New Insights into the Crystal Structure Hydration of Polysaccharides

15

N e w Insights i n t o the C r y s t a l S t r u c t u r e H y d r a t i o n of

Polysaccharides

T. BLUHM, Y. DESLANDES, R. H. MARCHESSAULT, and P. R. SUNDARARAJAN

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch015

Xerox Research Centre of Canada, Mississauga, Ontario L5L 1J9, Canada

With the exception of c e l l u l o s e and c h i t i n , plant polysaccharides are usually hydrated. Hydration often occurs in the c r y s t a l l i n e regions as well as in the amorphous areas. When water of hydration is found in the c r y s t a l l i t e s , i t may or may not affect the conformation of the polysaccharide backbone and in most cases, i t affects the u n i t - c e l l dimensions, while in a few cases, the water appears to have no effect on u n i t - c e l l dimensions. The structures of six hydrated neutral polysaccharides w i l l be examined with regards to the state of water of hydration in the structure. It w i l l be seen that water may occur as columns or as sheets in these structures. The structures that w i l l be discussed are (1->4)-ß-D-xylan, nigeran, amylose, galactomannan, (1->3)-ß-D-glucan and (1->3)-ß-D-xylan. The chemical structures of these polysaccharides are shown in Figure 1. The evidence to be used is based on x-ray d i f f r a c t i o n analysis and not on swelling or moisture regain studies. This allows rather specific conclusions about the nature of the polysaccharidewater interaction but eliminates macromolecu1ar aspects related to the osmotic phenomenon of polymerwater interaction, as in hydrogels. In polysaccharides, both aspects are important, for example: in seed germination, c r y s t a l l i n e hydration is a f i r s t step in the process, while g e l a t i n i z a t i o n of starch involves a hydrogel phenomenon with hydrate c r y s t a l l i t e s as pseudo-crosslinks. ( 1_) In 1 i g n o c e l l u l o s i c s , the extreme a f f i n i t y of dry wood for water is notorious. The Egyptians took advantage of the phenomenon to s p l i t large rocks by wedging one end of a beam in a c l e f t and placing the other end in contact with water. In a thermodynamic sense, this takes advantage of the swelling pressure [2) which is certainly related to the hydration of the 0-8412-0559-0/ 80/47-127-253505.00/ 0 © 1980 American Chemical Society Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Figure 1.

NIGERAN

(1 - 3 ) - β - D ~ X Y L A N

amy-

GALACTOMANNAN

(1 - 3 ) - β - ρ - G L U C A N

Chemical structures of (1 -> 4)-β-ο-χγίαη, (1 -* 3)-β-τ>-χγ1αη, (1 -> 3)^-D-glucan, lopectin, nigeran, and galactomannan

AMYLOPECTIN

(1-4)-6-p-XYLAN


15.

BLUHM ET AL.

255

Hydration of Polysaccharides

hemicelluloses. In f a c t , t h e same a f f i n i t y c o u l d be i n v o l v e d i n the f a m i l i a r problem of water t r a n s p o r t i n t r e e s , whereby water columns o f hundreds o f f e e t are stable.


Xylan

[Poly

(1^4)-g-D-XVLP]

