Structural Features Promoting Water Solubility in Carbohydrate

1 Structural Features Promoting Water Solubility in Carbohydrate Polymers J. E. Glass

Downloaded by 146.185.202.12 on May 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch001

Department of Polymers and Coatings, N o r t h D a k o t a State University, F a r g o , ND 58105

Although a large variety of carbohydrate polymers have a multitude of industrial uses, most carbohydrate polymers have limited applicability across a wide spectrum of applications. The structural features affecting water solubility in carbohydrate polymers are discussed. Methods of derivatizing the world's most abundant polymer, cellulose, are presented, and derivatization processes are compared with the complexities of synthesizing polymers by fermentation processes. On the basis of three criteria, (1) the parameters promoting solubility, (2) the known limitations of commercially available products, and (3) the solubility trends observed in a limited number of thoroughly investigated carbohydrate polymers, projections of what might be achieved with a structurally designed carbohydrate polymer, obtained from variations in the repeating unit and anomeric and positional bonding patterns, are postulated.

T H E C H E M I S T R Y O F C A R B O H Y D R A T E P O L Y M E R S is a n i n f r e q u e n t s u b ject i n college curricula. These macromolecules c a n b e a p p r o p r i a t e l y i n t r o d u c e d w i t h t h e c h e m i s t r y ( J , 2) o f the s i m p l e t h r e e - c a r b o n m o l e c u l e , g l y c e r a l d e h y d e , w h i c h is the m o n o m e l i c p r o t o t y p e f o r p o l y s a c c h a r i d e s . T h e m i d d l e c a r b o n is a s y m m e t r i c (i.e., f o u r d i f f e r e n t g r o u p s a r e b o n d e d t o the c e n t e r c a r b o n ) ; the t w o o p t i c a l i s o m e r s (dist i n g u i s h e d b y the d i r e c t i o n i n w h i c h t h e y r o t a t e p l a n e - p o l a r i z e d l i g h t ) that r e s u l t a r e i l l u s t r a t e d i n s t r u c t u r e I. I n the d e x t r o r o t a t o r y c o m p o u n d , d e s i g n a t e d b y a D p r e f i x , the h y d r o x y l g r o u p is p l a c e d to the right o f the a s y m m e t r i c c a r b o n ( C * ) w i t h the a l d e h y d e g r o u p p o s i t i o n e d a b o v e . I n t h e l e v o r o t a t o r y c o m p o u n d ( L p r e f i x ) , the h y d r o x y l f u n c t i o n is p l a c e d t o the l e f t o f the a s y m m e t r i c c a r b o n a t o m . A s a d d i t i o n a l c a r b o n s ( w i t h t h e i r h y d r o x y l f u n c t i o n s ) a r e a d d e d to this t h r e e - c a r b o n m o l e c u l e , t h e n u m b e r o f i s o m e r s increases b y 2", w h e r e n is t h e n u m b e r o f a s y m m e t r i c c a r b o n s . T h e D o r L c l a s s i f i c a t i o n c o n t i n u e s t o a p p l y to the 0(W5-2393/86/0213-0003$07.25/0 ® 1986 American Chemical Society

Glass; Water-Soluble Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1986.


4

W A T E R - S O L U B L E POLYMERS

s e c o n d to last c a r b o n i n the s t r u c t u r a l f o r m u l a , r e g a r d l e s s o f the n u m b e r o f c a r b o n s i n the m o l e c u l e . T h e s a c c h a r i d e s a r e p o l y h y d r o x y l i c c o m p o u n d s w i t h a p e n d a n t c a r b o n y l g r o u p a n d c a n b e r e p r e s e n t e d b y the f o r m u l a ( C H 0 ) „ . P e n t o s e s (n = 5) a n d hexoses (n = 6) a r e t h e m o s t a b u n d a n t ; the s i x - c a r b o n m o l e c u l e s are the m o s t i m p o r t a n t i n d u s t r i a l class. T h e l a r g e n u m b e r o f a s y m m e t r i c c a r b o n a t o m s g i v e s rise to 16 p o s s i b l e h e x o s e i s o m e r s (half a r e o f t h e D c o n f i g u r a t i o n , C h a r t I , a n d the r e m a i n d e r a r e o f the L c o n f i g u r a t i o n ) . 2

S u g a r s exist as c y c l i c s t r u c t u r e s i n w h i c h s i x - m e m b e r e d p y r a n o s e r i n g s a r e f a v o r e d o v e r f i v e - m e m b e r e d f u r a n o s e r i n g s . O p e n - c h a i n , free sugars h a v e n o t b e e n i s o l a t e d , b u t s o m e sugars h a v e b e e n d e t e c t e d i n trace amounts in solution e q u i h b r i u m w i t h their closed, ring forms. T h e c l o s e d , r i n g - f o r m sugars a r i s e b y i n t e r n a l c o n d e n s a t i o n o f a h y d r o x y l

CHO H-C-OH

CHO HO-C-H

CHO H-C-OH HO-C-H

CHO HO-C-H HO-C-H

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-6-0H

H-C-OH

H-C-OH

H-C-OH

H-C-OH

I

C H OH 2

Allose CHO H-C-OH H-C-OH HO-C-H H-^-OH

C H OH 2

Gulose

C H OH 2

Altrose CHO

C H OH 2

Glucose CHO H-C-OH

CH OH 2

Mannose CHO HO-C-H

HO-C-H I H-C-OH

HO-C-H

HO-C-H

HO-C-H

HO-C-H

HO-C-H

I

H-C-OH

I

C H OH 2

Idose

H-C-OH

I

C H OH 2

Galactose

I

H-C-OH CH OH 2

Talose

Chart I. D isomers of hexose.


1.

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Water Solubility in Carbohydrate Polymers

5

Scheme I. Equilibrium between D-glucose and a-D-glucopyranose (ring form; shown in the Haworth projection).


g r o u p w i t h t h e e a r b o n y l m o i e t y to f o r m a n a c e t a l g r o u p . C y c l i z a t i o n c r e ates a n a d d i t i o n a l a s y m m e t r i c c e n t e r at the r e a c t i v e c a r b o n p o s i t i o n , n o w designated the a n o m e r i c c a r b o n . T h u s , D-glucose cyclizes preferentially b y reaction of the h y d r o x y l g r o u p o n c a r b o n 5 ( n u m b e r e d sequentially d o w n the o p e n - c h a i n s t r u c t u r e f r o m the a l d e h y d e c a r b o n ) w i t h the a l d e h y d e e a r b o n y l to f o r m a s i x - m e m b e r e d r i n g (Scheme

I). T h e n e w l y created

h y d r o x y l g r o u p o n c a r b o n 1 c a n p o s i t i o n a b o v e o r b e l o w the p l a n e o f t h e r i n g . W i t h t h e p y r a n o s e r i n g d r a w n a c c o r d i n g to the H a w o r t h c o n v e n t i o n as i n S c h e m e I , t h e p e n d a n t h y d r o x y m e t h y l g r o u p o f D sugars lies a b o v e t h e r i n g p l a n e w h i l e the c o r r e s p o n d i n g g r o u p i n the L series lies b e l o w . W i t h i n either series, the i s o m e r w i t h the a n o m e r i c h y d r o x y l g r o u p b e l o w the r i n g is c a l l e d t h e a a n o m e r ; the c o r r e s p o n d i n g h y d r o x y l lies a b o v e the p l a n e i n the

anomer. T h e connection of rings through

t w o d i f f e r e n t a n o m e r i c b o n d s is a n i m p o r t a n t f e a t u r e i n c a r b o h y d r a t e polymers. W h e n l i n e a r h y d r o c a r b o n s are s t r u c t u r e d i n r i n g s , the b o n d angles b e t w e e n contiguous c a r b o n atoms i n a six-carbon ring prohibit planar projections. T h e rings b u c k l e a n d assume various conformations. S i x c a r b o n r i n g s are k n o w n to exist m o s t f r e q u e n t l y i n a c h a i r f o r m [struct u r e II (a = a x i a l a n d e = e q u a t o r i a l ) ] i n w h i c h s u b s t i t u e n t b o n d s to r i n g c a r b o n s , h y d r o g e n , o r h y d r o x y l f u n c t i o n s are a x i a l o r e q u a t o r i a l . A x i a l

a

II


6


b o n d s are those p a r a l l e l w i t h t h e r i n g s g e n e r a l axis o f s y m m e t r y . E q u a t o r i a l b o n d s l i e i n the a p p r o x i m a t e p l a n e o f the r i n g ( w i t h its greater c o n t i n u u m of electron density) a n d generally have different reactivities t h a n a x i a l b o n d s . O f t h e c o n f o r m a t i o n s p o s s i b l e f o r t h e p y r a n o s e r i n g , t h e o n e that m i n i m i z e s steric r e p u l s i o n s a m o n g a x i a l l y d i s p o s e d h y d r o x y l s a n d t h e h y d r o x y m e t h y l g r o u p is f a v o r e d . A c c o r d i n g l y , the c o n f o r m a t i o n d e p i c t e d i n s t r u c t u r e II is p r e f e r r e d b y b o t h

anomers

of D - g l u c o s e because it positions the b u l k y h y d r o x y m e t h y l a n d h y d r o x y l g r o u p s o n c a r b o n s 2, 3, a n d 4 i n e q u a t o r i a l p o s i t i o n s . T h e d i f f e r e n c e b e t w e e n a a n d /? l i n k a g e s is i l l u s t r a t e d b y u s i n g the g l u c o p y r a n o s y l u n i t i n t h e c o n s t r u c t i o n o f t w o d i s a c c h a r i d e s (structure


I l i a a n d b). If t w o g l u c o p y r a n o s y l molecules are j o i n e d through equatorial b o n d s , t h e l i n k a g e is r e f e r r e d to as p a n d t h e p r o d u c t is c e l l o b i o s e . I f g l u c o p y r a n o s y l units are joined t h r o u g h a n axial linkage o f one unit, a n a l i n k a g e is f o r m e d , a n d t h e p r o d u c t is m a l t o s e . C e l l o b i o s e is the b a s i c u n i t o f c e l l u l o s e ( s t r u c t u r e I l i a ) a n d m a l t o s e is the b a s i c u n i t o f s t a r c h (structure I l l b ) — t w o o f the w o r l d s m o s t a b u n d a n t p o l y m e r s .

