Linear Dextrins - ACS Symposium Series (ACS Publications)

Chapter 18

Linear Dextrins Solid Forms and Aqueous Solution Behavior S. G. Ring and M. A. Whittam

Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: April 30, 1991 | doi: 10.1021/bk-1991-0458.ch018

Institute of Food Research, Colney Lane, Norwich, Norfolk NR4 7UA, United Kingdom

This article reviews physical and physico - chemical studies on linear malto - dextrins. The various crystalline forms of the dextrins are described and the melting and glass transition behavior discussed. Solution conformations and the viscous behavior of dilute and concentrated aqueous solutions are considered. In addition factors affecting the interaction of small solutes with the dextrins in aqueous solution are discussed and compared with the known behavior of cyclodextrins.

A m y l o d e x t r i n s a r e c o n v e n i e n t l y p r e p a r e d by t h e l i m i t e d h y d r o l y s i s of starch t o produce fragments ranging i n degree o f polymerization f r o m 2 t o a p p r o x i m a t e l y 6 0 . A l t h o u g h b r a n c h e d and c y c l i c f o r m s may be p r o d u c e d , i n t h i s a r t i c l e we w i s h t o c o n s i d e r t h e a q u e o u s s o l u t i o n b e h a v i o r o f l i n e a r d e x t r i n s . T h e s e d e x t r i n s may be p r o d u c e d i n a number o f ways f o r e x a m p l e by t h e l i m i t e d a c i d h y d r o l y s i s o f s o l i d f o r m s o f s t a r c h , more p a r t i c u l a r l y s t a r c h g r a n u l e s . A l t e r n a t i v e l y t h e y may be p r o d u c e d by l i m i t e d o t - a m y l o l y s i s o f s o l u b l e f o r m s o f s t a r c h . More r e c e n t l y w i t h t h e p r o d u c t i o n o f t h e d e b r a n c h i n g enzymes p u l l u l a n a s e and i s o a m y l a s e and w i t h t h e i d e n t i f i c a t i o n o f α - a m y l a s e s w h i c h p r o d u c e o l i g o s a c c h a r i d e s o f f i x e d length from t h e i r h y d r o l y s i s o f s t a r c h (1.2) t h e r e i s i n c r e a s e d o p p o r t u n i t y f o r t h e p r o d u c t i o n o f d e f i n e d d e x t r i n s f r o m s t a r c h . In a d d i t i o n t h e r e i s p o t e n t i a l , t h r o u g h t h e a p p l i c a t i o n o f g e n e t i c a p p r o a c h e s and m o l e c u l a r b i o l o g y t o m a n i p u l a t e enzyme s t r u c t u r e and h e n c e f u n c t i o n t o o b t a i n a w i d e r r a n g e o f p r o d u c t s more e f f e c t i v e l y . T h e s e m o l e c u l e s h a v e a v a r i e t y o f i n d u s t r i a l u s e s b o t h i n t h e f o o d i n d u s t r y and e l s e w h e r e . T h e y may be u s e d a s a g e n t s f o r t h e m a n i p u l a t i o n o f l i q u i d t e x t u r e and a l s o t h e m e c h a n i c a l p r o p e r t i e s o f s o l i d - l i k e m a t e r i a l s . In b o t h of these instances the i n t e r a c t i o n o f the d e x t r i n with water i n f l u e n c e s u s e f u l n e s s . C y c l o d e x t r i n s h a v e v e r y many p o t e n t i a l u s e s as a g e n t s f o r m o l e c u l a r e n c a p s u l a t i o n , t h e e n c a p s u l a t e d s p e c i e s occupying the c y c l o d e x t r i n c a v i t y . Dextrins can a l s o form molecular complexes with guest molecules. In a d d i t i o n i t i s p o s s i b l e t o 0097-6156/91/0458-0273$06.25A) © 1991 American Chemical Society

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

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BIOTECHNOLOGY OF AMYLODEXTRIN OLIGOSACCHARIDES

encapsulate a c t i v e ingredients i n glassy matrices of the d e x t r i n ; i n t h e s e m a t r i c e s i t i s p o s s i b l e t o p r o t e c t a c t i v e s p e c i e s and s u b s e q u e n t l y o b t a i n r e l e a s e on p l a s t i c i z a t i o n . F o r t h e s e a p p l i c a t i o n s i n t e r a c t i o n of the d e x t r i n with water again influences u s e f u l n e s s . C u r r e n t r e s e a r c h on t h e s e d e x t r i n s i s e x a m i n i n g t h e e f f e c t o f w a t e r on m a t e r i a l and m o l e c u l a r p r o p e r t i e s as a f u n c t i o n o f c o m p o s i t i o n , t e m p e r a t u r e and d e g r e e o f p o l y m e r i z a t i o n . In t h i s way i t i s h o p e d t o e s t a b l i s h p r e d i c t i v e r e l a t i o n s h i p s f o r s t r u c t u r e and f u n c t i o n w h i c h w i l l h e l p t a r g e t t h e a p p l i c a t i o n o f biotechnology to t h e i r production.