P r o b a b l y t h e s e c o n d most a b u n d a n t p o l y s a c c h a r i d e i n t h e w o r l d , x y l a n (3) i s f o u n d i n a s s o c i a t i o n w i t h c e l l u l o s e i n h a r d w o o d s and s o f t w o o d s as w e l l as i n g r a s s e s . I t i s s e l d o m f o u n d as a h o m o g l y c a n , b u t r a t h e r w i t h a s u b s t i t u t e d backbone 4-0-methyl-D-glucuronic a c i d and a r a b i n o - f u r a n o s e b e i n g t h e common g l y c o s i d i c s u b s t i t u e n t s . O - A c e t y l groups f r e q u e n t l y occur along t h e c h a i n as n a t u r a l e s t e r g r o u p s . O f t e n , x y l a n s are found with only arabinose substituents and these m a t e r i a l s a r e r e f e r r e d t o as a r a b i n o - x y l a n s . They a r e i m p o r t a n t c o m p o n e n t s o f c e r e a l g r a i n s and p l a n t c o r m s f r o m w h i c h t h e y c a n be e x t r a c t e d t o be u s e d as g u m s . Xylans have a l w a y s been c l a s s i f i e d with the h e m i c e l l u l o s e s , the n o n - c e l l u l o s i c p o l y s a c c h a r i d e s of woody m a t e r i a l s . In t h e p a p e r - m a k i n g p r o c e s s , t h e y a r e assigned a h y d r a t i o n r o l e , i . e . , w i t h o u t them, s t r o n g interfiber bonds do not form unless extensive mechanical stock r e f i n i n g is used. A c c o r d i n g l y , they a r e c l a s s i f i e d as h y d r o c o l l o i d s o r gums and a r e t h o u g h t t o a c t as a g l u e e n c o u r a g i n g t h e s t i c k i n g t o g e t h e r o f f i b e r s in paper. Although it was not r e a l i z e d at first, the h y d r a t i n g p r o p e r t i e s of xylan extend to i t s c r y s t a l l i n e state. The u n i t - c e l l i s h e x a g o n a l (4) w i t h : a = b = 9 . 1 6 Â and c ( f i b e r

repeat)

=

14.9%

The b a s e p l a n u n i t - c e l l p r o j e c t i o n ( F i g u r e 2) shows t h e p o s i t i o n s o f t h e w a t e r m o l e c u l e s i n t h e u n i t - c e l l as deduced f r o m an a n a l y s i s o f t h e equatorial x-ray d i f f r a c t e d i n t e n s i t i e s f o r a f i b e r diagram r e c o r d e d at a b o u t 50% r e l a t i v e h u m i d i t y . The w a t e r molecules c l u s t e r i n d i s t i n c t a r e a s and f o r m a h e l i c a l c o l u m n whose s y m m e t r y m a t c h e s t h a t o f t h e x y l a n c h a i n s . In f a c t , t h e w a t e r o f h y d r a t i o n may d i c t a t e t h e s y m m e t r y o f t h e x y l a n c h a i n s . The e n e r g e t i c a l l y ( t h e o r e t i c a l ) m o s t s t a b l e conformation of the x y l a n c h a i n i n v o l v e s twof o l d symmetry, whereas i n the hydrated crystalline e n v i r o n m e n t , as d e d u c e d f r o m x - r a y d i f f r a c t i o n , the x y l a n c h a i n s possess t h r e e - f o l d symmetry. The w a t e r m o l e c u l e s s t a b i l i z e t h e t h r e e - f o l d s t r u c t u r e by t h e f o r m a t i o n of hydrogen bonds. This structure i s an example o f columnar h y d r a t i o n which a l l o w s a symmetric


256


WATER IN P O L Y M E R S

Figure 2.

Top: projection in the ab plane of (1 -» 4)-β-Ό-χγΙαη unit cell. Bottom: projection in the 110 plane of (1 -> 4)-β-Ό-χγΙαη unit cell


15.

B L U H M

ET A L .

Hydration

of

257

Polysaccharides

expansion of the unit-cell as the humidity and corresponding degree of hydration increase. At 58% r e l a t i v e humidity, the u n i t - c e l l contains one water molecule per xylose unit while at 90% r e l a t i v e humidity there are two water molecules per xylose u n i t . At very low r e l a t i v e humidities, the c r y s t a l 1 i n i t y is poor. Table I shows the u n i t - c e l l dimensions of the various polymorphs.


Table I U n i t - C e l l Parameters of (1+4) -g-Q-Xy 1 an Polymorphs "Dry" a = b (A) c {%)

"Hydrate"

8.8 14.85

γ (deg.)