Classification of Polysaccharides by Natural Function: Storage, Structural, and Extracellular In general, the s i m p l e distinction of a versus 0 linkages b e t w e e n

mono-

m e r units determines the f u n c t i o n of polysaccharides i n plant cells. Storage polysaccharides p r o v i d e an energy s u p p l y for plant or a n i m a l cells. G e n e r a l l y , s t o r a g e p o l y s a c c h a r i d e s possess a n a x i a l - e q u a t o r i a l b o n d (i.e., a n a l i n k a g e ) b e t w e e n t h e 1- a n d 4 - p o s i t i o n s o f a d j a c e n t g l u c o s e units. I n p l a n t c e l l s , this e n e r g y s o u r c e is s t a r c h , e i t h e r a m y l o s e ( l i n e a r m a c r o m o l e c u l e s o f m a l t o s e units) o r a m y l o p e c t i n (a b r a n c h e d m a l t o s e m a c r o -

Illa

CH OH 2

Illb


1.

GLASS


7


m o l e c u l e ) . I n a n i m a l c e l l s , the e n e r g y s o u r c e is a h i g h l y b r a n c h e d a r r a n g e m e n t o f m a l t o s e units, g l y c o g e n . S t r u c t u r a l p o l y s a c c h a r i d e s p r o v i d e the r i g i d i t y a n d e l a s t i c i t y n e e d e d to p r o t e c t c e l l s . G e n e r a l l y , s t r u c t u r a l p o l y s a c c h a r i d e s are c h a r a c t e r i z e d b y e q u a t o r i a l - e q u a t o r i a l b o n d s (i.e., j3 l i n k a g e s ) b e t w e e n g l u c o p y r a n o s y l units a n d , l i k e storage polysaccharides, their m a c r o m o l e c u l a r structure v a r i e s w i t h v a r i a t i o n i n l i f e f o r m . U n m o d i f i e d c e l l u l o s e is the p r i m a r y s t r u c t u r a l p o l y s a c c h a r i d e o f p l a n t s . T h e (J l i n k a g e p r o d u c e s a n e a r l y l i n e a r , e x t e n d e d m a c r o m o l e c u l a r c o n f o r m a t i o n (structure I V ) , w h i c h p e r m i t s c l o s e p a c k i n g o f p o l y m e r c h a i n s ; this c l o s e p a c k i n g i n t u r n encourages intermolecular h y d r o g e n b o n d i n g (and crystallinity) bet w e e n a d j a c e n t c h a i n s w i t h m i n i m u m e n t r o p y loss t o p r o d u c e the r i g i d i t y r e q u i r e d i n a s t r u c t u r a l m a t e r i a l . T h e a l i n k a g e o f storage p o l y s a c c h a r i d e s i m p a r t s a h e l i c a l c o n f o r m a t i o n to the g l u c o p y r a n o s y l c h a i n ( s t r u c t u r e V ) . T h e h e l i x i n h i b i t s e x t e n s i v e i n t e r c h a i n associations; t h e r e f o r e , t h e h e l i x is u n f i t as a s t r u c t u r a l e n t i t y b u t e x c e l l e n t as a n e n e r g y s o u r c e b e c a u s e r a p i d d e g r a d a t i o n f o r e n e r g y release is n o t h a m p e r e d b y the necessity o f first b r e a k i n g s t r o n g i n t e r m o l e c u l a r a t t r a c t i o n s .

V


8



I n the l o w e r a n i m a l f o r m s (insects, s p i d e r s , a n d c r a b s ) , the C - 2 h y d r o x y l g r o u p i n c e l l u l o s e is r e p l a c e d w i t h a n N - a c e t y l a m i n o g r o u p ( s t r u c t u r e VI) to p r o d u c e c h i t i n ( 3 - 5 ) . I n m a n y l i f e f o r m s , this p o l y m e r f u n c t i o n s as the a d h e s i v e i n a p r e d o m i n a n t l y c a l c i u m c a r b o n a t e m a t r i x . I n h i g h e r l i f e f o r m s , a m i n o a c i d s are a t t a c h e d to the C - 3 h y d r o x y l .

VI C e r t a i n m o n o s a c c h a r i d e s o c c u r m o r e f r e q u e n t l y t h a n others i n i n d u s t r i a l l y i m p o r t a n t c a r b o h y d r a t e p o l y m e r s . T h e m o n o m e r i c units are g l u c o s e , m a n n o s e , a n d g a l a c t o s e . I n the g l u c o p y r a n o s y l r i n g , the h y d r o x y l g r o u p s are e q u a t o r i a l l y p o s i t i o n e d i n the f a v o r e d c o n f o r m a t i o n . M a n n o p y r a n o s e is the C - 2 e p i m e r o f g l u c o s e (i.e., the h y d r o x y l is a x i a l n o t e q u a t o r i a l ) ; g a l a c t o p y r a n o s e is a C - 4 e p i m e r o f g l u c o p y r a n o s e . Intricate structural variations w i l l affect solubility a n d occasion solut i o n p r o p e r t i e s n o t o b s e r v e d i n the structures d i s c u s s e d p r e v i o u s l y ; these c h a r a c t e r i s t i c s are f o u n d i n p o l y s a c c h a r i d e s p r o d u c e d b y m i c r o o r g a n i s m s t h r o u g h f e r m e n t a t i o n synthesis. T h e s e e x t r a c e l l u l a r p o l y m e r s are s e c r e t e d b y m i c r o o r g a n i s m s e i t h e r as (1) a c a p s u l e l a y e r o f p o l y s a c c h a r i d e that c l i n g s to the o u t s i d e o f the c e l l w a l l o r as (2) a s l i m e c o m p o s e d o f p o l y s a c c h a r i d e that a c c u m u l a t e s n e a r the c e l l b u t e v e n t u a l l y diffuses i n t o the a q u e o u s m e d i u m . T h e s l i m e has greater c o m m e r c i a l p o t e n t i a l b e c a u s e o f the ease o f r e c o v e r y . F e r m e n t a t i o n p o l y s a c c h a r i d e s c a n b e d i v i d e d i n t o t w o classes: (1) the s i m p l e p o l y s a c c h a r i d e h o m o p o l y m e r s p r o d u c e d b y the a c t i o n o f a single e n z y m e i n the p r e s e n c e o f the m i c r o o r g a n i s m to p r o d u c e a w a t e r - s o l u b l e p o l y m e r ; a n d (2) the s t r u c t u r a l l y c o m p l e x h e t e r o p o l y s a c c h a r i d e s r e q u i r i n g the s e q u e n t i a l actions o f a g r o u p o f e n z y m e s p r o d u c e d b y the m i c r o o r g a n i s m . T h e synthesis o f c a r b o h y d r a t e p o l y m e r s b y f e r m e n t a t i o n p r o c e s s e s represents a m o r e c o m p l e x c o m m e r c i a l p r o c e s s t h a n those u s e d i n o b t a i n i n g w a t e r - s o l u b l e p o l y m e r s b y a s l u r r y p r o c e s s (e.g., i n the d e r i v a t i z a t i o n o f c e l l u l o s e o r g u a r a n d i s c u s s e d b e l o w ) b e c a u s e o f the g e n e r a t i o n o f h i g h l y v i s c o u s s o l u t i o n s . T h e v i s c o s i t y arises b o t h f r o m the c o n v e r s i o n o f m o n o m e r (e.g., g l u c o s e ) to p o l y m e r a n d f r o m a n i n c r e a s e i n the n u m b e r o f m i c r o o r g a n i s m b o d i e s . T h e i n c r e a s e i n v i s c o s i t y , p a r t i c u l a r l y the p s e u d o p l a s t i c i t y o f the s o l u t i o n s , i m p o s e s m i x i n g p r o b l e m s b e y o n d the g e n e r a l c a p a b i l i t y o f s t a n d a r d p r o d u c t i o n e q u i p m e n t to ensure the


1.

GLASS

9


n e c e s s a r y t r a n s p o r t o f o x y g e n a n d o t h e r i n g r e d i e n t s to the m i c r o o r g a n i s m . T h e r e a d e r is r e f e r r e d to o t h e r treatises (6, 7) f o r these a n d o t h e r c o m p l e x i t i e s i n f e r m e n t a t i o n processes. T h e s o l u t i o n p r o p e r t i e s o f f e r m e n t a t i o n p o l y m e r s that f o r m h e l i c a l a g g r e g a t e s i n a q u e o u s s o l u t i o n s are u n i q u e w i t h r e s p e c t to the v i s c o s i t y r e t e n t i o n at h i g h e r t e m p e r a t u r e s a n d to m e c h a n i c a l ( d i s c u s s e d i n C h a p -


ter 11), t h e r m a l - o x i d a t i v e a n d a c i d - c a t a l y z e d d e g r a d a t i o n , a n d f e r m e n t a t i o n p o l y m e r s h a v e t h e r e f o r e f o u n d c o s t - e f f e c t i v e use i n m a n y areas o f application. I n the r e m a i n i n g sections of this c h a p t e r , d e r i v a t i z a t i o n t e c h n i q u e s a n d f a c t o r s a f f e c t i n g t h e p o s i t i o n o f a d d u c t s u b s t i t u t i o n i n the m o d i f i c a t i o n o f c e l l u l o s e a n d the s t r u c t u r a l features that a f f e c t w a t e r s o l u b i l i t y i n u n d e r i v a t i z e d c a r b o h y d r a t e p o l y m e r s are d i s c u s s e d .