C r y s t a l l i n e Forms S o l i d a m y l o d e x t r i n s may be i n t h e g l a s s y / r u b b e r y o r c r y s t a l l i n e s t a t e ; c o n s i d e r a b l y more s t u d i e s h a v e been c a r r i e d o u t on t h e l a t t e r , so c r y s t a l s t r u c t u r e s a r e q u i t e w e l l u n d e r s t o o d . W h i l s t a m y l o p e c t i n i s t h o u g h t t o be t h e m a i n c r y s t a l l i n e component i n s t a r c h granules, amylose f r a c t i o n s ranging i n degree o f p o l y m e r i z a t i o n f r o m DP100 t o DP2700 h a v e been c r y s t a l l i z e d f r o m w a t e r i n t h e f o r m o f f i b r i l s ( 3 ) . S h o r t e r c h a i n a m y l o s e s o f DP15 and 3 5 h a v e been shown t o f o r m more p e r f e c t s i n g l e c r y s t a l s (4). O l i g o m e r s o f g l u c o s e h o w e v e r between h e p t a m e r and t r i m e r a r e v e r y d i f f i c u l t t o c r y s t a l l i z e . A l t h o u g h o l i g o m e r s as s h o r t a s m a l t o h e x a o s e h a v e been c o - c r y s t a l l i z e d i n t h e p r e s e n c e o f l o n g e r c h a i n s , t h e minimum c h a i n l e n g t h n e c e s s a r y f o r d o u b l e h e l i x f o r m a t i o n a p p e a r s t o be DP10 ( 5 . 6 ) . C r y s t a l s t r u c t u r e s o f g l u c o s e and m a l t o s e h a v e l o n g been known, b u t i t i s o n l y r e c e n t l y t h a t a c r y s t a l l i n e s t r u c t u r e f o r m a l t o t r i o s e has b e e n r e p o r t e d (2). D e g r e e o f c r y s t a l l i n i t y can v a r y w i d e l y , f r o m r e t r o g r a d e d a m y l o s e w h i c h i s p r e d o m i n a n t l y amorphous, c o n t a i n i n g p e r h a p s 10% c r y s t a l l i n e m a t e r i a l , t o h i g h l y c r y s t a l l i n e s i n g l e c r y s t a l s (4) o r s p h e r u l i t e s (δ) p r e p a r e d f r o m s h o r t e r c h a i n a m y l o s e s . S i m i l a r l y , s i n g l e c r y s t a l s o f V a m y l o s e have been r e p o r t e d (4^9) as w e l l as amorphous a m y l o s e c o m p l e x e s w h i c h do n o t g i v e r i s e t o X - r a y d i f f r a c t i o n p a t t e r n s (Ifi). Since the f i r s t p u b l i c a t i o n of X-ray d i f f r a c t i o n p a t t e r n s o f A , Β and C s t a r c h e s s i x t y y e a r s ago (H), s e v e r a l m o d e l s h a v e been p r o p o s e d f o r t h e i r c r y s t a l s t r u c t u r e s . U n t i l r e c e n t l y , a r i g h t handed, p a r a l l e l stranded arrangement o f double h e l i c e s , with a n t i p a r a l l e l packing of the h e l i c e s themselves was f a v o r e d f o r b o t h A and Β s t r u c t u r e s ( 1 2 . 1 3 ) . In t h e l a s t two y e a r s , h o w e v e r , s t u d i e s c o m b i n i n g X - r a y and e l e c t r o n d i f f r a c t i o n m e t h o d s o n s i n g l e c r y s t a l s o f a m y l o s e DP15, t o g e t h e r w i t h c o m p u t e r e n e r g y c a l c u l a t i o n s h a v e been c a r r i e d o u t ( 1 4 . 1 5 ) . From t h e s e s t u d i e s , a l e f t handed, p a r a l l e l stranded double h e l i x formation seems more l i k e l y , w i t h h e l i c e s p a c k e d p a r a l l e l t o one a n o t h e r i n the c r y s t a l . L e f t handed o r r i g h t handed, i t i s g e n e r a l l y accepted t h a t A and Β s t r u c t u r e s c o n s i s t o f s i m i l a r d o u b l e h e l i c e s ; t h e d i f f e r e n c e l i e s i n the s p a t i a l arrangement of these double h e l i c e s w i t h i n t h e c r y s t a l . In t h e Β s t r u c t u r e , s i x d o u b l e h e l i c e s a r e hexagonally arranged around a c e n t r a l c a v i t y c o n t a i n i n g water m o l e c u l e s . A i s more c o m p a c t , t h e c e n t r a l c a v i t y c o n t a i n i n g a n o t h e r double h e l i x , leading to a c r y s t a l structure including l e s s water t h a n t h a t o f B. L e s s c o n t r o v e r s y s u r r o u n d s t h e s t r u c t u r e o f V a m y l o s e , now



18.