120

"Dihydrate"

9.16

9.64

14.85

14.95

120

120

The influence of the arabinose substituent on hydration is i l l u s t r a t e d by the u n i t - c e l l recorded for the gummy polysaccharide from corm sacs of Watsonia pyramidata where there seems to be two arabino-furanose units per backbone xylose u n i t . In this case, the hexagonal base plane expands to ( : 0

a = b = 14.OA c(fiber

0

repeat) = 14.9A

Since this sample loses c r y s t a l 1 i n i t y on drying, i t is clear that water is involved in maintenance of the organized structure. Nigeran

[Poly ( 1+4) - - D - G L C P - ( 1+3) -a-D-GLCP] a

One of the polysaccharides most e a s i l y extracted from fungal c e l l walls is nigeran. It is soluble in warm water (^60°C) and was shown to be an alternating copolysaccharide composed of (l+3)-a and ( l + 4 ) - linked D-glucose u n i t s . (6,7) The polymer is highly c r y s t a l l i n e in the ceTl wall and was f i r s t isolated (6) in 1914. Its role in c e l l walls or mycelia is not clear at this time and by suitably adjusting the growth medium, one can obtain mycelia with as much as 40% by weight of nigeran.(8) Other polysaccharide constituents of the fungal c e l l wall are t y p i c a l l y : c h i t l n , (l+3)-a-D-glucan, and ( 1 + 3 ) - ρ - D - g 1 u c a n , a l l of which are water^insoluble. a


258


The detection of c r y s t a l l i n e nigeran in fungal c e l l walls can be accomplished r e a d i l y by x-ray d i f f r a c t i o n . Even for low percentages of nigeran (3-6%), the c r y s t a l 1 i n i t y is high and the r e f l e c t i o n s are e a s i l y recognized and accurately measured.(9J By working with p u r i f i e d m a t e r i a l , i t was shown that two d i s t i n c t polymorphs can be i d e n t i f i e d ; the "dry" and "hydrate" forms whose orthorhombic u n i t - c e l l dimensions are given i η Table 11.


Table II U n i t - C e l l Dimension of "Dry" and "Hydrate" Nigeran "Dry"

Hydrate

a (A)

17.76

17.6

b (A)

6.0

c ( f i b e r repeat)

(A)

14.62

7.35 13.4

The major dimensional changes in the u n i t - c e l l are in the b and £ d i m e n s i o n s . Since the l a t t e r is related to the cTiain conformation, its interpretation is important. Recent studies (10,1_1) have shown that there is no change in the 2 heTTx symmetry on going from "hydrate" to "dry" form. The conclusion is that a s l i g h t extension of the hel i χ takes place while the contract ion in the t) dimension indicates a major structural transformation. Since only one dimension of the base plane is changing, one is tempted to describe the water of hydration in nigeran as s h e e t - l i k e , with the sheets running p a r a l l e l to the a^ axis. Figure 3 shows electron micrographs of nigeran single c r y s t a l s . These are lamellae grown from d i l u t e solutions and do not necessarily bear any r e l a t i o n s h i p to the c r y s t a l l i n e morphology found in the fungal c e l l wall. Nevertheless, they have been invaluable in deriving the c r y s t a l l i n e parameters of nigeran and are v i s i b l e proof of the high chemical r e g u l a r i t y of t h i s material. Furthermore, the systems of p a r a l l e l marks which are c l e a r l y v i s i b l e in the "dry" form are macroscopic evidence of the stresses that occur in one d i r e c t i o n of the u n i t - c e l l base plane as dehydration takes place on the grid of the electron microscope. When the water is removed by solvent exchange to methanol, the smoother uncracked surface is seen. χ


B L U H M ET A L .

Hydration

of

Polysaccharides

259


15.

Figure 3.

Top: nigeran single crystals grown from ether. Bottom: nigeran single crystals after solvent exchange with methanol.