Solubilization through Derivatization A s n o t e d p r e v i o u s l y , c e l l u l o s e is i n s o l u b l e i n a q u e o u s s o l u t i o n s b e c a u s e o f i n t r a - a n d i n t e r m o l e c u l a r h y d r o g e n b o n d i n g . D i s r u p t i o n o f the p o l a r b o n d i n g sequences can b e achieved through esterification or etherificat i o n (of the a n h y d r i d e s ) to c e l l u l o s e . N o n u n i f o r m a d d i t i o n o c c u r s u n t i l the b a c k b o n e is h i g h l y s u b s t i t u t e d . T o a c h i e v e w a t e r s o l u b i l i t y , the f u l l y s u b s t i t u t e d c e l l u l o s e ester m u s t b e p a r t i a l l y h y d r o l y z e d . T h e c o m m e r c i a l l y i m p o r t a n t esters are u s e d i n p l a s t i c s a n d o t h e r a p p l i c a t i o n s , n o t as w a t e r soluble polymers. W a t e r - s o l u b l e c e l l u l o s e ethers are c o m m e r c i a l l y i m p o r t a n t . T h e i r p r o d u c t i o n i n v o l v e s the b a s e - c a t a l y z e d a d d i t i o n of o n e o r a c o m b i n a t i o n o f t w o of the f o l l o w i n g f o u r a d d u c t s : m e t h y l c h l o r i d e ( C H 3 C I ) , the s o d i u m salt o f a - c h l o r o a c e t i c a c i d ( a - C l C H C 0 ~ N a ) , e t h y l e n e o x i d e , a n d p r o p y l e n e o x i d e . A s i n the c o m m e r c i a l l y i m p o r t a n t esters, m i x e d ether a d d u c t s are p r o d u c e d to m e e t s p e c i f i c a p p l i c a t i o n r e q u i r e m e n t s . 2

+

2

T w o o f the a d d u c t s , C H C 1 a n d o > C l C H C 0 ~ N a , are a d d e d u n d e r m o l e e q u i v a l e n t c a u s t i c c o n d i t i o n s to p r o d u c e m e t h y l c e l l u l o s e a n d (carb o x y m e t h y l ) c e l l u l o s e ( C M C ) . T h e d e g r e e of s u b s t i t u t i o n ( D S ; see C h a p ter 4 f o r N M R analysis) r e q u i r e d to a c h i e v e w a t e r s o l u b i l i t y is l o w e r w i t h the a n i o n i c g r o u p i n g (1.3 versus 0.6). T h e d e g r e e o f s u b s t i t u t i o n a c h i e v e d i n the c a u s t i c - c a t a l y z e d a d d i t i o n o f e t h y l e n e o r p r o p y l e n e o x i d e is d i f f i c u l t to d e t e r m i n e e x p e r i m e n t a l l y b e c a u s e the g e n e r a t i o n o f n e w r e a c t i o n sites w i t h the a d d i t i o n o f e a c h a d d u c t . T h e m o l a r s u b s t i t u 3

2

2

+

tion ( M S ) of adduct per glucopyranose group can be quantified experim e n t a l l y (8) a n d the a m o u n t o f s u b s t i t u t i o n i n o x i d e d e r i v a t i v e s is r e p o r t e d as a n M S v a l u e . T h e r e l a t i v e r e a c t i v i t i e s (8) of three of these a d d u c t s w i t h the three carbon-containing h y d r o x y l positions ( C - 2 , C - 3 , a n d C-6) available o n e a c h r e p e a t i n g u n i t u n d e r h i g h c a u s t i c c o n d i t i o n s are as f o l l o w s [substitu e n t , r e l a t i v e r e a c t i v i t y {krk$:k%:k , w h e r e k is the r e l a t i v e r e a c t i v i t y o f x

x


10


p o s i t i o n s g e n e r a t e d b y the a d d i t i o n o f e t h y l e n e o x i d e t o a n y o f the p y r a n o s e h y d r o x y l s ) ] : m e t h y l c h l o r i d e , 5:1:2; s o d i u m c h l o r o a c e t a t e , 2.0:1.0:2.5; a n d e t h y l e n e o x i d e , 5:1:8:12. T h e a d d i t i o n of C H C 1 a n d a - C l C H C 0 2 ~ N a requires equivalent amounts of caustic, generating 1 m o l of N a C l per m o l of adduct reacted. T h e a d d i t i o n o f e t h y l e n e o r p r o p y l e n e o x i d e is c a t a l y t i c a n d e x o t h e r m i c . T h i s c h a r a c t e r i s t i c w i t h the e x p l o s i v e p o t e n t i a l o f o x i d e s , a n d the g e n e r a t i o n o f a n e w , m o r e r e a c t i v e a n i o n w i t h e a c h o x i d e a d d e d (the n e w , m o r e reactive anion promotes chaining a n d nonuniformity of substitut i o n ) , has r e s u l t e d i n t h e use o f a s l u r r y p r o c e s s f o r the a d d i t i o n o f o x i d e units to c e l l u l o s e p r o d u c t s . F o r a c h i e v e m e n t o f u n i f o r m s u b s t i t u t i o n i n o x i d e d e r i v a t i z a t i o n s , a m o d e r a t e c o n c e n t r a t i o n o f c a u s t i c is r e q u i r e d . T h e m o d e r a t e c a u s t i c c o n c e n t r a t i o n (e.g., a n a l k a l i - t o - c e l l u l o s e r a t i o o f 0.37 o r 6.8 M ) c o m p l e m e n t e d b y the p r e s e n c e o f a s u i t a b l e w a t e r c o n c e n t r a t i o n ( d i s c u s s e d later) d i s r u p t s the h y d r o g e n b o n d i n g p r e s e n t i n c e l l u l o s e a n d p r o m o t e s the a v a i l a b i l i t y o f a l l h y d r o x y l f u n c t i o n s f o r s u b s t i t u t i o n . T h e s i m i l a r i t y i n r e a c t i v i t y ratios b e t w e e n h i g h m o l e c u l a r w e i g h t ( h y d r o x y e t h y l ) c e l l u l o s e p r e p a r e d v i a the s l u r r y p r o c e s s (9) a n d that observed f r o m l o w molecular weight, water-soluble, regenerated cellulose (JO) s u p p o r t s this h y p o t h e s i s .


3

2

+

V a r i a t i o n i n e t h y l e n e o x i d e r e a c t i v i t y w i t h the three p y r a n o s e h y d r o x y l s w i t h c a u s t i c c o n c e n t r a t i o n w a s also d e m o n s t r a t e d i n the l a t t e r studies. T h e w a t e r s o l u b i l i t y e n s u r e d the a v a i l a b i l i t y o f a l l h y d r o x y l g r o u p s , w h i c h is n o t t r u e i n s o m e c o m m e r c i a l d e r i v a t i z a t i o n processes. T h e r e l a t i v e r e a c t i v i t i e s as a f u n c t i o n o f s o d i u m h y d r o x i d e c o n c e n t r a t i o n o b s e r v e d i n this s t u d y a r e as f o l l o w s [ s o d i u m h y d r o x i d e c o n c e n t r a t i o n ( M ) , r e a c t i v i t y r a t i o (k :k3:k :k )]: 0.75 M , 3.0:1.0:3.0:1.5; 2.50 M , 4.0:1.0:5.5:4.0; a n d 4.50 M , 4.7:1.0:8.5:12.0. A t the m o d e r a t e l y h i g h c a u s t i c c o n c e n t r a t i o n (4.50 M ) , a d e c i d e d p r e f e r e n c e f o r the a d d i t i o n o f e t h y l e n e o x i d e to the p r i m a r y h y d r o x y l p o s i t i o n (i.e., C - 6 a n d the o x y a n i o n s g e n e r a t e d o n o x i d e a d d i t i o n ) is o b s e r v e d ; as the c a u s t i c l e v e l is d e c r e a s e d , l i t t l e s e l e c t i v i t y i n a d d i t i o n o c c u r s . A c o m b i n a t i o n o f t w o m e c h a n i s m s w a s o f f e r e d f o r this v a r i a n c e : (1) a d e c r e a s e i n r e a c t i v i t y o f the C - 6 h y d r o x y l w i t h d e c r e a s i n g b a s e ( 2 0 , 2 2 ) d u e t o a s h e a t h o f w a t e r a r o u n d the p r i m a r y h y d r o x y l a n d (2) a decrease i n reactivity w i t h increasing caustic of the secondary h y d r o x y l s d u e t o a d d u c t f o r m a t i o n b e t w e e n the b a s e a n d the v i c i n a l d i o l s o f the 2a n d 3 - c a r b o n p o s i t i o n s (20, 2 2 ) . A d d i t i o n a l c o m p l e x i t i e s d u e to a n e i g h b o r i n g g r o u p e f f e c t s h a v e also b e e n i n d i c a t e d (23). A t h i r d m e c h a n i s m h a s b e e n r e c e n t l y (14) s u g g e s t e d f o r this v a r i a n c e . A d e c r e a s i n g c o n t r i b u t i o n o f i n t r a m o l e c u l a r h y d r o g e n b o n d i n g o f the C - 3 h y d r o x y l to the C - 2 o x y a n i o n is p r o p o s e d w i t h i n c r e a s i n g c a u s t i c . T h i s results i n h i g h a c i d i t y o f the C - 2 h y d r o x y l at l o w c a u s t i c a n d a l o w a c i d i t y at h i g h c a u s t i c . 2

6

x



1.

