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Please release me - but slowly.

Please Release Me--But Slowly

There's a family of compounds called cyclodextrins that are adept at letting go.

The three compounds, differing in molecular ring size and made from cornstarch, serve as "hosts" or traps for other substances, which they slowly release.

The substances that have been encapsulated in this way include pharmaceuticals, pesticides, perfumes, cosmetics, and foods. Their gradual release can be triggered in several ways, including exposure to moisture.

Cyclodextrins are ideal for shielding drugs or nutrients from premature deterioration caused by oxygen and for masking undesirable flavors.

Until now, however, it hasn't been practical to encapsulate certain products. Although cornstarch is inexpensive, cyclodextrins have been costly to make and purify, says ARS chemist Jacob A. Rendleman, Jr., of the National Center for Agricultural Utilization Research, Peoria, Illinois.

Especially costly is gamma-cyclodextrin with a "donut hole" larger than those of alpha- and beta-cyclodextrins. Its cavity is easily capable of binding cyclic molecules that have 12 or more atoms in their rings.

Prices for gamma-cyclodextrin have been about 100-fold greater than those of beta-cyclodextrin over the past 10 years, but they are drifting downward with prospects of market competition, according to an industry spokesperson.

Rendleman's studies may lead to cheaper production and increased use of gamma-cyclodextrin. Until he recently found a way to convert up to 50 percent of starch into gamma-cyclodextrin, a typical yield was 5 percent unless processes were involved that require toxic or more expensive materials.

Currently, worldwide sales of beta-cyclodextrin are estimated at 2 million pounds, mostly for encapsulating food additives in Japan. But the fastest market growth--estimated at 20 to 30 percent annually--is in pharmaceuticals, where 3 to 5 percent of the specialty product is used.

Cyclodextrins are formed when an enzyme from the bacterium Bacillus macerans breaks up starch and reassembles it into the donut-shaped molecules. Biosynthesis begins to shut down with increasing concentration of product in the reaction medium.

Rendleman enhanced the yield of the gamma form by adding a nontoxic 12-carbon-ring compound called cyclododecanone to the reaction medium where it captured and insolubilized the cyclodextrin as it was formed.

Then, in a process called azeotropic distillation, he separated cyclodextrin from the cyclododecanone--which could then be reused.

PHOTO : Chemist Jacob Rendleman uses molecular models to determine the best shape and size for a molecule to bind snugly within the cavity of the donut-shaped gamma-cyclodextrins.
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Title Annotation:Agricultural Research Service research into production and use of gamma-cyclodextrins
Author:Hardin, Ben
Publication:Agricultural Research
Date:Oct 1, 1991
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