Biotechnology of Amylodextrin Oligosaccharides - ACS Publications

Chapter 1

Helical and Cyclic Structures in Starch Chemistry

Downloaded by UNIV OF MEMPHIS on September 10, 2012 | http://pubs.acs.org Publication Date: April 30, 1991 | doi: 10.1021/bk-1991-0458.ch001

J. Szejtli Cyclodextrin Research and Development Laboratory, Cyclolab, 1026 Budapest, Endrdódi S. 38/40, Hungary

Two opposing hypotheses attempted to describe the conformation of the α - l , 4 - l i n k e d glucopyranoside polymers in ne utral aqueous solutions: the random coil and the segmented helix structure hypotheses. The former one was based mainly on hydrodynamic studies of amylose solutions, the latter on many very different observations, but mainly on the formation and properties of amylose-helix complexes. The formation of cyclodextrins, catalysed by cyclizing enzy mes, delivers further proof for the helical structure, and simultaneously is the source of a new technology: the molecular encapsulation of different compounds by cyc lodextrin complexation. The significance of the cyclodext rins and their derivatives in commercial applications will be discussed. When two D-glucopyranose units are linked by bonds.

Fig.2.

Only the " c i s " and the "trans" conformations can e x i s t .

amylose Fig.3.

cellulose

Repeating the " c i s " (=maltose) conformation r e s u l t s i n a h e l i c a l structure while the "trans" (=cellobiose) leads to a zig-zag chain.

In Biotechnology of Amylodextrin Oligosaccharides; Friedman, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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Fig.4.


The r i g i d , r o d - l i k e h e l i x , the "segmented" f l e x i b l e c o i l l i k e h e l i c a l structure, and the random c o i l .

Fig.5.

The extended h e l i x i s contracted to a t i g h t - h e l i x upon i n c l u s i o n complex formation - e.g. with iodine.

o CGT

enzyme

Qj

O 0

O x 1 starch

Fig.6.

cyclic and

'

J

acyclic dextrins

Degradation of starch to a mixture of c y c l i c and a c y c l i c dextrins by cyclodextrin glycosyl-transferase enzyme.



Fig.7.

Structure of the cyclodextrins.


1.

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The i n c l u s i o n complex formation In t h i s c y c l i c structure an i n t e r e s t i n g , and p r a c t i c a l l y very important assymetry of the glucopyranose units i s revealed: the i n t e r n a l cavity has an apolar character, because i t i s " l i n e d " by hydrogen atoms, and ether-oxygen atoms. (Fig.8.) One rim of the molecule consists of the primary hydroxyl groups, the other rim of the secondary hydroxyl groups. The dimensions of these "empty cylinders" are considerable. (Fig.9.) Dissolving cyclodextrin i n water, the cavity-included water molecules w i l l contact an apolar cavity-surface, which i s energetically unfavorable. Adding any substance to an aqueous cyclodextrin solution which has also an apolar - water repellent - character, and which can t i g h t l y f i t into the c a v i t y , an i n c l u s i o n complex is formed(Fig.lO). The cyclodextrin "host" can accomodate molecules of very d i f f e r e n t "guest" compounds. The association constants are very d i f f e r e n t , (Fig.11.) but generally such i n c l u s i o n complexes can be i s o l a t e d i n m i c r o c r y s t a l l i n e form. The composition of the i s o l a t e d complexes again depends on various f a c t o r s , and only occassionally are of s t r i c t l y stoichiometric r a t i o s , but i n most cases a nearly 1:1 (CD:guest), or 2:1, or 3:2 etc. composition i s found. (Fig.12.) In such molecularly encapsulated form the physical-chemical properties of the "guest" substances are strongly modified. I n d u s t r i a l aspects of molecular encapsulation by CDs These e f f e c t s can be u t i l i z e d f o r many i n d u s t r i a l purposes. For example benzaldehyde i s a l i q u i d compound, which very rapidly oxidized to benzoic acid by atmospheric oxygen. The (iCDbenzaldehyde complex can be stored even i n pure oxygen atmosphere, without s i g n i f i c a n t oxidation. V o l a t i l e l i q u i d s become stable, also. This i s the base of the s t a b i l i z a t i o n of flavours and fragrances by CD-complexation. Upon contact with water, these complexes immediately begin to d i s s o c i a t e , i . e . the entrapped substances w i l l be released r a p i d l y . Such products are marketed already i n several countries. Unstable, poorly soluble drugs can be s t a b i l i s e d , or t h e i r b i o a v a i l a b i l i t y can be improved by CD-complexation. Several drugs are marketed i n CD-complexed form already. With appropriate cyclodextrins, i n j e c t a b l e aqueous solutions can be prepared from insoluble drugs. Light sensitive substances can be protected against decomposition by CD-complexation. For example, l i g h t - s e n s i t i v e pyrethroids are i n s e c t i c i d e s which can be s t a b i l i z e d f o r an extended period of time. In biotechnological processes the CDs can act as s o l u b i l i z e r s (e.g. i n microbiological s t e r o i d conversion) or as t o x i c i t y reducing agents (e.g. i n waste-water detoxication). The t o t a l number of CD-papers and patents - which i s already over 5000 - shows an explosionlike increase. Actually yearly some 650 new papers and patents are published. (7). More than 800 patents are dedicated to some sort of iricTustrial u t i l i z a t i o n of cyclodextrins. The CD production began j u s t 10 years ago, p r a c t i c a l l y on laboratory scale, but now i t increases r a p i d l y . The approximately 1000 ton/year production i s expected to grow up to a several ten thousand ton/year CD-market within several years.