260



A simple but descriptive picture of this hydrate structure (12) is as a sandwich with water of hydration in between T â y e r s of polysaccharides (Figure 4). This is c l e a r l y o v e r - s i m p l i f i e d , but in view of the observed morphological changes in the single c r y s t a l lamellae, i t helps to understand the large scale dimensional changes which occur due to the additive effect of the small changes in J3 in each of the u n i t - c e l l s which make up a c r y s t a l . The water of hydration also has an effect on the chain conformation, causing a contraction, even though the symmetry of the chain does not change. Amylose [Poly ( 1+4)-α-13-GLCP] Amylose is the linear homopolysaccharide of (1-* 4 ) - α -D-glucose which when associated with the branched ^homopolysaccharide amylopectin (Figure 1) [(1-> 4 ) - α - D - g l u c a n with (1-* 6 ) - a -D-glucose branch points] forms the commonly occurring polysaccharide, starch. Starch occurs as c r y s t a l l i n e granules in nature which give r i s e to three types of x-ray d i f f r a c t i o n diagrams indicating the existance of three polymorphic structures; A, Β and C. Since x-ray diffractograms obtained from pure amylose exhibit characteristic r e f l e c t i o n s i d e n t i c a l to those obtained from A, Β and C starch granules, it is considered that the c r y s t a l 1 i nity in starch is due to the amylose portion or to the linear branches of amylopectin. Native Α - s t a r c h occurs predominantly in cereal grains, B-starch is found in certain tuberous plants, Cstarch is a rare form found in some plants and may actually be a combination of A- and B-starches. P a r t i a l conversion of B-starch to Α - s t a r c h can be accomplished by adjusting temperature and humidity conditions, but complete conversion has never been achieved. When starch is dissolved in hot water, i t spontaneously undergoes gelation and subsequent c r y s t a l l i z a t i o n in the well-known process of r é t r o g r a d â t ion. (Γ3) The polymorphic form of retrograded starch is B-starch, i r r e s p e c t i v e of the form of starch i n i t i a l l y dissolved. Recent c r y s t a l structure proposals for A- and B-amylose (14,15) consist of parallei-stranded, r i g h t - h a n d e d T o û ¥ l e helices (Figure 5) packed in an a n t i - p a r a l l e i fashion. Each strand is comprised of six α-Q-glucose units in the stable 4 c conformation. The u n i t - c e l l of A-amylose is orthorhombic with: x

a = 11.90A, b = 17.70A and c ( f i b e r

repeat)

= 10.52À



15.

BLUHM ET AL.

Figure 4.

Hydration

of

Polysaccharides

261

Schematic of nigeran unit cell, showing approximate location of water of hydration

Figure 5.

Double helical conformation of amylose


262


The fiber repeat of 10.52A for the double helix indicates that each single chain has a repeat of 21.04A. The u n i t - c e l l of B-amylose is hexagonal with: ο 0 a = b = 18.50A and c(fiber repeat) = 10.40A


0

In this case the single chain repeat is 20.8A, p r a c t i c a l l y identical with the chain in A-amylose. The double h e l i c a l conformation of A- and B-amylose are nearly i d e n t i c a l , however, the two polymorphs d i f f e r s i g n i f i c a n t l y in degree and type of hydration. The water content of c r y s t a l l i n e B-amylose varies from 5% to 27% depending on the relative humidity of the surroundings whi le A-amylose contains a nearly constant 6% water at various r e l a t i v e humidities. In neither Anor B-amylose, do the u n i t - c e l l constants change with degree of hydration, however, the i n t e n s i t i e s of diffracted x-rays change with various degrees of hydration in B-amylose. Upon inspection of the c r y s t a l structures of A- and B-amylose, shown schematically in Figure 6, one sees that the amylose double helices pack in hexagonal arrays in both structures; the major difference in the structures being that in B-amylose the centre of the hexagonal array is occupied by water of hydration, whereas in A-amylose this l a t t i c e s i t e is f i l l e d with another amylose double h e l i x . The water in B-amylose is l a b i l e , moving in and out of the structure with great ease. This water may in fact be transported in tuberous plants. In A-amylose, the water of hydration is more sheet-like and more t i g h t l y bound to the surrounding amylose double h e l i c e s . Hence, the A-amylose structure is not as sensitive to the surrounding humidity as is the B-amylose structure. Conversion of B- to A-amylose on the molecular level occurs with the loss of s i g n i f i c a n t amounts of water followed by a movement of amylose chains into the l a t t i c e s i t e vacated by the columnar water of hydration. Starch polymorphism in plants may be a result of the environment in which synthesis occurs. Synthesis and subsequent c r y s t a l l i z a t i o n may occur as follows: amylose single strands are synthesized f i r s t , the strands then intertwine about each other forming the amylose double h e l i x . C r y s t a l l i z a t i o n then occurs in either the A or Β polymorphic form depending on the amount of water in the environment. This mechanism probably implies low degree of c r y s t a l l i n i t y in the f i n a l m a t e r i a l , which is generally the case.



