
A methyl red-modified beta-cyclodextrin (1) and a p-methyl red-modified one (2), both being the structural isomers with a positional difference of one substituent in the appending dye part, were synthesized in order to attain color changes induced by host-guest complexation as well as to investigate their conformations and binding properties. The structures of 1 and 2 were investigated by UV-visible absorption-and induced circular dichroism spectroscopy in a 10% ethylene glycol aqueous solution. Host 1 accommodates the methyl red moiety in its own beta-CD cavity with an orientation parallel to the CD axis, forming an intramolecular complex, while 2 partially includes the p-methyl red moiety in its own beta-CD cavity with an orientation perpendicular to the CD axis. In neutral media, both 1 and 2 exist as azo forms, which have the dye part unprotonated. In acidic medium, 1 prefers the ammonium form, which is the form protonated on the dimethylamino group in the methyl red moiety, while 2 adopts the azonium form, which is the form protonated on the azo group. Their pK(a)'s are estimated to be 1.25 for 1 and 2.08 for 2. Upon addition of guest species, 1 and 2 in acidic media near their pK(a) values (pH, 1.60 for 1;2.52 for 2) changed the locations of the dye parts from inside to outside the cavities, forming 1:1 inclusion complexes with the guests. The guest-induced conformation changes of 1 and 2 caused the color changes from yellow to red for 1 and from orange to red for 2, which occur associated with the conversion of the dye parts of 1 and 2 from the azo form to the azonium one. The guest-binding properties, however, were markedly affected by structural difference of 1 and 2, as shown by the fact that the binding constants of 2 are larger than those of 1 in acidic media and 1 loses the binding ability in neutral medium, where 2 still has binding ability. The guest selectivity of 2 observed in acidic media is remarkable for the adamantane derivatives. These results demonstrated that various color changeable indicators for molecules may be constructed on the basis of the appropriate structural design of the dye part in chromophore-modified cyclodextrins.
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页码:6297 / 6303
页数:7
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