GLASS


11

T h e c a u s t i c s e n s i t i v i t y o f e t h y l e n e o x i d e p l a c e m e n t n o t e d i n the a b o v e g l u c o p y r a n o s e o l i g o m e r s t u d y has b e e n u s e d i n c o n t r o l l i n g the placement of ethylene oxide i n derivatizing high molecular weight cellulose v i a a s l u r r y p r o c e s s (9) (the d i s p e r s a n t is g e n e r a l l y a n a l c o h o l o r k e t o n e ) . T h e s l u r r y p r o c e s s is u s e d to m a i n t a i n v i s c o s i t y c o n t r o l . I n the c a t a l y z e d e t h o x y l a t i o n o f c e l l u l o s e , 6.8 M N a O H is e m p l o y e d . A w a t e r c o n c e n t r a t i o n b e t w e e n 9 a n d 13 w t % c o n t a i n i n g 6.8 M c a u s t i c is e f f e c t i v e i n disrupting crystallinity and intra- and intermolecular hydrogen bonding t o f a c i l i t a t e t h e a v a i l a b i l i t y o f a l l h y d r o x y l g r o u p s . I f this c o m b i n a t i o n is u s e d i n t h e s l u r r y a d d i t i o n o f e t h y l e n e o x i d e to c e l l u l o s e a n d the a d d u c t is a d d e d to a n M S o f 1.0, the c a u s t i c c o n c e n t r a t i o n c a n t h e n b e d e c r e a s e d to 0.1 M a n d t h e e t h o x y l a t i o n c o n t i n u e d to a c h i e v e a w a t e r - s o l u b l e ( h y d r o x y ethyl)cellulose. D i r e c t ethoxylation of cellulose i n a slurry process using a 0.1 M c a u s t i c c o n c e n t r a t i o n t h r o u g h o u t results i n n o n u n i f o r m a d d i t i o n a n d a water-insoluble (hydroxyethyl)cellulose even if h i g h l y substituted. Part i a l n e u t r a l i z a t i o n at a n i n t e r m e d i a t e M S o f a s t a n d a r d 6.8 M s l u r r y f a c i l i tates the a v a i l a b i l i t y o f a l l g l u c o p y r a n o s e h y d r o x y l s i f the i n t e r m e d i a t e m o l a r s u b s t i t u t i o n p r o d u c t is n o t d r i e d . C o n t i n u e d e t h o x y l a t i o n u n d e r l o w e r c a u s t i c c o n c e n t r a t i o n s f a c i l i t a t e s e q u a l s u b s t i t u t i o n a m o n g the t h r e e a v a i l a b l e p o s i t i o n s . T h i s a p p r o a c h p e r m i t s greater s u b s t i t u t i o n at the C - 2 p o s i t i o n , a n d t h e r e b y , greater s t a b i l i t y to e n z y m a t i c d e g r a d a t i o n (9,15,16). T h e i m p o r t a n c e o f the a m o u n t o f w a t e r a d d e d to the s l u r r y is e v i d e n t i n the s u b s t i t u t i o n p a t t e r n s (9) r e f l e c t e d i n the p e r c e n t a g e o f u n s u b s t i t u t e d v i n c i n a l d i o l a n d o f u n s u b s t i t u t e d a n h y d r o g l u c o s e units as a f u n c t i o n o f t h e m o l a r s u b s t i t u t i o n o f e t h y l e n e o x i d e . T h e r e is a d r a m a t i c d i f f e r e n c e i n the u n s u b s t i t u t i o n p a t t e r n s b e t w e e n 4 a n d 7 to 10 w t % w a t e r l e v e l s o r b e t w e e n the 7 to 10 a n d 13 w t % c o n c e n t r a t i o n s ( F i g u r e 1). T h e f o r m e r t r a n s i t i o n reflects the a m o u n t o f w a t e r r e q u i r e d (17) to c o m p l e m e n t c a u s t i c i n a f f e c t i n g the a v a i l a b i l i t y o f a l l o f the g l u c o p y r a n o s y l h y d r o x y l s ; t h e l a t t e r t r a n s i t i o n is b e l i e v e d to b e r e l a t e d t o a c h a n g e i n t h e r e a c t i v i t y ratios b y a l o w e r c a u s t i c c o n c e n t r a t i o n i n the c e l l u l o s e - w a t e r m a t r i x o f the d i s p e r s e d s l u r r y at the h i g h e r w a t e r l e v e l . T h e i m p o r t a n c e o f w a t e r i n c h a n g i n g the d i s t r i b u t i o n p a t t e r n o f o x y e t h y l e n e p l a c e m e n t suggests that the r e l a t i v e p l a c e m e n t o f CH3CI a n d a - C l C H 2 C 0 2 ~ N a substituents w o u l d f o l l o w s i m i l a r s e l e c t i v i t y f o r t h e C - 6 h y d r o x y l s i f these a d d u c t s w e r e a d d e d at a c a u s t i c l e v e l . T h i s s e l e c t i v i t y has b e e n o b s e r v e d (14). +

W h e n a n a n i o n is a d d e d to p r o p y l e n e o x i d e , o n l y 2 - 4 2 o f the o x y a n i o n g e n e r a t e d is p r i m a r y ; the p r e d o m i n a n t species is s e c o n d a r y ( S c h e m e I I ) . D u e to steric a n d i n d u c t i v e e f f e c t s , the s e c o n d a r y o x y a n i o n s h o u l d b e less r e a c t i v e ; m o r e u n i f o r m s u b s t i t u t i o n i n c e l l u l o s e d e r i v a t i z a t i o n results. T h i s u n i f o r m s u b s t i t u t i o n is r e f l e c t e d i n the r e a c t i v i t y p r o f i l e ( F i g u r e 2). T h e results a r e i n p a r t r e f l e c t i v e o f the greater s e l e c t i v i t y i n


12


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Figure 1. Reaction profile (percent unsubstituted diol, closed symbols; percent unsubstituted glucopyranose units, open symbols; as a function of molar substitution) for (hydroxyethyl)cellulose in slurries containing (A) 4, (B)7,(C) 10, and (D)13wt% water. The alkali-to-cellulose ratio was 0.37.


1.

GLASS

13

Water Solubility in Carbohydrate Polymers 0" F

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96-98

RO-CH -CH-CH 2

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Scheme II. S#2 reaction intermediate of propylene oxide.

o x y a n i o n r e a c t i v i t y o f p r o p y l e n e o x i d e r e l a t i v e to that o f e t h y l e n e o x i d e . T h i s d i f f e r e n c e is i l l u s t r a t e d i n t h e s e l e c t i v e r e a c t i o n (14) o f p r o p y l e n e o x i d e w i t h the o x y e t h y l e n e e n d g r o u p s o f a n i n t e r m e d i a t e m o l a r s u b s t i t u t i o n ( h y d r o x y e t h y l ) c e l l u l o s e ( M S = 1.0). B o t h a m y l o s e a n d g u a r a n a r e d e r i v a t i z e d to l o w m o l a r s u b s t i t u t i o n l e v e l s . H i g h d e g r e e s o f s u b s t i t u t i o n e x c e p t i n l i m i t e d a p p l i c a t i o n areas are n o t n e c e s s a r y ; t h e p r e c u r s o r s are w a t e r s o l u b l e , a n d l o w levels o f s u b s t i t u t i o n w i l l i m p r o v e their t e m p e r a t u r e s o l u b i l i t y o r f a c i l i t a t e a d d i t i o n a l p u r i f i c a t i o n o f the p r e c u r s o r s . T w o p r i m a r y p o l y m e r classes that

J

L

MS Figure 2. Reaction profile (percent unsubstituted diol, closed symbols; percent unsubstituted glucopyranose units, open symbols; as a function of molar substitution) for (hydroxypropyl)cellulose in slurries containing 13 wt % water. The alkali to cellulose ratio was 0.37.


14

WATER-SOLUBLE POLYMERS


a r e d e r i v a t i z e d f o r s u c h p u r p o s e s a r e s t a r c h a n d the g a l a c t o m a n n a n s . D i r e c t derivatization of such p o l y m e r s i n their h i g h m o l e c u l a r weight f o r m w o u l d result i n s i g n i f i c a n t v i s c o s i t y d u e to t h e i r s o l u b i l i t y . I n g u a r g u m ( g u a r a n , d i s c u s s e d i n a l a t e r section) d e r i v a t i z a t i o n s , b o r a t e ions (at a 1 0 0 - p p m c o n c e n t r a t i o n ) c o m p l e x w i t h the 2,3-cis d i o l s (i.e., a x i a l - e q u a t o r i a l s t e r e o c h e m i s t r y ) o n b o t h the m a n n o - a n d g a l a c t o p y r a n o s e units p r e s e n t i n g u a r a n to m i n i m i z e s o l u b i l i t y . T h i s r e a c t i v i t y o f the v i c i n a l d i o l s t r u c t u r e , p a r t i c u l a r l y w i t h b o r a t e s a n d titanates, represents a s i t u a t i o n w h e r e a m o l e c u l e ' s s t e r e o c h e m i s t r y is c l o s e l y r e l a t e d w i t h its u t i l i z a t i o n . T h i s r e l a t i o n is u s e d i n a p p l i c a t i o n s w h e r e v i s c o u s gels u n d e r h i g h t e m p e r a t u r e a n d p r e s s u r e c o n d i t i o n s are r e q u i r e d , f o r e x a m p l e , i n f r a c t u r i n g o f p e t r o l e u m a n d gas w e l l s f o r i n c r e a s e d p r o d u c t i v i t y . T h i s a p p l i c a t i o n is d i s c u s s e d i n C h a p t e r 13.