15.

B L U H M ET A L .

Figure 6.

Hydration

of

Polysaccharides

Schematic of A- and B-amylose unit cell in the ab plane; amylose double helices are shown as circles.


263

264


Galactomannan Ivory nut man nan, poly ( 1+4) -$-D-mannose, is a we 11known reserve polysaccharide which is water insoluble (16) and highly c r y s t a l l i n e . It occurs also in date seeds where i t was shown to disappear upon germination, a c h a r a c t e r i s t i c of seed endosperms. The native u n i t c e l l , mannan I, is orthorhombic ( Γ 7 ) with:


a = 7.21A, b = 8.82A and c ( f i b e r repeat)

= 10.3A

Intermolecular hydrogen bonds hold the adjacent chains t i g h t l y together, indeed, the density of native mannan generally surpasses that of native c e l l u l o s e . Because of this high cohesion, mannan I is water insoluble and unhydrated. Substitution along the polysaccharide backbone increases water s o l u b i l i t y since bulky groups disrupt the f i t and regular hydrogen bonding scheme between adjacent molecules, thereby increasing the a c c e s s i b i l i t y of hydroxyls to water molecules. Galactomannans are a family of seed endosperm polysaccharides (16) with a mannan backbone and appended (1+6)- α -D-galactose substituents which render them water soluble (see Figure 1). A wide variety of galactomannans with different mannose/galactose (M/G) r a t i o s have been studied. Typical of these commercially available gums are: Guar, Locust and Tara galactomannans where the r a t i o s M/G are 1.9, 3.2 and 3.7, r e s p e c t i v e l y . C r y s t a l l i n e , oriented films of these materials can be obtained by evaporation of an aqueous solution of these polysaccharides.(18,19) Table

III

ο Orthorhombic U n i t - C e l l Parameters (in A) from Fiber Diagrams of Galactomannans Sample 0% RH Guar Locust Tara at 58% RH Guar Locust* Tara at 78% RH Guar Locust Tara

a

b

c

13.5 11.6

8.7 8.7 —

10.4 10.4

24.0 24.0 24.2

8.9 8.9 9.0

10.4 10.4 10.4

33.2 30.6 28.3

9.0 9.0 9.1

10.4 10.4 10.4

at


15. BLUHM ET AL.

Hydration

of

Polysaccharides

265


X-ray fiber diagrams recorded at different r e l a t i v e humidities showed three important features (Table I I I ) : ο

New d i f f r a c t i o n spots develop above 20% r e l a t i v e humidity necessitating adoubling of the u n i t - c e l l (a. dimension). It seems that water induces a variation of the u n i t - c e l l by modifying the r e l a t i v e orientation of the chains.

ο

The cJ-spacings of the most intense equatorial r e f l e c t i o n (related to the a_ dimension only) increase as r e l a t i v e humidity increases; the b and c dimensions do not change with r e l a t i v e humicTity; Tn f a c t , those values are the same as found in pure mannan I, where galactose is t o t a l l y absent.

ο

F i n a l l y , the a dimension of these three samples generally increases with the degree of galactose substitution with b_ and £ remaining constant.