RING & WHTTTAM

Linear Dextrins

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w i d e l y a c c e p t e d as a s i x f o l d s i n g l e h e l i x , a l t h o u g h w i t h i n t h e c l a s s i f i c a t i o n "V" s e v e r a l c r y s t a l l i n e v a r i a n t s a r e f o u n d . Vh and Va, t h e h y d r a t e d and a n h y d r o u s f o r m s o f c o m p l e x e d a m y l o s e , c a n be i n t e r c o n v e r t e d by m a n i p u l a t i n g m o i s t u r e o r h u m i d i t y c o n d i t i o n s . When 1 - b u t a n o l i s u s e d as t h e c o m p l e x i n g a g e n t , a t h i r d c r y s t a l s t r u c t u r e i s o b s e r v e d ( 1 6 ) , w h i l s t s e v e n f o l d and e i g h t f o l d h e l i c a l c o n f o r m a t i o n s h a v e been r e p o r t e d f o r a m y l o s e c o m p l e x e s w i t h b r a n c h e d c h a i n a l c o h o l s and α-naphthol r e s p e c t i v e l y ( 9 ) . One q u e s t i o n w h i c h r e m a i n s t o be r e s o l v e d i s t h a t o f t h e e x a c t l o c a t i o n o f t h e c o m p l e x i n g a g e n t w i t h i n t h e V s t r u c t u r e . In s o l u t i o n , t h e r e i s a body o f e v i d e n c e t o s u g g e s t t h a t t h e a m y l o s e h e l i x encapsulates the complexing "guest" molecule w i t h i n i t s core, g i v i n g r i s e , f o r example, t o the f a m i l i a r b l u e c o l o r o f the a m y l o s e - i o d i n e c o m p l e x . E v i d e n c e f r o m X - r a y and e l e c t r o n d i f f r a c t i o n t h a t s e v e n and e i g h t f o l d h e l i c e s a r e f o r m e d when i n c r e a s i n g l y l a r g e c o m p l e x i n g m o l e c u l e s a r e u s e d (2) l e n d s w e i g h t t o t h e i n c l u s i o n t h e o r y . However, an a l t e r n a t i v e e x p l a n a t i o n f o r t h e l a r g e r u n i t c e l l s ( a s c r i b e d t o s e v e n and e i g h t f o l d h e l i c e s ) i s that the complexing molecules are present i n the i n t e r s t i c e s between t h e i n d i v i d u a l h e l i c e s (lé). X - r a y d i f f r a c t i o n a n a l y s i s has g i v e n l i t t l e i n f o r m a t i o n on t h e l o c a t i o n o f t h e c o m p l e x e d m o l e c u l e s . T h i s may be b e c a u s e t h e s e m o l e c u l e s a r e r a n d o m l y p l a c e d and c o n s e q u e n t l y f o r m no r e g u l a r l a t t i c e f o r d i f f r a c t i o n , o r i t may be t h a t d e s p i t e b e i n g c o n f i n e d w i t h i n t h e h e l i c e s g u e s t m o l e c u