Structural Features That Affect Water Solubility in Underivatized Carbohydrate Polymers F o u r s t r u c t u r a l features polymers.

i m p a r t a q u e o u s s o l u b i l i t y to

carbohydrate

T h e first is b r a n c h i n g . B r a n c h e s o n the m a c r o m o l e c u l a r c h a i n w i l l d i s r u p t i n t e r m o l e c u l a r associations a n d t h e r e b y p r o m o t e s o l v a t i o n . T h e s e c o n d is i o n i z i n g g r o u p s . I o n i z i n g g r o u p s s u c h as c a r b o x y l a t e , s u l f o n a t e , o r sulfate a n i o n s are r e a d i l y s o l v a t e d b y w a t e r , a n d s u c h g r o u p s i n h i b i t i n t e r m o l e c u l a r associations t h r o u g h e l e c t r o s t a t i c repulsions. T h e t h i r d is i n t e r u n i t p o s i t i o n a l b o n d i n g . T h e m o s t a b u n d a n t p o l y s a c c h a r i d e s a r e f o r m e d b y c o n d e n s a t i o n t h r o u g h the 1- a n d 4 - c a r b o n units ( d e s i g n a t e d 1 - 4 ) . T h i s b o n d i n g p r o v i d e s a h i g h l y s y m m e t r i c a l s t r u c t u r e a n d f a c i l i t a t e s i n t e r m o l e c u l a r a s s o c i a t i o n b e t w e e n units o f d i f f e r e n t c h a i n s (i.e., i f the c o n n e c t i n g b o n d is a / J l i n k a g e ) . B o n d i n g t h r o u g h the 1- a n d 3 - c a r b o n u n i t s (1—3) i m p a r t s less s y m m e t r y a n d p r o d u c e s s l i g h t l y b e t t e r s o l u b i l i z a t i o n (e.g., s o l u b i l i t y i n a q u e o u s - c a u s t i c m e d i a ) . T o o f e w e x a m p l e s o f 1—2 b o n d i n g exist to m a k e a g e n e r a l i z a t i o n c o n c e r n i n g s o l u b i l i t y c h a r a c t e r i s t i c s . L i n k a g e t h r o u g h the 1- a n d 6c a r b o n positions dramatically improves aqueous solubility. T h e 5-6 carb o n - c a r b o n b o n d is e x t e r n a l to the p y r a n o s y l r i n g a n d p r o v i d e s a n e n t r o p i c c o n t r i b u t i o n to s o l u b i l i z a t i o n t h r o u g h i n c r e a s e d r o t a t i o n a l f r e e d o m i n the s o l u b i l i z e d state. T h e l i n k a g e e x t e r n a l to the p y r a n o s y l r i n g also p r o v i d e s a greater d i s t a n c e b e t w e e n r i n g s . T h e f o u r t h is n o n u n i f o r m i t y i n the r e p e a t i n g s t r u c t u r e . N o n u n i f o r m i t y c a n b e o b t a i n e d b y (1) a l t e r n a t i o n o f the t y p e o f m o n o s a c c h a r i d e units l i n k e d (i.e., to p r o d u c e a h e t e r o p o l y s a c c h a r i d e ) , (2) v a r i a t i o n o f the l i n k a g e p o s i t i o n (e.g., 1—4 a l t e r n a t i n g w i t h 1 — 3 , 1 — 2 , a n d 1—6), a n d (3) v a r i a t i o n i n the a n o m e r l i n k a g e (i.e., a l t e r n a t i n g a w i t h 0 ) .


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T h e first t h r e e s o l u b i l i z a t i o n c o n c e p t s are s u p p o r t e d w i t h s p e c i f i c examples. Branching. « ( 1 — 4 ) - G L U C O P Y R A N O S E . Storage polysaccharides o f f e r t h e b e s t e x a m p l e o f the e f f e c t i v e n e s s o f b r a n c h i n g i n p r o v i d i n g solubilization. T h e a l i n k a g e of glucose units does not confer true solub i l i t y to a m y l o s e (the m i n o r c o m p o n e n t o f m o s t starches) b u t d o e s create an i m p r o v e m e n t o f s e v e r a l o r d e r s o f m a g n i t u d e o v e r 0 - l i n k e d units. A m y l o p e c t i n ( s t r u c t u r e V I I ) , g l y c o g e n (structure V I I I ) , a n d a m y l o s e (structure V ) a l l c o n t a i n a b a c k b o n e c o m p o s e d of a ( l — 4 ) linkages; a m y l o p e c t i n a n d g l y c o g e n d i f f e r o n l y b y the f r e q u e n c y o f b r a n c h e d c h a i n s f r o m t h e 6 - c a r b o n h y d r o x y l , a n d t r u e s o l u b i l i t y is a c h i e v e d i n these p o l y m e r s (18). W h e n a m a c r o m o l e c u l a r c h a i n is h i g h l y b r a n c h e d as i n a m y l o p e c t i n o r g l y c o g e n , the t h i c k e n i n g p o w e r o f the p o l y m e r is c o m p r o m i s e d . T h e e f f e c t i v e s w e e p v o l u m e f o r a g i v e n m o l e c u l a r w e i g h t is l o w e r t h a n that of the linear m a c r o m o l e c u l e . T h e p r o b l e m of inadequate thickening b y

VIII


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s t a r c h is i n p a r t e c o n o m i c . S t a r c h is a b u n d a n t a n d c a n b e i s o l a t e d w i t h a m o l e c u l a r w e i g h t o f 1 m i l l i o n , b u t c o m m e r c i a l starches h a v e l o w m o l e c ular weights, since low-cost rectification procedures result i n m o l e c u l a r w e i g h t d e g r a d a t i o n . M a n y c o m m e r c i a l starches h a v e v e r y l o w m o l e c u l a r w e i g h t s b e c a u s e o f i n t e n t i o n a l d e g r a d a t i o n (e.g., o x i d a t i o n ) to a c h i e v e effects other than thickening. 0(1—4)-MANNOPYRANOSE. I f the b r a n c h i n g consists o f short segments, spaced r a n d o m l y , solubility can be achieved without c o m p r o m i s i n g t h e m a c r o m o l e e u l e ' s a b i l i t y to t h i c k e n . L o c u s t b e a n a n d g u a r g u m s a r e e x a m p l e s o f this t y p e o f c a r b o h y d r a t e p o l y m e r ; the b a c k b o n e s t r u c t u r e is a r e p e a t i n g m a n n o p y r a n o s y l u n i t . T h e u n b r a n c h e d 2 - c a r b o n e p i m e r o f c e l l u l o s e , i v o r y n u t m a n n a n , u n l i k e c e l l u l o s e is n o t e a s i l y s w o l len b y caustic a n d water. T h e placement of one-unit galactopyranosyl b r a n c h e s f r o m the 6 - c a r b o n o f the r e p e a t i n g m a n n o p y r a n o s y l units (in l o c u s t b e a n a n d g u a r g u m s ) causes s o l u b i l i z a t i o n w i t h o u t c o m p r o m i s i n g the r h e o l o g i c a l p r o p e r t i e s o f the m a c r o m o l e e u l e . L o c u s t b e a n g u m is o b t a i n e d f r o m c a r o b trees a n d c o n t a i n s a n a v e r age o f six m a n n o p y r a n o s y l u n i t s f o r e v e r y g a l a c t o p y r a n o s y l b r a n c h . T h e p o l y m e r achieved c o m m e r c i a l acceptance i n a w i d e variety of a p p l i c a t i o n s , b u t the l i m i t e d g r o w t h p o t e n t i a l o f c a r o b trees r e s t r i c t e d its a v a i l a b i l i t y . W h e n i t w a s r e a l i z e d that the p o l y s a c c h a r i d e i n g u a r g u m seeds w a s s t r u c t u r a l l y s i m i l a r to that i n l o c u s t b e a n g u m , a n d that the p o t e n t i a l a v a i l a b i l i t y o f g u a r a n w a s s i g n i f i c a n t l y greater, the u t i l i z a t i o n o f l o c u s t b e a n g u m d i m i n i s h e d . T h e m a n n o s e - t o - g a l a c t o s e r a t i o (2:1) i n g u a r a n is l o w e r t h a n that i n l o c u s t b e a n g u m (6:1). O r i g i n a l l y , the g a l a c t o s e b r a n c h e s i n g u a r a n w e r e t h o u g h t to b e a l t e r n a t i n g (as i l l u s t r a t e d i n s t r u c t u r e I X ) . R e s e a r c h o v e r the past d e c a d e (19-22)—

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c o m b i n i n g p e r i o d a t e o x i d a t i o n s w i t h statistical analysis a n d d e t a i l e d c h r o m a t o g r a p h i c s e p a r a t i o n o f e n z y m a t i c a l l y d e g r a d e d p o l y m e r residues (23)—has s h o w n that the g a l a c t o s e b r a n c h e s are r a n d o m l y p l a c e d ; s e q u e n c e r u n s w i t h n o b r a n c h a t t a c h m e n t s a n d sections w h e r e the b r a n c h e s o c c u r i n c o n t i g u o u s s e q u e n c e s exist. H o w e v e r the g a l a c t o p y r a n o s y l r e s i d u e s are s p a c e d , the units are e f f e c t i v e i n d i s r u p t i n g i n t e r c h a i n associations a n d t h e r e b y p r o m o t i n g s o l u b i l i z a t i o n w i t h o u t c o m p r o m i s i n g the i n h e r e n t o v e r a l l t h i c k e n i n g p o t e n t i a l o f the m a i n c h a i n .