These three observations lead to the conclusion that a good c r y s t a l l i n e model consists of a sheet-like arrangement of chains p a r a l l e l to the u n i t - c e l l b> axis as f i r s t proposed by Palmer and Ballantyne.(19) By increasing the galactose content or by increasing the r e l a t i v e humidity, a "repulsive force" between the chains in the à d i r e c t i o n is f e l t and the u n i t - c e l l expands accordingly. The packing forces in the b^ d i r e c t i o n are the same no matter what the level of galactose substitution and t h i s force seems responsible for the sheet-like hydration mechanism. Since c r y s t a l l i n e mannan I is not hydrated, i t seems clear that the role of the (l->6)-a-D-galactose substituent is to encourage hydration and p l a s t i c i t y . In wood c e l l s where the galactog1ucomannan (glucose present in the backbone) is a matrix substance, p i a s t i c i z a t i o n is probably the desired property. In seeds where galactomannan is a constitutent of an endosperm, controlled hydration to f a c i l i t a t e attack by some enzyme is probably the important feature. The f a c i l i t y of hydration can control the s p e c i f i c time of germination of seeds especially in a desert envi ronment. As is expected with this type of hydrated structure, the best x-ray patterns (best ordered sample) are obtained when the r e l a t i v e humidity is in the middle range (40% to 80%). At 98% r e l a t i v e humidity, the amount of water begins to s o l u b i l i z e the chains, thus destroying the c r y s t a l 1 i n i t y . At 0% r e l a t i v e


266


humidity, the lack of water l i m i t s the mobility of the polysaccharide elements and the chains have d i f f i c u l t y finding the regular arrangement of the c r y s t a l l i n e state.


(l+3)-3-P-Glucan and Xylan Another polysaccharide which displays interesting hydration phenomena is (l+3)-3-D-glucan, often c a l l e d paramylon (20), curdlan (2JJ or laminaran (16). The molecular crystalline arrangement oT" this polysaccharide consists of a t r i p l e helix formed by three intertwining 6 he! ices . (22!,23) Two polymorphs (22) are observed when x-ray ïïTagrams of wellc r y s t a l l i z e d fibers are recorded at different r e l a t i v e humidities. If the sample is placed under vacuum, the "dry" polymorph is obtained. The "hydrated" polymorph is found at 75% r e l a t i v e humidity and contains two water molecules per glucose residue. The transformation between the two forms is r e v e r s i b l e , the c r i t i c a l r e l a t i v e humidity being around 20% depending on sample history. The u n i t - c e l l of both polymorphs is hexagonal with the parameters shown in Table IV. The s t r i k i n g variation i n £ c a n be e a s i l y explained in terms of a loss of symmetry i η the c r y s t a l line structure when going from the "dry" form to the "hydrate" form. Since no physical modification of the fibers is observed, i t is u n l i k e l y that the increase in fiber repeat is caused by an actual physical stretching of the chain. The t r i p l e h e l i c a l structure is composed of three equivalent strands related by a three-fold symmetry operation. The repeat of such a structure is 1/3 of the repeat of a single chain (Figure 7). However, i f the strands are not i d e n t i c a l , the three-fold symmetry is lost and the fiber repeat of the whole structure is that of the single strand, i . e . , that found in the "hydrate". χ

Table IV U n i t - C e l l Parameters of the Polymorphs of (l+3)-3-D-Glucan and (l+3)-3-D-Xylan (l+3)-g-D-Glucan 0

a = b (A) c (A) Helix Pitch (A)

Dry

Hydrated

( 1 + 3 ) - β - D - X y 1 an Dry 13.7

Hydrated

14.6

15.6

15.4

5.8

18.6

5.88

6.12

17.34

18.6

17.64

18.36


15.

B L U H M ET A L .

Hydration

of

267

Polysaccharides

A l l of the u n i t - c e l l dimensions increase when converting from the "dry" form to the "hydrate". The a b plane expands equally in both directions and i t is believed that thewater of hydration is disposed between the t r i p l e helices in a columnar fashion (Figure 8). Interhelix hydrogen bonds are probably broken and/or replaced by new bonds involving water molecules. This mechanism could lead to the loss of three-fold symmetry. The three strands are no longer equivalent, possibly due to different hydroxymethy1 group rotameric p o s i t i o n s . (l+3)-3-D-xylan behaves s i m i l a r l y to the glucan even though i t s C(5) carbon lacks the CH 0H group. Nevertheless, the c r y s t a l structures of these two polysaccharides are very s i m i l a r , consisting of t r i p l e h e l i c e s . ( 2 £ ) The xylan u n i t - c e l l is hexagonal with the parameters given in Table IV. The effect of hydration is about the same as in the corresponding glucan s t r u c t u r e . However, in the case of the xylan, the three-fold symmetry is not lost in the hydrated form. The c o r r e l a t i o n of t h i s phenomenon with the absence of the hydroxymethy1 group seems obvious. The presentation of three-fold symmetry is an indication that no water molecules can be accommodated in the middle of the triple helix. It is impossible to introduce three coplanar water molecules and r e t a i n the three-fold symmetry in a cavity of approximately 3A diameter. In conclusion, i t is very l i k e l y that the hydration of both (l+3)-3-0-glucan and ( 1+3)-3-D-xy 1 an is in the form of columns between the t r i p l e helices which bring about an increase in the u n i t - c e l l dimensions of the hydrate.