l e s p o s s e s s t o o much r o t a t i o n a l and t r a n s l a t i o n a l f r e e d o m t o be i d e n t i f i a b l e by X - r a y d i f f r a c t i o n . I f c o m p l e x i n g m o l e c u l e s a r e removed f r o m t h e a m y l o s e c r y s t a l s by d r y i n g , the V s t r u c t u r e remains i n t a c t , i n d i c a t i n g t h a t the c o m p l e x a n t i s n o t a n e c e s s i t y f o r c r y s t a l s t a b i l i t y . However t h e r e i s some e v i d e n c e f r o m X - r a y d i f f r a c t i o n t o g e t h e r w i t h p o t e n t i a l energy c a l c u l a t i o n s t h a t dimethyl sulphoxide molecules are l o c a t e d i n s i d e t h e a m y l o s e h e l i x w i t h one DMSO m o l e c u l e f o r e v e r y t h r e e g l u c o s e r e s i d u e s (1Z). F u r t h e r work i s o b v i o u s l y needed t o d e t e r m i n e u n e q u i v o c a l l y where t h e i n c l u d e d m o l e c u l e s a r e s i t u a t e d w i t h i n the V amylose s t r u c t u r e . Computer m o d e l l i n g i s being used i n c r e a s i n g l y t o g a i n i n s i g h t i n t o the e n e r g i e s o f d i f f e r e n t m o l e c u l a r c o n f o r m a t i o n s . H e l i c a l forms o f a m y l o d e x t r i n s h a v e been m o d e l l e d u s i n g t h e d i m e r m a l t o s e as t h e s t a r t i n g u n i t ( 1 8 ) . Minimum e n e r g y c o n f o r m a t i o n s o f t h e d i s a c c h a r i d e a r e f o u n d by c a l c u l a t i n g t h e e n e r g y as t h e two s u g a r r i n g s a r e r o t a t e d a b o u t t h e g l y c o s i d i c l i n k a g e . P o l y m e r s b u i l t up f r o m t h e s e low e n e r g y m a l t o s e u n i t s a r e f o u n d t o have h e l i c a l s t r u c t u r e s w h i c h c o r r e s p o n d t o conformations observed f o r maltodextrins i n the c r y s t a l l i n e state. T h u s c o m p u t e r s t u d i e s complement X - r a y a n a l y s i s , and may h e l p i n t h e f u t u r e t o r e s o l v e t h e q u e s t i o n o f where c o m p l e x e d m o l e c u l e s a r e l o c a t e d w i t h i n c r y s t a l l i n e V amylose. The p o l y m o r p h i c f o r m o f a m y l o s e o r a m y l o d e x t r i n d e p e n d s on s e v e r a l f a c t o r s . F o r an a m y l o d e x t r i n o f a g i v e n l e n g t h i n a q u e o u s s o l u t i o n , formation of the A c r y s t a l type i s favored over Β at high t e m p e r a t u r e and h i g h c o n c e n t r a t i o n . C h a i n l e n g t h a l s o has an