0(1—3)-GLUCOPYRANOSE. S h o r t - c h a i n b r a n c h i n g i n this f a m i l y o f c a r b o h y d r a t e p o l y m e r s affects s o l u b i l i t y w i t h d r a m a t i c v i s c o s i f y i n g p r o p e r t i e s . F o r o r g a n i z a t i o n a l p u r p o s e s , this class w i l l b e d i s c u s s e d under Interunit Positional B o n d i n g . I o n i z i n g G r o u p s . C a r b o x y l a t e , s u l f o n a t e , a n d sulfate g r o u p s are the m o s t f r e q u e n t l y f o u n d i o n i c substituents i n p o l y s a c c h a r i d e s , w i t h the f o r m e r the m o r e a b u n d a n t . C a r b o x y l a t e g r o u p s ( - C 0 H ) are w e a k a c i d s ; t h e i r salt f o r m s are r e a d i l y s o l v a t e d b y w a t e r a n d the charges i n h i b i t i n t e r m o l e c u l a r a s s o c i a tions b e t w e e n c h a i n s . O n e o f the m o s t w i d e l y u s e d c o m m e r c i a l p o l y m e r s , c a r b o x y m e t h y l c e l l u l o s e ( C M C ) , relies o n s u c h s o l u b i l i z i n g g r o u p s . T h e t e c h n o l o g y a s s o c i a t e d w i t h the d e r i v a t i z a t i o n o f c e l l u l o s e to f o r m C M C is d i s c u s s e d u n d e r S o l u b i l i z a t i o n t h r o u g h D e r i v a t i z a t i o n . T w o n a t u r a l l y o c c u r r i n g s t r u c t u r a l a n a l o g u e s , p e c t i n i c a n d a l g i n i c a c i d s , are i l l u s t r a t e d i n structures X a n d X I , r e s p e c t i v e l y . 2

Pectinic a n d alginic acids, like C M C , have a backbone comprised of 1—4 p o s i t i o n a l l i n k a g e s b e t w e e n m o n o s a c c h a r i d e units (24, 2 5 ) . U n l i k e C M C , the c a r b o x y l a t e f u n c t i o n is the C - 6 c a r b o n a n d the a c i d f u n c t i o n is n o t l i n k e d to the 6-, 3-, o r 2 - c a r b o n s as i n C M C . W h e n the a c i d f u n c t i o n -

X


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XI


a l i t y is a t t a c h e d t o t h e 5 - c a r b o n , t h e s t r u c t u r e is d e s i g n a t e d as a u r o n i c acid. P e c t i n i c a c i d is i s o l a t e d f r o m c i t r u s f r u i t s . T w o t y p e s o f p r o d u c t s are a v a i l a b l e c o m m e r c i a l l y . A s a t h i c k e n e r a n d g e l l i n g agent, p e c t i n i c a c i d is c o m m e r c i a l l y a v a i l a b l e i n l o w - a n d h i g h - m e t h o x y l g r a d e s . I n the h i g h - m e t h o x y l p r o d u c t , at least 50% o f the C - 5 a p p e n d a g e exists as m e t h y l esters u n i t s . T h e h i g h - m e t h o x y l g r a d e s a r e g e l l e d t h r o u g h h y d r o p h o b i c b o n d i n g . T h i c k e n i n g is o b t a i n e d i n a n a c i d m e d i u m , w h i c h l o w ers the free c a r b o x y l i o n c o n t e n t ; the l o w e r free c a r b o x y l i o n c o n t e n t p e r m i t s g r e a t e r i n t e r c h a i n a s s o c i a t i o n , w h i c h is p o t e n t i a t e d b y d e h y d r a t i n g agents (sucrose, g l y c e r i n e , e t c ) . T h e d i l u t e a q u e o u s s o l u t i o n p r o p e r ties o f p e c t i n s a r e d i s c u s s e d i n C h a p t e r 3. A s e c o n d e x a m p l e o f a n a t u r a l l y o c c u r r i n g c a r b o x y l a t e d p o l y m e r is a l g i n i c a c i d , i s o l a t e d f r o m g i a n t k e l p , f o u n d i n the P a c i f i c O c e a n o f f southern California a n d Australia. A l g i n i c acid can be p r o d u c e d b y a n u m b e r o f d i f f e r e n t a l g a e ; the b e s t k n o w n is Macrocystis pyrifera. L i k e s t a r c h , a l g i n i c a c i d c a n b e i s o l a t e d i n h i g h m o l e c u l a r w e i g h t , i f c a r e is t a k e n i n its r e c t i f i c a t i o n . A l g i n i c a c i d c o n t a i n s u n i q u e b l o c k a r r a n g e m e n t s of 0 - D - m a n n o p y r a n o s y l u r o n i c a c i d a n d a - L - g u l o p y r a n o s y l u r o n i c a c i d m o n o m e r units ( s t r u c t u r e X I ) . T h e s t r u c t u r e o f g u l o s e is g i v e n i n C h a r t I. T h i s unusual c o p o l y m e r arrangement was p r o d u c e d b y nature m i l l e n n i u m s b e f o r e the uniqueness of b l o c k c o p o l y m e r s was r e c o g n i z e d b y synthetic p o l y m e r chemists. A l g i n i c a c i d , l i k e p e c t i n i c a c i d , c a n b e u s e d as a g e l l i n g agent t h r o u g h e i t h e r the p r e s e n c e o f d i v a l e n t c a t i o n s o r the use o f a n a c i d i c m e d i u m . I n g e n e r a l , a m a x i m u m v i s c o s i t y is r e a l i z e d at a p H o f 3.0-3.5. A l g i n i c a c i d p o l y m e r s d i f f e r i n structure w i t h different types of algae, w h i c h cause v a r i a t i o n s i n the r a t i o o f m a n n u r o n i c to g u l u r o n i c a c i d units. B o t h the mannose a n d gulose m o n o m e r units of alginic a c i d have the 2 - c a r b o n h y d r o x y l g r o u p i n a x i a l p l a c e m e n t ; i n a H a w o r t h c o n v e n t i o n o f the p y r a n o s y l r i n g , the 2 , 3 - v i c i n a l h y d r o x y l g r o u p s a r e i n a cis p o s i t i o n (i.e., b o t h a b o v e the p l a n e o f the r i n g ) . T h i s p o s i t i o n is i n c o n trast t o t h e trans a r r a n g e m e n t o f the v i c i n a l h y d r o x y l g r o u p s i n the glucose- a n d galactose-based polysaccharides discussed previously, w h e r e the 2 - c a r b o n h y d r o x y l is e q u a t o r i a l l y p o s i t i o n e d . T h e cis v i c i n a l h y d r o x y l a r r a n g e m e n t also is o b s e r v e d i n g u a r a n p o l y m e r s ; the cis


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a r r a n g e m e n t is v e r y r e a c t i v e to m u l t i v a l e n t ions (e.g., c a l c i u m , b o r a t e s , a n d titanates). C a r b o x y l a t e a n i o n s are the salts o f w e a k a c i d s a n d are t h e r e f o r e s u s c e p t i b l e to s i g n i f i c a n t c h a r g e s h i e l d i n g i n saline solutions. W i t h o u t h i g h c h a r g e densities b e t w e e n r e p e a t i n g units that m a i n t a i n the e x t e n d e d p o l y i o n r o d c o n f o r m a t i o n t h r o u g h c h a r g e r e p u l s i o n s , the p o l y m e r c o l lapses ( i n h i g h l y saline solutions) a n d the v i s c o s i t y o f the s o l u t i o n d e c r e a s e s . I f the a n i o n is the salt o f a s t r o n g a c i d (e.g., a s u l f o n a t e o r s u l f a t e g r o u p ) , saline s o l u t i o n s are less e f f e c t i v e i n d e c r e a s i n g v i s c o s i t i e s , i n c r e a s i n g a d s o r p t i o n , etc. S u c h c a r b o h y d r a t e p o l y m e r s are k n o w n (e.g., the c a r r a g e e n a n s ) a n d a r e d i s c u s s e d u n d e r N o n u n i f o r m i t y i n R e p e a t i n g S t r u c t u r e . A m o d i f i e d c e l l u l o s i c ( c e l l u l o s e s u l f a t e ester) of this t y p e has b e e n r e p o r t e d (26, 27). U n i f o r m s u b s t i t u t i o n c a n b e a c h i e v e d o n c e l l u l o s e b y first n i t r a t i n g a n d t h e n e x c h a n g i n g the substituents w i t h sulfate g r o u p s . T h e r e a c t i o n s e q u e n c e is i l l u s t r a t e d i n S c h e m e I I I . T h e e c o n o m i c s o f p r o d u c t i o n f o r a c o m m e r c i a l synthesis of this t y p e w i l l b e less f a v o r a b l e t h a n those e n c o u n t e r e d w i t h C M C . S o m e s u p e r i o r c h a r a c t e r i s tics f o r c e l l u l o s e s u l f a t e ester solutions h a v e b e e n o b s e r v e d a n d are d i s c u s s e d i n C h a p t e r 11. Interunit Positional Bonding. A t h i r d f a c t o r i m p o r t a n t to a c h i e v i n g w a t e r s o l u b i l i t y is v a r i a t i o n i n the i n t e r u n i t b o n d i n g p o s i t i o n . T h e s y m m e t r i c a l 0(1—4) b o n d i n g f o u n d i n c e l l u l o s e a n d i v o r y n u t m a n n a n p r o m o t e s a n e x t e n d e d , n e a r - p l a n a r m a c r o m o l e c u l a r c o n f o r m a t i o n that f a c i l itates i n t e r - a n d i n t r a m o l e c u l a r h y d r o g e n b o n d i n g , c r y s t a l l i t e f o r m a t i o n , a n d i n s o l u b i l i t y . D e r i v a t i z a t i o n to o b t a i n a q u e o u s s o l u t i o n s o l u b i l i t y o f c e l l u l o s e w a s d i s c u s s e d i n the p r e c e d i n g s e c t i o n . W h e n the i n t e r u n i t b o n d i n g is 0(1—3) as n o t e d i n the f e r m e n t a t i o n p o l y m e r c u r d l a n , s o l u b i l i t y is a p p a r e n t i n h y d r o g e n - b o n d b r e a k i n g a n d a l k a l i n e s o l u t i o n s . A 0(1—3) b o n d i n g a r r a n g e m e n t p r o m o t e s h e l i c a l

+ RONO or HONO Scheme III. Synthesis of cellulose sulfate ester.