2

Pi scussion The observations discussed above were d e l i b e r a t e l y restricted to neutral glycans in order to avoid hydrating phenomena related to polyelectrolyte behaviour. The l a t t e r effect is found in the mucopolysaccharides {25) and sulfated algal polysaccharides. (2j>_) The glycans of this review are from the c e l l walls of f 1 owering plants, algaeor fungi. Their roles are c l e a r l y s t r u c t u r a l , m a t r i c i a l or reserve (Table V). Present understanding of the shortrange non-bonded interaction allows one to predict general features of single chain conformation, but d e t a i l s such as multiple helix formation or hydration and its effect on comformation are s t i l l beyond theoretical prediction. In general, hydration of polysaccharides is an element of s t r u c t u r a l adaptation. Structures that hydrate w i l l show stress relaxation under tension and Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.



Figure 8.

ab projection of (1 -» 3)-fi-O-glucan anhydrous unit cell



Structural,

(l+3)-3-D-Xylan

Matrix Matrix ? Reserve

(l-*4)-3-D-Galactomannan

(l-*4)-3-D-Xylan

Nigeran

Amylose

Reserve

Structural, Reserve

Role

(l->3)-3-D-Glucan

Polysaccharide

Columnar, Sheet-Like

Sheet-Like

Columnar

Sheet-Like

Columnar

Columnar

Hydration

Hexagonal, Orthorhomb ic

Orthorhombic

Hexagonal

Orthorhombic

Hexagonal

Hexagonal

Unit-Cel 1

Role in Nature, Hydrate Form, U n i t - C e l l Type and Conformational Symmetry of Various Polysaccharides

Table V


61

2i

3i

2i

61

6χ

Conformational Symmetry


270


the rate w i l l be a function of r e l a t i v e water content. Of the two categories discussed, columnar hydration is more reversible and less morphologically damaging. It is the only kind expected in a structural material. Sheet-like hydration causes uneven expansion or contraction of the material and thereby destroys the structural cohesion. The effect can be gauged by the appearance of the vacuum dried single c r y s t a l s of nigeran shown in Figure 2a. The system of p a r a l l e l cracks created by the drying stresses are i r r e v e r s i b l e and are of a size to allow enzymes to access the inner surfaces of a c e l l w a l l . Thus, mere drying of a fungal wall containing nigeran increases i t s s u s c e p t i b i l i t y to enzyme attack. ( 27) The s t r a i n effects due to dehydration are nowhere better i l l u s t r a t e d than in the precautions that must be taken to dry and season wood before its use. This problem relates to water removal from c a p i l l a r i e s but c e r t a i n l y the dehydration of the hemicelluloses which are in a p a r a c r y s t a l l i n e order at the surface of the m i c r o f i b r i l s (2j8) must play a r o l e . Xylan ( 4 ) in hardwoods and galactoglucomannans (1_8) in softwoods have the characteristics of columnar and sheet hydration, r e s p e c t i v e l y . Seed germination is a phenomenon which requires moisture, hence hydration of the polysaccharide in the endosperm. Galactomannans seem t a i l o r e d to adapt to the environmental requirements of plants located in tropical areas where moisture is seasonal. By comparison, starch hydration is probably more gradual and r e v e r s i b l e , a situation more in keeping with a temperate climate. From a thermodynamic point of view, one might expect that polysaccharide c r y s t a l l i t e s would display d i s t i n c t hydrates and not show continuous variation in water content as a function of r e l a t i v e humidity. So f a r , the continuous variation in u n i t - c e l l parameters as a function of r e l a t i v e humidity seems to be the r u l e . However, the variation of c e l l parameters with r e l a t i v e humidity seems to follow the shape of the moisture sorption curve, (IS) In a l l p r o b a b i l i t y , the fine structure factor introduces l o c a l i z e d strain effects which prevents detection of a unique hydrate at a given r e l a t i v e humidity.