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e f f e c t , s h o r t e r c h a i n s g i v i n g r i s e t o A s t r u c t u r e s and l o n g e r chains ( i n c l u d i n g amylose r e t r o g r a d e d under o r d i n a r y c o n d i t i o n s ) forming Β s t r u c t u r e s . Intermediate c o n d i t i o n s give r i s e to C-type c r y s t a l f o r m a t i o n . B e l o w DP10, s o l u t i o n s o f a m y l o d e x t r i n s d o n o t c r y s t a l l i z e e v e n a t 4°C and 5 0 % w/w, i m p l y i n g a l o w e r l i m i t o f c h a i n l e n g t h n e c e s s a r y f o r d o u b l e h e l i x f o r m a t i o n ( 5 ) . The c r y s t a l l i z a t i o n , m e l t i n g and d i s s o l u t i o n b e h a v i o r o f a m y l o d e x t r i n s may g i v e i n s i g h t i n t o t h e b e h a v i o r o f w h o l e s t a r c h e s , S i n c e i t i s the short chains of amylopectin which g i v e r i s e to the c r y s t a l l i n e r e g i o n s w i t h i n s t a r c h g r a n u l e s and w h i c h a r e b e l i e v e d t o m e l t and r e c r y s t a l l i z e d u r i n g g e l a t i n i z a t i o n and r é t r o g r a d a t i o n r e s p e c t i v e l y . These t r a n s i t i o n s are important i n terms o f f u n c t i o n a l p r o p e r t i e s o f d e x t r i n s and s t a r c h e s . A d d i t i o n o f an a l c o h o l , f o r e x a m p l e e t h a n o l o r i s o - p r o p a n o l , t o an a m y l o d e x t r i n s o l u t i o n e n c o u r a g e s t h e f o r m a t i o n o f A- r a t h e r t h a n B- t y p e c r y s t a l s . Above a c e r t a i n a l c o h o l c o n c e n t r a t i o n , V-type c r y s t a l s then begin to form p r e f e r e n t i a l l y . W h i l s t i t i s d i f f i c u l t t o p r e p a r e h i g h l y c r y s t a l l i n e s a m p l e s o f A and Β s t r u c t u r e using long c h a i n amylose, V amylose c r y s t a l s w i l l form r e l a t i v e l y e a s i l y under the r i g h t c o n d i t i o n s . S u i t a b l e complexing a g e n t s i n c l u d e f a t t y a c i d s , l i n e a r and b r a n c h e d a l c o h o l s , m o n o g l y c e r i d e s and v a r i o u s o t h e r o r g a n i c compounds. C a r e i s r e q u i r e d when c r y s t a l l i z i n g h i g h m o l e c u l a r w e i g h t a m y l o s e c o m p l e x e s ; f o r e x a m p l e a r a p i d c o o l i n g r a t e can l e a d t o f o r m a t i o n o f amorphous as w e l l as c r y s t a l l i n e m a t e r i a l ( 1 0 ) . S u c c e s s f u l technique r e s u l t s in p l a t e l e t - l i k e c r y s t a l s c o n s i s t i n g of stacked l a m e l l a e , t h e m s e l v e s f o r m e d f r o m a m y l o s e m o l e c u l e s f o l d e d b a c k and f o r t h b e t w e e n t h e b o t t o m and t o p s u r f a c e s o f t h e l a m e l l a ( 9 . 1 0 ) . For s y n t h e t i c polymer c r y s t a l l i z a t i o n i t i s found t h a t l a m e l l a r t h i c k n e s s , and h e n c e c r y s t a l s t a b i l i t y , d e p e n d s s t r o n g l y on p r e p a r a t i o n c o n d i t i o n s s u c h as t e m p e r a t u r e o f c r y s t a l l i z a t i o n and c o o l i n g r a t e . F u t u r e s t u d i e s may i n d i c a t e s i m i l a r r e l a t i o n s h i p s f o r these biopolymer c r y s t a l s . I f i t i s accepted t h a t the complexed molecules i n V amylose are i n c l u d e d w i t h i n the h e l i x , then the complexation p r o c e s s can be t h o u g h t o f as m o l e c u l a r e n c a p s u l a t i o n . S t a b i l i t y o f t h e s e c o m p l e x e s has been s t u d i e d by o b s e r v a t i o n o f t h e m e l t i n g / d i s s o l u t i o n b e h a v i o r o f amorphous and c r y s t a l l i n e c o m p l e x e s . D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) shows an e n d o t h e r m i c t r a n s i t i o n o n h e a t i n g amylose complexes; the value at which i t occurs depends on the n a t u r e o f t h e c o m p l e x . T h i s t r a n s i t i o n c o r r e s p o n d s t o m e l t i n g and d i s s o c i a t i o n o f t h e a m y l o s e - g u e s t c o m p l e x . Amorphous c o m p l e x e s h a v e been shown t o m e l t a t l o w e r t e m p e r a t u r e s t h a n t h e i r c r y s t a l l i n e counterparts, demonstrating the s t a b i l i z i n g i n f l u e n c e of the crystal l a t t i c e (Table I ) . The n a t u r e o f t h e g u e s t m o l e c u l e a l s o a f f e c t s c o m p l e x s t a b i l i t y ; f o r example complexes formed u s i n g guest molecules o f i n c r e a s i n g hydrocarbon chain length d i s s o c i a t e at i n c r e a s i n g l y h i g h e r temperatures. Thus the l o n g e r hydrocarbon c h a i n s c o n f e r g r e a t e r s t a b i l i t y on t h e a m y l o s e c o m p l e x s t r u c t u r e .