20


structures as w a s n o t e d i n the a(1 — 4 ) - g l u c o p y r a n o s y l p o l y m e r ( a m y lose). A n a ( l — 6) b o n d i n g u n i t p e r m i t s greater r o t a t i o n a l f r e e d o m t h a n b o n d i n g t h r o u g h a r i n g h y d r o x y l , a n d the C - 6 p o s i t i o n also increases the distance b e t w e e n g l u c o p y r a n o s y l units; b o t h factors favor water solubility.


T w o polysaccharides made b y single-enzyme action and demons t r a t i n g the i m p o r t a n c e o f p o s i t i o n a l s u b s t i t u t i o n patterns i n p r o m o t i n g a q u e o u s s o l u b i l i t y are the d e x t r a n s (3-5) a n d 1 — 6 - b o n d e d m a l t o t r i o s e p o l y m e r (3-5). D e x t r a n is a p o l y s a c c h a r i d e that e x h i b i t s 1—6 u n i t b o n d i n g ; d e x t r a n is p r o d u c e d f r o m sucrose b y the a c t i o n o f g l u c o s y l t r a n s f e r a s e , a n e n z y m e f r o m the b a c t e r i u m Leuconostoc dextranium, w h i c h transfers the g l u c o s e u n i t t o the g r o w i n g d e x t r a n p o l y m e r . T h e m o s t c o m m o n d e x t r a n consists o f g l u c o s e m o n o m e r s , 95% o f w h i c h are l i n k e d b y a ( l — 6 ) b o n d s (structure X I I ) ; the r e m a i n i n g 5% are g e n e r a l l y l i n k e d b y 1—4 b o n d s , p r o d u c i n g a b r a n c h e d s t r u c t u r e . A p p r o x i m a t e l y 80% o f the s i d e b r a n c h e s are o n l y o n e g l u c o s e u n i t ; the r e m a i n i n g 2Q% are c h a i n s of v a r i a b l e l e n g t h w h e n p r o d u c e d f r o m B-511 e n z y m e ( U . S . D e p a r t m e n t o f A g r i c u l t u r e l a b o r a t o r y c o d e s y s t e m ) . T h e s p e c i f i c e n z y m e systems o f d i f f e r e n t b a c t e r i a p r o d u c e the s t r u c t u r a l v a r i a t i o n s d e s c r i b e d . C o m p l e x i n g a b i l i t i e s o f d e x t r a n a r e d e t e r m i n e d p r i m a r i l y b y the p o s i t i o n o f the b r a n c h e d u n i t s a n d a r e o f i m p o r t a n c e i n b i o m e d i c a l a p p l i c a t i o n s w h e r e d e x t r a n is u s e d as a p l a s m a e x t e n d e r , a d r u g c a r r i e r , a n d as a s o l u b i l i z e d salt c o m p l e x e r . A n o t h e r e x a m p l e o f 1—6 i n t e r u n i t b o n d i n g is i l l u s t r a t e d b y p o l y [ a - ( l — 6 ) - D - m a l t o t r i o s e ] (structure XIII) p r o d u c e d f r o m s t a r c h b y the y e a s t Pullularia pullulans. T h e a ( l — 4 ) l i n k a g e s o f s t a r c h are r e t a i n e d i n the t h r e e - u n i t s e g m e n t s ; the t h r e e - u n i t s e q u e n c e constitutes a b o u t 94% o f t h e m a c r o m o l e c u l e . A p p r o x i m a t e l y 6% o f the segments are f o u r units i n l e n g t h (not i l l u s t r a t e d ) . T h e a m b i e n t t e m p e r a t u r e s o l u b i l i t y n o t r e a l i z e d o n a l o n g - t e r m basis i n amylose a n d r e a l i z e d o n l y w i t h a cost-thickening


1.

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XIII p e n a l t y i n a m y l o p e c t i n is o b t a i n e d t h r o u g h the 1—6 p o s i t i o n b o n d i n g w h i c h o c c u r s after e v e r y three o r f o u r 1—4 b o n d i n g units. As noted under Branching, poly-0(1—3)-glucopyranose is s o l u b l e i n a l k a l i n e s o l u t i o n s , b u t c e l l u l o s e [i.e., p o l y - / ? ( l — 4 ) - g l u c o p y r a n o s e ] is n o t s o l u b l e . S o l u b i l i t y i n n e u t r a l o r a c i d i c s o l u t i o n s i n the p o l y - 0 ( 1 — 3 ) - g l u c o p y r a n o s e f a m i l y c a n b e a c h i e v e d i f b r a n c h i n g is i n t r o d u c e d . S u c h a p o l y s a c c h a r i d e is s y n t h e s i z e d b y the e n z y m e s o f the yeast Sclerotium glucanium ( S G P S ) (28). T h e s t r u c t u r a l f o r m u l a is i l l u s t r a t e d i n structure X I V ; a glucopyranosyl b r a n c h occurs on every third m a i n c h a i n u n i t . T h e associations i n S G P S l e a d i n g to h e l i c a l aggregates are s u f f i c i e n t to p r o m o t e t h r e e - d i m e n s i o n a l structures that are n o t s o l u b l e i n s a l i n e o r l o w - t e m p e r a t u r e s o l u t i o n s (50 ° C ) . I n a d d i t i o n , X C P S is s e n s i t i v e to b o r a t e i o n c r o s s - l i n k i n g , as is a l g i n i c a c i d , d u e to the p r e s e n c e o f cis v i c i n a l d i o l s i n the b r a n c h e d u n i t s . T h e u r o n i c a c i d p o l y m e r s , p e c t i n i c a n d a l g i n i c acids, are unstable i n alkaline solutions. T h e C - 6 c a r b o n y l group activ a t e s t h e p r o t o n a t t a c h e d to the C - 5 c a r b o n , a n d a j 3 - e l i m i n a t i o n o f the C - 4 - b o n d e d r e s i d u e ( S c h e m e V ) . C e l l u l o s e s u l f a t e ester is n o t as s e n s i t i v e to d i v a l e n t i o n s . T h i s ester's r i g i d i t y i n f r e s h w a t e r solutions p r o v i d e s m a n y s o l u t i o n p r o p e r t i e s a s s o c i a t e d w i t h the h e l i c a l f e r m e n t a t i o n p o l y m e r s , b u t the ester g r o u p i n c e l l u l o s e sulfate ester is n o t h y d r o l y t i c a l l y stable.

CH OH

OH

2

/

HO

XV



1.

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Scheme IV. Schematic of egg-crate complex formation of alginic or pectinic acid.

1

OMe

Scheme V. Instability of uronic acid structure. D e s p i t e these l i m i t a t i o n s the s t r u c t u r a l v e r s a t i l i t y a v a i l a b l e i n c a r b o h y d r a t e p o l y m e r s a p p e a r s t o o f f e r the p o s s i b i l i t y o f o b t a i n i n g w a t e r - s o l u b l e p o l y m e r s c a p a b l e o f m e e t i n g the p e r f o r m a n c e d e m a n d s i n m a n y a p p l i c a t i o n s . W i t h this t h o u g h t , the f o u r t h a l t e r n a t i v e to a c h i e v i n g w a t e r s o l u b i l i t y i n c a r b o h y d r a t e p o l y m e r s is c o n s i d e r e d . N o n u n i f o r m i t y i n R e p e a t i n g S t r u c t u r e . V a r i a t i o n i n the r e p e a t i n g u n i t (i.e., h e t e r o p o l y s a c c h a r i d e r a t h e r t h a n h o m o p o l y s a c c h a r i d e ) a n d


24


a l t e r n a t i o n i n the i n t e r u n i t b o n d i n g p a t t e r n ( c o m b i n i n g b o t h a n o m e r i c a n d p o s i t i o n a l changes) s h o u l d i n c o n c e p t p r o v i d e the m e a n s to o b t a i n aqueous solution solubility without 1. l o s i n g v i s c o s i f y i n g p o w e r as n o t e d i n the a ( l - * 6 ) h i g h l y b r a n c h e d a ( l - 4 ) structures;

o r the

2. the salt s e n s i t i v i t y i n h e l i c a l structures n o t e d i n the j8(l—4) p o l y e l e c t r o l y t e s (i.e., X C P S ) ; a n d