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8

BIOTECHNOLOGY OF AMYLODEXTRIN OLIGOSACCHARIDES

A P O L A R CAVITY

SECONDARY HYDROXYLS

"PRIMARY HYDROXYLS


Fig.8.

Functional schematic representation of a cyclodextrin ''cylinder".

174

A

3

262 A

427

A

3

ccCD in one mol: 104 ml in one g • 0,10 ml Fig.9.

0,14 ml

0,20 ml

The cavity volumes i n the cyclodextrin "capsules"

o 0 0 00 O O O O O O O O O O O O O O O O O O O n O ° ° 0 ° 0 o0 ° b O O O o o ° o o0 0

0

ooo °

0

0

J ° o 00 0° ° o o H

H

0

S3 °

0

O

O

/^CC>so

f

u

o o o

0

o^o

C H

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o

oooo? ooop o o o o o oooo U

o o- o ° ' ° ° " o o o ' oo o o o o o o o o o o o o 0

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u

ooo o ° o oo

oo°oo o

ooo

Fig.10. The "host-guest" i n t e r a c t i o n : molecular encapsulation of p-xylene by |3-cyclodextrin.



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BENCYCLAN-CD Fig.11. The assumed structures, the determined association constants f o r the bencyclan (a cerebral vasodilator) cyclodextrin complexes.

COOH

Fig.12. Various stoichiometrics of cyclodextrin complexes.


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BIOTECHNOLOGY OF AMYLODEXTRIN OLIGOSACCHARIDES

Literature Cited 1. 2.


3.

4. 5. 6. 7.

E l i e l , E . L . , Allinger N.L., Angyal S . J . , Morrison G.A. Conformational Analysis. Interscience Publishers, New York, London, Sydney 1965. (Third Printing 1967). Stoddart J . F . Stereochemistry of Carbohydrates. Wiley-Intersciences, New York. 1971. p.249. Rao V.S.R., Sundararajan P.R., Ramakrishnan C., Ramachandran G.N. Conformation of Biopolymers. (Papers read at an International Symposium held at the University of Madras. 1967. jan. 18-21). ed.: Ramachandran G.N. Academic Press, London, New York 1967. p. 721-33. Szejtli J., Richter M., Augustat S. Biopolymers, 1967, 5, 5-16, 17-26. (1967) Vetter D. Workshop on Supramolecular Organic Chemistry and Photochemistry, Universität des Saarlandes, Saarbrücken, 27 aug.1. sept. 1989. Szejtli J . Cyclodextrin Technology, Kluwer Academic Publisher, Dordrecht, 1988. Cyclodextrin News, published by FDS Publications, Trowbridge U.K. Vol. 3. 1989.

RECEIVED October 19, 1990


Biotechnology of Amylodextrin Oligosaccharides - ACS Publications

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