18.

RING & WHTTTAM

277

Linear Dextrins

T a b l e I . M e l t i n g t e m p e r a t u r e s f o r c r y s t a l l i n e and amorphous complexes o f amylose with l i n e a r a l c o h o l s Alcohol Chain Length

M e l t i n g Temperature


C r y s t a l 1 i ne C4 C5 C6 C7 C8 CIO

(K) Amorphous

341 354 364 368 372 377

321 334 344 348 352 358

A t t e m p t s h a v e been made t o d e s c r i b e t h e d i s s o l u t i o n o f s t a r c h c r y s t a l l i t e s on g e l a t i n i z a t i o n u s i n g t h e F l o r y e q u a t i o n f o r p o l y m e r / d i l u e n t systems (H): 1

_ 1

=

R

V Γ

XV,

(1)

where Tm i s t h e o b s e r v e d m e l t i n g t e m p e r a t u r e , Tm° i s t h e m e l t i n g t e m p e r a t u r e o f t h e u n d i l u t e d p o l y m e r , AHu i s t h e e n t h a l p y o f f u s i o n o f t h e r e p e a t i n g u n i t ( g l u c o s e ) , vyV- i s t h e r a t i o o f t h e m o l a r volume o f the r e p e a t i n g u n i t ( g l u c o s e ; t o t h a t o f the d i l u e n t ( w a t e r ) , ν i s t h e v o l u m e f r a c t i o n o f t h e d i l u e n t and χ i s t h e Flory-Huggins polymer- s o l v e n t i n t e r a c t i o n parameter. W h i l s t meeting with l i m i t e d success, the a p p l i c a t i o n o f the F l o r y equation t o w h o l e s t a r c h must be v i e w e d w i t h c a u t i o n s i n c e i t i s v a l i d o n l y f o r e q u i l i b r i u m m e l t i n g o f p u r e c r y s t a l l i n e p h a s e s . The p r e s e n c e o f g l a s s y o r amorphous m a t e r i a l w i t h i n t h e g r a n u l e w o u l d t e n d t o a f f e c t c r y s t a l l i t e melting temperature, thus j e o p a r d i z i n g the a p p l i c a b i l i t y of the F l o r y equation. Spherulites of amylodextrins i n A o r Β f o r m may be c o n s i d e r e d m o d e l s o f a m y l o p e c t i n c r y s t a l l i t e s , and i n t h i s c a s e a p p l i c a t i o n o f t h e F l o r y e q u a t i o n i s more j u s t i f i e d . S t u d i e s a r e c u r r e n t l y i n p r o g r e s s on t h e m e l t i n g b e h a v i o r o f A- and B - t y p e c r y s t a l s and t h e e f f e c t o f w a t e r as a diluent. B i o t e c h n o l o g y w i l l soon p e r m i t t h e b r e e d i n g o f p l a n t s w i t h s t a r c h e s o f p r e - d e t e r m i n e d m o l e c u l a r c o m p o s i t i o n . In a d d i t i o n t o v a r i a t i o n s i n a m y l o s e / a m y l o p e c t i n r a t i o , i t w i l l be p o s s i b l e t o v a r y t h e l e n g t h o f t h e a m y l o p e c t i n c h a i n s . I t has a l r e a d y been shown t h a t v a r i a t i o n i n a m y l o p e c t i n c h a i n l e n g t h has a s i g n i f i c a n t e f f e c t on s t a r c h r é t r o g r a d a t i o n b e h a v i o r ( 2 0 ) . In o r d e r f o r technology to e x p l o i t these v a r i a t i o n s f u l l y , i t i s important to understand the underlying molecular processes. This w i l l permit a r a t i o n a l a p p r o a c h t o t h e use o f g e n e t i c a l l y m a n i p u l a t e d s t a r c h e s and w i l l l e a d t o i m p r o v e d u n d e r s t a n d i n g o f c r y s t a l l i z a t i o n b e h a v i o r


278


o f amylodextrins i n g e n e r a l . An example might be f r a c t i o n a t i o n o f amylodextrins u s i n g knowledge o f t h e i r r e s p e c t i v e c r y s t a l l i z a t i o n temperatures i n a given s o l u t i o n .