15. BLUHM ET AL.

Hydration of Polysaccharides

111


Literature Cited

(1)

Flory, P. J., Faraday Discussions of the Chemical Society, 1974, 57, 7.

(2)

Freundlich, H., "Kapillarchemie", V2, Akademische Verlagsgesellschaft, M8H, Leipsig, Germany, 1930, p. 568.

(3)

Timell, 247.

(4)

Nieduszynski, I. Α.; Marchessau1t, Biopolymers, 1972, 11, 1335.

(5)

L e l l i o t t , C . ; Atkins, E. D. T . ; Juritz, J . W.; Stephen, Α. Μ., Polymer, 1978, 19, 363.

(6)

Barker, S. Α.; Carrington, T. R., J . Chem. Soc., 1953, 3588.

(7)

Barker, S. Α.; Bourne, E. J.; O'Mant, D. M.; Stacey, Μ. Α . , J . Chem. Soc., 1957, 2448.

(8)

Dox, A. W.; Niedig, R. E., J . Biol. Chem., 1914, 18, 167.

(9)

Reese, E. T . ; Mandels, M., Can. J . Microbiol., 1964, 10, 103.

(10)

Sundararajan, P. R.; Marchessault, R. H.; Quigley, G. J.; Sarko, Α . , J. Am. Chem. Soc., 1973, 95, 2001.

(11)

Perez, S., Roux, M.; Revol, J. F.; Marchessault, R. H . , J . Mol. B i o l . , 1979, 129, 113.

(12)

Taylor, Κ. J., Montreal, 1976.

(13)

Whistler, R. L.; Smart, C. L., "Polysaccharide Chemistry", Academic Press, New York, 1953.

(14)

Wu, H-C. H . ; Sarko, Α . , Carbohyd. Res., 1978, 61, 7.

(15)

ibid, 26.

Τ. Ε . , Adv. Carbohydrate Chem., 1964, 19,

Ph.D. Thesis,

R. H . ,

Universite


de



272

(16)

Aspinal, G. O., "Polysaccharides", Press, Elmford, New York, 1970.

(17)

Nieduszynski, I . ; Marchessault, Chem., 1972, 50, 2130.

(18)

Marchessault, R. H . ; Buleon, Α . ; Deslandes, Y . ; Goto, T., J. C o l l o i d and Interface Sci., in press.

(19)

Palmer, K. J.; 1950, 72, 736.

(20)

Barras, D. R.; Stone, Β. Α . , "The Biology Euglena", Academic Press, New York, 1969.

(21)

Harada, T., Process Biochem., 1974,

(22)

Marchessault, R. H . ; Deslandes, Y . ; Ogawa, Κ.; Sundararajan, P. R . , Can. J. Chem., 1977, 55, 300.

(23)

Bluhm, T. L.; Sarko, Α . , Can. J. Chem., 1977, 293.

(24)

Atkins, E. D. T . ; Parker, K. D . , J . Polym. Part C, 1968, 28, 69.

(25)

Winter, W. T . ; Smith, P. J . J . Mol. Biol., 1975, 99, 219.

(26)

Arnott, S.; Scott, W. E . ; McNab, C. G. Α . , J. Mol. Biol.,

(27)

Nordin, J.; Bobbitt, T . , unpublished

(28)

Marchessault, R. H . ; Liang, C. Υ., J. Polym. 1962, 59, 357.

Ballantyne,

J.

Pergamon

R. H . , Can. J .

Am. Chem. Soc.,

9,

of

21.

55, Sci.,

C . ; Arnott,

S.,

Rees, D. Α . ; 1974, 90, 253. result.

R E C E I V E D January 4, 1980.


Sci.,

New Insights into the Crystal Structure Hydration of Polysaccharides

Recommend Documents