Advanced Technologies for Cyclodextrin Chemistry (Toshiyuki Kida)
What are Cyclodextrins?
Cyclodextrins (CDs), which are produced from starch by a enzyme ‘cyclodextrin
glucanotransferase’, are a class of cyclic oligosaccharides consisting
of several α-(1,4)-linked D-glucopyranose units. CDs composed of 6, 7,
and 8 glucosidic units, called α-, β-, and γ-CD, respectively, are commercially
available, and have been studied extensively. They have a doughnut-shaped
cavity into which a guest molecule of an appropriate size and shape can
be included. The ability of CDs to form inclusion complexes with organic
molecules has been studied academically, and, in addition, it has found
applications in many areas, including the food, cosmetic, and pharmaceutical
(1) Development of Skeleton-Modified CDs
Since it has been believed that flexible design of CD hosts to fit the
shape and size of the desired guest molecules would be difficult, the types
of guest molecules applied for conventional CD hosts are relatively limited.
In order to overcome this drawback and to extend the utility of CDs, we
designed ‘spacer-inserted CDs’ whose cavity size and shape are freely controllable,
and succeeded in synthesizing them in two steps starting from perrmethylated
CDs. These spacer-inserted CDs were found to show unique inclusion ability.
‘Bond-converted CDs’ bearing a β-1,4-glucosidic bond were also synthesized
from permethylated CDs. The bond-converted CD derived from perrmethylated
α-CD formed an inclusion complex with not only conventional polymeric guests
such as polytetrahydrofuran, poly(ε-caprolactone), and poly(propylene glycol)
but also with poly(acrylic acid) to generate novel pseudopolyrotaxanes.
1) T. Kida et al., Chem. Commun. 2002, 1596-1597. 2) T. Kida et al., Chem. Commun. 2003, 3020-3021. 3) A. Kikuzawa et al., J. Org. Chem. 2005, 70, 1253-1261. 4) T. Kida et al., Tetrahedron 2005, 61, 5763-5768. 5) A. Kikuzawa et al., Org. Lett. 2007, 9, 3909-3912. 6) A. Kikuzawa et al., Macromolecules 2008, 41, 3393-3395.
(2) Guest Inclusion by CDs in Oils and Nonpolar Solvents
CDs can form inclusion complexes with a variety of molecules in aqueous media or in several kinds of polar organic media via the incorporation of the guests into the CD cavities. On the other hand, it has been believed that inclusion complex formation between CDs and guest molecules in nonpolar media would be extremely difficult, because the main driving force for the inclusion of guest molecules within the CD cavity is hydrophobic interactions and/or van der Waals interactions between the guests and the CD cavity, and the enormous amount of nonpolar solvents become a strong competitor for inclusion within the CD cavity. We found that 6-O-modified CDs effectively form inclusion complexes with guest molecules in nonpolar solvents, and demonstrated that these CD hosts functioned as a selective adsorbent for polychlorinated biphenyls (PCBs) contaminated in insulating oils. We also succeeded in the efficient separation and concentration of PCBs from oils by passing the PCBs-contaminated oils through a column loaded with the CD hosts.
More than 500,000 tons of PCB-contaminated oils are still stored in Japan, and thus the development of appropriate treatment methods for these oils is highly required. Our technique with a CD-loaded column can be expected to contribute to a large reduction in the stockpile of PCBs-contaminated insulating oils in Japan. We are now studying how to apply this technique early, in collaboration with NEOS Co., Ltd.
1) T. Kida et al., Anal. Chem. 2008, 80, 317-320. 2) T. Kida et al., Org. Lett. 2009, 11, 5282-5285. 3) T. Kida et al., Org. Lett., 2011, 13, 4570-4573.
(3) Creation and Functions of CD Supramolecular Structures
In crystalline structures, CDs adopt three types of assembly modes: ‘cage’ type, ‘channel’ type, and ‘layer’ type. We found that channel-type assemblies of γ-CD (γ-CDchannel), which were prepared according to Tonelli’s method (A.E. Tonelli et al., Langmuir 2002, 18, 10016.), formed unique cubic microstructures by scanning electron microscopic (SEM) observation. We also succeeded in the preparation of CD structures with various morphologies by varying the fabrication conditions, such as the γ-CD concentration in the aqueous solution and the type of poor solvent. It was found that the channel-type assemblies of γ-CDs possess excellent oil dispersion ability, and can form organogels in a variety of oils and organic solvents at ambient temperature.
1) T. Kida et al., Chem. Commun. 2009, 3889-3891. 2) Y. Marui et al., Chem. Mater., 2010, 22, 282-284. 3) Y. Marui et al., Langmuir 2010, 26, 11441-11445. 4) T. Kida et al., Chem. Lett. 2010, 39, 1206-1208.
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