3. the m u l t i p l i c i t y o f p r o b l e m s c i t e d i n 1 a n d 2 w i t h the h e l i c a l / J ( l - * 3 ) class o f c a r b o h y d r a t e p o l y m e r s . R e a l i z i n g an o p t i m u m i n aqueous solution properties through m u l t i p l e b o n d i n g p a t t e r n s w i t h v a r i a b l e p y r a n o s y l units a p p e a r s r e a s o n a b l e , b u t p r o v i d i n g s p e c i f i c e x a m p l e s i n s u p p o r t o f the c o n c e p t is a n e l u s i v e task. T h e o r e t i c a l efforts (37-40) that c o n s i d e r c o n f o r m a t i o n a l e n e r g y , p r i m a r i l y e s t i m a t i n g the c o n f i g u r a t i o n a l e n t r o p y p e r r e s i d u e a n d e s t i m a t i n g the d e g r e e o f r o t a t i o n a l f r e e d o m o f t o r s i o n a l angles (structure X V I ) , c o u l d o f f e r i n s i g h t i f the m e t h o d s w e r e q u a n t i t a t i v e tools. U n f o r t u n a t e l y , the state o f the art i n s u c h efforts is u n r e f i n e d . A l i m i t e d e x a m p l e o f the i r r e g u l a r i t y i n s t r u c t u r e t h o u g h t to o v e r c o m e the l i m i t a t i o n s d i s c u s s e d is f o u n d i n n i g e r a n (structure X V I I ) . T h i s c a r b o h y d r a t e p o l y m e r c o n t a i n s v a r i a t i o n i n the i n t e r u n i t p o s i t i o n a l b o n d i n g p a t t e r n , b u t the r e p e a t i n g m o n o m e r a n d a n o m e r i c l i n k a g e are c o n stant. S t u d i e s o f this p o l y m e r h a v e b e e n l i m i t e d p r i m a r i l y to the s o l i d state (41, 42). T h e c a r r a g e e n a n s , e x t r a c t e d f r o m s e a w e e d a n d h a r v e s t e d o n t h e coasts o f M a i n e a n d M a s s a c h u s e t t s , a r e o t h e r e x a m p l e s . A l t h o u g h the r e p e a t i n g u n i t is c o n s t a n t ( g a l a c t o p y r a n o s e ) , the a n o m e r i c l i n k a g e

XVI

XVII


1.

GLASS


25

v a r i e s w i t h the i n t e r u n i t b o n d i n g p a t t e r n s ; the a n o m e r i c a n d p o s i t i o n a l p a t t e r n s a l t e r n a t e as 0(1—3) a n d a ( l — 4 ) l i n k a g e s . T h i s class, h o w e v e r , is also n o t e n t i r e l y s u i t a b l e i n that the m a i n c h a i n ( s t r u c t u r e K - c a r r a g e e n a n , R = H ; i - c a r r a g e e n a n , R = S03~)

XVIII:

c o n t a i n s sulfate ester

g r o u p s . F r o m the g e n e r a l s o l u b i l i t y c o n c e p t s d i s c u s s e d , i t m i g h t b e e x p e c t e d that the carrageenans w o u l d b e r e a d i l y soluble i n aqueous solutions a n d s a l i n e i n s e n s i t i v e . T h i s class o f p o l y m e r , h o w e v e r , r e a d i l y gels w i t h salts o f m o n o v a l e n t i o s (e.g., K

+

or N H

4

+

) . G e l a t i o n is r e l a t e d t o

helix f o r m a t i o n a n d subsequent desolvation.


A g a r o s e ( s t r u c t u r e X I X ) is w e a k l y a n i o n i c a n d is s t r u c t u r a l l y s i m i l a r to the c a r r a g e e n a n s . T h e p o l y m e r is s o l u b l e i n h o t (100 ° C )

aqueous

s o l u t i o n s b u t gels at a m b i e n t t e m p e r a t u r e s f r o m m o d e r a t e l y

concen-

t r a t e d s o l u t i o n s . A g a r o s e c o n t a i n s residues o f

3,6-anhydro-a-L-galactose

that a r e c o n s i d e r e d i n the c a r r a g e e n a n s to p r o m o t e g e l a t i o n (36). T h u s , agarose is n o t a s u i t a b l e e x a m p l e o f w h a t m i g h t b e c o n c e p t u a l l y p o s s i b l e in aqueous solubility a n d solution properties.

Conclusions T h e p r i m a r y p r o b l e m i n projecting structural relationships for water solu b i l i t y i n c a r b o h y d r a t e p o l y m e r s is e x e m p l i f i e d i n the b e h a v i o r o f p o l y 0 ( 1 — 6 ) - g l u c o p y r a n o s e . T h i s p r o b l e m relates to r e s i d u a l c r y s t a l l i n i t y . T h e w a t e r s o l u b i l i t y o f the a n o m e r i c a ( l — 6) i s o m e r (i.e., d e x t r a n ) is j u s t i f i e d o n its p o s i t i o n a l s u b s t i t u t i o n p a t t e r n . T h a t the d i f f e r e n c e i n a n o m e r i c b o n d i n g w o u l d n o t a f f e c t i n s o l u b i l i t y is e x p e c t e d ; h o w e v e r , this p o l y m e r a p p e a r s to h a v e a n o p t i m u m g e l f o r m a t i o n t e m p e r a t u r e i n the 5 - 2 5 ° C

OR XVIII

XIX


26



r a n g e . S t i p a n o v i c p r o p o s e d (43) that the g e l a t i o n m e c h a n i s m i n v o l v e s c r y s t a l l i n e r e g i o n s that act as c r o s s - l i n k s o r j u n c t i o n z o n e s to e s t a b l i s h a three-dimensionally stabilized n e t w o r k structure. T h e naturally occurr i n g 0 i s o m e r c o n t a i n s a p p r o x i m a t e l y 10% a c e t y l a t i o n a n d d o e s n o t g e l . T h e l o w d e g r e e o f g l u c o p y r a n o s e b r a n c h i n g m i g h t also b e a f a c t o r i n i n h i b i t i n g g e l a t i o n i n the a n o m e r i c (a) p o l y m e r (i.e., d e x t r a n ) . L o n g usage o f c a r b o h y d r a t e p o l y m e r s has n o t to d a t e p r o v i d e d suff i c i e n t f u n d a m e n t a l i n s i g h t s to p r o j e c t w h a t m i g h t b e a c h i e v e d w i t h structural variations i n carbohydrate p o l y m e r s . C o n t r o l l e d synthetic p r o d u c t s to d e l i n e a t e s t r u c t u r e - a q u e o u s s o l u t i o n p r o p e r t i e s w o u l d p r o v i d e direction a n d advance our understanding sufficiently for theoretical p r o j e c t i o n s to b e o f v a l u e . T h e t y p e o f insights p o s s i b l e w i t h c u r r e n t i n s t r u m e n t a l t e c h n i q u e s [ b e a u t i f u l l y d e m o n s t r a t e d i n the 0 ( 1 — 6 ) - g l u c o p y r a n o s e s t u d y (43)] o f f e r s the p o s s i b i l i t y o f a d d i n g s i g n i f i c a n t l y to w h a t has b e e n i n h e r i t e d o v e r the m i l l e n i u m s i n this f a m i l y o f m a c r o m o l e c u l e s . T h e r e a d e r w h o desires g r e a t e r d e t a i l o n e a c h class o f c a r b o h y d r a t e p o l y m e r d i s c u s s e d i n this b r i e f c h a p t e r is r e f e r r e d to the r e c e n t series o n The Polysaccharides, b y A s p i n a l l (44).

Literature Cited 1. Lehninger, A. L. Biochemistry; Worth Publishers: New York, 1975, 2nd ed., p 250. 2. Morrison, R. T.; Boyd, R. N . Organic Chemistry; Allyn and Bacon: Boston, 1983; 4th ed., Chapter 28. 3. Whistler, R. L . Industrial Gums; Whistler, R. L., E d . ; Academic: New York, 1973; 2nd ed., Chapter 1. 4. Bolker, H . I. Natural and Synthetic Polymers; Dekker: New York, 1974. 5. Elias, H - G . Macromolecules; Plenum: New York, 1977; Vol. II, Chapter 31. 6. Purification of Fermentation Products; LeRoth, D.; Shiloach, J.; Leahy, T. J., Eds.; ACS Symposium Series 271: American Chemical Society: Washington, D C , 1984. 7. Saeir, H . H . Mechanisms and Regulation of Carbohydrate Transport in Bacteria; Academic: Orlando, F L ; 1985. 8. Croon, I. Sven. Papperstin. 1960, 63, 247. 9. Glass, J. E.; Buettner, A. M . ; Lowther, R. G.; Young, C. S.; Cosby, L. A.; Carbohydr. Res. 1980, 84, 245. 10. Ramnas, O.; Samuelson, O. Sven Papperstin. 1968, 71, 829. 11. Rowland, S. P.; Roberts, E . J.; Wade, C. P. Text. Res. J. 1969, 39, 530. 12. Roberts, E . J. Wade, C. P.; Rowland, S. P. Carbohydr. Res. 1971, 17, 393. 13. Roberts, E . J.; Wade, C. P.; Rowland, S. P.; Carbohydr. Res. 1972, 21, 357. 14. Seneker, S. D. Ph.D. Thesis, North Dakota State University, 1986. 15. Klug, E . D.; Winquist, D. P.; Lewis, C. A.; In Water Soluble Polymers; Bikalis, N . M , E d . ; Plenum: New York, 1973; p 401. 16. Klop, W.; Kooiman, P. Biochim. Biophys. Acta 1965, 99, 102. 17. Seneker, S. D.; Glass, J. E. Proc. Am. Chem. Soc. Div. Polym. Mater.: Sci. Eng. 1984, 51, 248.



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