Amorphous Forms W h i l e d e x t r i n s c a n o f t e n be e n c o u n t e r e d as c r y s t a l l i n e s o l i d s i n many c a s e s t h e y can be p r e p a r e d as p a r t i a l l y c r y s t a l l i n e o r w h o l l y amorphous m a t e r i a l s . F o r t h e s e m a t e r i a l s , t h e g l a s s t r a n s i t i o n b e h a v i o r o f t h e amorphous r e g i o n s i s r e l e v a n t . A g l a s s has many o f the c h a r a c t e r i s t i c s of a l i q u i d except that of m o b i l i t y , i t s importance t o t e c h n o l o g i c a l a p p l i c a t i o n stems from the d r a m a t i c c h a n g e s i n m a t e r i a l p r o p e r t i e s e.g. m e c h a n i c a l and d i f f u s i v e b e h a v i o r , w h i c h o c c u r i n t h e r e g i o n o f t h e g l a s s t r a n s i t i o n . The t e c h n o l o g i c a l importance o f the g l a s s t r a n s i t i o n f o r polymeric m a t e r i a l s , i n o r g a n i c g l a s s e s and more r e c e n t l y b i o m a t e r i a l s has been r e v i e w e d ( 2 1 - 2 4 ) . A n e x a m p l e on how i t m i g h t a f f e c t t h e u s a g e o f d e x t r i n s w i l l i l l u s t r a t e some o f t h e p r i n c i p l e s . G l a s s y m a t r i c e s may be u s e d f o r e n c a p s u l a t i o n and p r o t e c t i o n o f a c t i v e i n g r e d i e n t s . The m e c h a n i s m o f p r o t e c t i o n i s somewhat d i f f e r e n t t o t h a t o b t a i n e d f r o m t h e f o r m a t i o n o f m o l e c u l a r i n c l u s i o n c o m p l e x e s and d o e s n o t r e l y upon t h e m o l e c u l a r r e c o g n i t i o n o f a c t i v e i n g r e d i e n t and h o s t . In t h e g l a s s , t h e r a t e o f d i f f u s i o n o f compounds i s s e v e r e l y h i n d e r e d . Hence, t h e g l a s s y m a t r i x can p r o t e c t t h r o u g h l i m i t i n g the d i f f u s i o n a l encounter of r e a c t i n g s p e c i e s . With p l a s t i c i z a t i o n of the matrix d i f f u s i o n w i l l a c c e l e r a t e ; through manipulation o f plasticization controlled release is possible. Although a g l a s s i s metastable with respect to the c r y s t a l l i n e f o r m , i t c a n r e m a i n i n a g l a s s y f o r m f o r many y e a r s ; a b o v e t h e g l a s s t r a n s i t i o n c r y s t a l l i z a t i o n i s much more r a p i d . G l a s s e s a r e t h e r e f o r e p r e p a r e d by c o o l i n g a t r a t e s s u f f i c i e n t l y f a s t t o a v o i d c r y s t a l l i z a t i o n . The r e q u i r e d c o o l i n g r a t e d e p e n d s o n t h e m a t e r i a l . F o r e x a m p l e , t o v i t r i f y w a t e r , c o o l i n g r a t e s i n e x c e s s o f 10 Ks" are necessary (25.26). Polymeric m a t e r i a l s c r y s t a l l i z e l e s s r e a d i l y and c o o l i n g r a t e s o f < l K s a r e o f t e n s u f f i c i e n t f o r v i t r i f i c a t i o n . A s a l i q u i d i s s u p e r c o o l e d , m o l e c u l a r m o t i o n and t h e l o c a l r e a r r a n g e m e n t o f m o l e c u l e s becomes p r o g r e s s i v e l y s l o w e r and the v i s c o s i t y increases. For D-glucose the v i s c o s i t y increases by 13 o r d e r s o f m a g n i t u d e on c o o l i n g f r o m a m e l t a t 413K t o t h e g l a s s a t 290K ( 2 7 . 2 8 ) . W i t h c o o l i n g , t h e p r o b a b i l i t y o f a m o l e c u l e o v e r c o m i n g an a c t i v a t i o n e n e r g y t o e s c a p e f r o m i t s n e i g h b o r s d e c r e a s e s . At the g l a s s t r a n s i t i o n t h e r e i s a sudden " f r e e z i n g ' O f m o t i o n w i t h a c o n s e q u e n t s h a r p f a l l i n h e a t c a p a c i t y . The g l a s s t r a n s i t i o n t e m p e r a t u r e , Tg, i s a f f e c t e d by c o o l i n g r a t e , t h e " f r e e z i n g i n " o f l i q u i d s t r u c t u r e w i l l occur at p r o g r e s s i v e l y lower t e m p e r a t u r e s as t h e c o o l i n g r a t e d e c r e a s e d , a l w a y s a s s u m i n g t h a t c r y s t a l l i z a t i o n d o e s n o t i n t e r v e n e . The o b s e r v e d Tg i s t h u s p r o f o u n d l y a f f e c t e d b y k i n e t i c f a c t o r s ( 2 1 ) . However, i t i s a r g u e d i n v a r i o u s t h e o r e t i c a l a p p r o a c h e s (£9) t h a t u n d e r l y i n g t h e e x p e r i m e n t a l l y observed t r a n s i t i o n i s a t r u e phase t r a n s i t i o n . The Tg o f amorphous m a t e r i a l s can be d e t e r m i n e d by m e a s u r i n g h e a t c a p a c i t y as a f u n c t i o n o f t e m p e r a t u r e and o b s e r v i n g t h e s h a r p i n c r e a s e i n heat c a p a c i t y , i n d i c a t i v e o f a g l a s s t r a n s i t i o n , which o c c u r s at Tg. At the p r e s e n t time t h i s i s c o n v e n i e n t l y performed 5

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u s i n g d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y t o d e t e r m i n e T g a n d ACp ( f i g u r e 1 ) . In a d d i t i o n by v a r y i n g both h e a t i n g and c o o l i n g r a t e s and o b s e r v i n g t h e c h a n g e i n T g i t i s p o s s i b l e t o o b t a i n an " a c t i v a t i o n enthalpy" f o r t h e r e l a x a t i o n process a t Tg (30.31). T h i s e n t h a l p y c a n be u s e f u l l y compared w i t h t h a t f r o m v i s c o s i t y data (3Û). The g l a s s t r a n s i t i o n b e h a v i o r o f c a r b o h y d r a t e s i n g e n e r a l i s r e c e i v i n g r e c e n t a t t e n t i o n . T h e T g ' s o f amorphous mal t o o l i g o m e r s f r o m m a l t o s e t o m a l t o h e x a o s e was r e c e n t l y d e t e r m i n e d ( 3 2 ) a n d r a n g e d f r o m 364K f o r m a l t o s e t o 448K f o r m a l t o h e x a o s e . D a t a f o r h i g h e r o l i g o m e r s was n o t o b t a i n a b l e d u e t o t h e r m a l d e g r a d a t i o n , t h e h i g h m o l e c u l a r w e i g h t l i m i t o f T g o b t a i n e d b y e x t r a p o l a t i o n was 500±10K. A t low d i l u e n t c o n c e n t r a t i o n s t h e a d d i t i o n o f water s t r o n g l y d e p r e s s e d T g , f o r example t h e T g f o r m a l t o h e x a o s e f e l l 100K on a d d i t i o n o f 1 0 % w/w w a t e r . T h e c o m p o s i t i o n a l v a r i a t i o n o f Tg w i t h w a t e r c o n t e n t was i n v e s t i g a t e d a t w a t e r c o n t e n t s s o g u e s t m o l e c u l e s may o r i e n t t h e m s e l v e s i n s i d e t h e c y c l o d e x t r i n c a v i t y w i t h t h e i r own d i p o l e moment o r i e n t e d a n t i p a r a l l e l t o that o f the cyclodextrin molecule, with a r e s u l t i n g a t t r a c t i v e d i p o l e - d i p o l e f o r c e . In c a s e s where t h e g u e s t m o l e c u l e has no p e r m a n e n t d i p o l e moment, i t s p o l a r i z a b i l i t y w i l l d e t e r m i n e t h e s i z e o f d i p o l e moment i n d u c e d by t h e p e r m a n e n t d i p o l e o f t h e c y c l o d e x t r i n m o l e c u l e and t h u s i n f l u e n c e t h e s t r e n g t h o f i n t e r a c t i o n between t h e two. Van d e r Waal s ' f o r c e s , i n c l u d i n g b o t h d i p o l e - d i p o l e i n t e r a c t i o n a n d London d i s p e r s i o n f o r c e s , a r e t h u s l i k e l y t o play a part i n the i n c l u s i o n process. Evidence f o r t h i s comes f r o m t h e f a r g r e a t e r a f f i n i t y o f α c y c l o d e x t r i n f o r t h e 4 - n i t r o p h e n o l a t e anion than f o r 4-nitrophenol (68) L i n e a r m a l t o d e x t r i n h e l i c e s p r e s u m a b l y a l s o p o s s e s s a d i p o l e moment, a l t h o u g h t h e e f f e c t m i g h t be e x p e c t e d t o be s m a l l e r t h a n t h a t o f t h e c y c l o d e x t r i n d u e t o t h e more e x t e n d e d f o r m o f t h e l i n e a r maltodextrin. A c l o s e s p a t i a l f i t between c y c l o d e x t r i n a n d g u e s t m o l e c u l e i s g e n e r a l l y r e c o g n i z e d as important f o r good c o m p l e x a t i o n , which s u p p o r t s t h e e v i d e n c e t h a t v a n d e r Waal s' i n t e r a c t i o n s , w h i c h a r e s h o r t r a n g e , may s t a b i l i z e t h e c o m p l e x . B e c a u s e o f t h i s r e q u i r e m e n t f o r a c l o s e f i t between g u e s t and h o s t , p e r h a p s , c y c l o d e x t r i n s o f d i f f e r e n t s i z e s complex p r e f e r e n t i a l l y w i t h d i f f e r e n t guest m o l e c u l e s a n d may be u s e d f o r t h e s e p a r a t i o n o f s p e c i f i c m o l e c u l e s f r o m a m i x t u r e . L i n e a r m a l t o d e x t r i n s , on t h e o t h e r hand, show l e s s s p e c i f i c " r e c o g n i t i o n " o f m o l e c u l e s , and a r e b e l i e v e d t o f o r m h e l i c a l c o m p l e x e s w i t h 6,7 o r 8 g l u c o s e u n i t s p e r t u r n , d e p e n d i n g on t h e s i z e o f t h e g u e s t m o l e c u l e . No s i n g l e mode o f i n t e r a c t i o n , t h e r e f o r e , seems t o be r e s p o n s i b l e f o r c o m p l e x a t i o n , b u t a v a r i e t y of interactions are operational, their relative contributions v a r y i n g w i t h t h e n a t u r e o f t h e g u e s t m o l e c u l e . T h i s e x p l a i n s why such a wide range o f molecules can form i n c l u s i o n complexes w i t h e i t h e r c y c l i c o r l i n e a r g l u c a n s , and may a l s o h e l p t o s h e d l i g h t on why, f o r e x a m p l e t h e c o m p l e x i n g a b i l i t i e s o f c y c l o d e x t r i n s a r e 2-3 orders o f magnitude higher than those o f t h e corresponding non-cyclic analogs (69). Acknowledqments The a u t h o r s w i s h t o t h a n k t h e M i n i s t r y o f A g r i c u l t u r e , F i s h e r i e s and F o o d f o r f i n a n c i a l s u p p o r t . Literature Cited 1.

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