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8/17/2019 Dilute Solution Viscosity of Red Microalga Exopolysaccharide
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P e r g a m o n
Chemical Engineering Science Vol. 51, No. 9, pp. 1487 1494, 1996
Copyright © 1996 Elsevier Science t d
Printed in Great Britain. All rights reserved
0009 2509/96 15.00 + 0.00
0009-2509(95)00305-3
D I L U T E S O L U T I O N V I S C O S I T Y O F R E D M I C R O L G
E X O P O L Y S C C H R I D E
EDWARD ETESHOLA, MOSHE GOTTLIEB*t and SHOSH ANA (MALIS) ARAD *
tDepartment of Chemical Engineering and *Department of Life Sciences and The Institutes for Applied
Research, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
First received 4 November
1994;
revised manuscript received and accepted
24
April
1995)
Abstract The
red microalga
Porphyridium sp P.
sp) is encapsulated in a sulphated polysaccharide. The
external part of this capsule dissolves n the growth medium. This extracellular polysaccharide is a hetero-
polyelectrolyte with molecular weight of ~ 6 x 106 Da. The effect of solvent, counterion and pH on chain
flexibilityand structural features in dilute solution of the exopolysaccharide was investigated by intrinsic
viscometry. From the dependence of the intrinsic viscosity [q] on ionic strength, it was estimated that the
stiffness of P. sp polysaccharide chains is in the same range as that of xanthan and DNA. The effect of the
counterion on [q] is found to be specific and dependent on the type and valence of the counterion. The
polyelectrolyte behaviour of the polymer is confirmed by the decrease of [q] with the addition of salt
without any observable order-disorder conformational transition in aqueous salt solutions in the com-
monly used range of ionic strength (0.01 1.0). At considerably lower ionic strength (< 0.01) there is an
indication of a transition in the P. sp polyion conformation, most likely reflecting a contraction of the
polymer chain from a highly stretched to a stiff, wormlike chain. It is hypothesized from the overall dilute
solution features that the P. sp biopolymer chain molecules adopt stiff ordered conformation in solution.
I N T R O D U C T I O N
P o r p h y r i d i u m
sp. is a red microalga which is encap-
sulated in a sulfated polysaccharide. The external part
of this capsule dissolves in the growth medium. This
extracellular mucil aginous material (hereafter referred
to as PspP, the second P standing for polysacchar-
ide/polymer) carries carboxyl and half ester sulfate
groups o n its glycosidic backbo ne which confers on it
the properties of a biopolyelectrolyte. The exocellular
polymer of P. sp contains different sugars, including
xylose, galactose, glucose, mannose, arabinose,
methyl hexoses and methyl pentoses in various
amou nts and ratios (Jones, 1962; Ramus, 1973; Med-
calf et
al.,
1975; Heaney-Kiera s and Chapm an, 1976;
Heaney-Kieras
et al.,
1976; Percival and Foyle, 1979;
Geresh et al., 1992). In addition, the polysaccharide
conta ins ca 9 glucuroni c acid, ca 10 half ester
sulfate, and a protein moiety (Heaney-Kieras
et al.,
1976). The molecular weight of the P. sp polysacchar-
ide was estimated as 5-7 x 106 Da (Simon
et al.,
1992).
The polysaccharide produced by the red microalga
P. sp is of great interest because at low polymer
concentrations it yields highly viscous aqueous solu-
tions with unique theological properties comparable
with those of xanthan and carrageenan, and thus
suitable for technological applications (Savin, 1978;
Ramus, 1986; Ramus
et al.,
1989; Geresh and Arad,
1991). For example, dilute aqueous solutions of the
polymer have been shown to be effective in drag
*Corresponding author.
CE
5 1 : 9 -
reducti on in capillary pipe flow (Ramus, 1986; Ramus
et al.,
1989). Due to its special properties, PspP aque-
ous solutions have been reported to be compatible
with various salts and stable under varying pH, high
temperature, and accelerated flow rates (Savin, 1978;
Ramus, 1986; Ramus
et al.,
1989; Geresh and Arad,
1991). Preliminary studies in our laboratory indicate
that P. sp polysaccharide has also interesting surface
activity properties and has potential use as a pharma-
ceutical material.
Most of the work published in the literature on this
polysaccharide is concerned with glycosyl content
and chemistry and hardly touches upo n the relation-
ships between conformat ional structure and rheologi-
cal properties (Geresh and Arad, 1991). In order to
develop the extracellular polysaccharide of P. sp, we
need to u nderstand their structure- function relation-
ships, which presupposes a detailed knowledge of
their molecular behaviour and physico-chemical
properties (Lee and Chandrasekaran, 1992). Accord-
ingly, in the work described in this paper we examined
the dilute solutio n properties of PspP as a function of
the nature of the solvent (water, aqueous salt solu-
tions, pH), and the type of counterion. Our purpose
was to determine the influence of these experimental
variables on the chain flexibility and on the conforma-
tional features of the polymer in dilute solutions as
manifested by intrins ic viscosity values.
EXPERIMENTAL
A l g a a n d g r o w t h c o n d i t i o n s
P o r p h y r i d i u m
sp. (UTEX 637) was obtained from
the culture collection of algae at the University of
1487
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1488
E ETESHOLAet a l .
Texas, Austin. The cells were grown in batch culture
in 1-1 column s 6 cm in diameter at 24 + I°C in artifi-
cial seawater according to Jones e t a l . (1963). The
cultures were illuminated cont inuou sly with fluor-
escent cool-white lamps at a n ir radiance of 150 micro-
einsteins m- 2 s- ~. The med ium was aerated with ster-
ile air cont aini ng 3% COg (Arad e t a l . , 1988). At the
stationary phase of growth (after 2 weeks), the cells
were separated from the growth medium by centrifu-
gation at 10,000 rpm for 20 min using a Sorvall centri-
fuge model RC2-B (Sorvall Instruments Du Pont).
The super natant medium cont aining the polymer was
collected and dialyzed in Visking size 98 32/32 dialy-
sis tubing (Medicell International Ltd., London)
against distilled water and then exhaustively against
bidistilled water at 4°C. The resultant solution was
then freeze dried in order to isolate the water soluble
polysaccharide fraction.
P r e p a r a t i o n o f p o l y s a c c h a r i d e s o l u ti o n s
The •eze- drie d exopolysaccharide was dissolved
in bidistilled water or in aqueous salt solution of the
desired concentration by prolonged gentle stirring
with a magnetic stirrer. For studies on the effect of
pH, test solutions were made by judicio us adjustment
of the pH with a few drops of either 8 M HC1 or
NaO H solutions so as to mai ntain constant polysac-
charide concentration.
I n t r i n s i c v i s c o s i t y
The viscosities of the polymer solutions were meas-
ured with a size 75 Cannon- Fens kesemi-micro capillary
viscometer (Cannon Instrument Co., State College, PA,
USA). The shear rate experienced by the polymer solu-
tions in this viscometer was estimated to be in the range
of 175-670 s - 1. The effect of shear ra te was examined by
the use of a narrower capillary (size 50, 110-450 s). The
difference in [~/] values was insignificant.
The viscometer was suspended in a thermostati-
cally controlled water bath maintaine d at the required
temperature to within + 0.2°C, in the temperature
range 25-85°C. Equilibration time of 15-20 min was
allowed before measurements were made since the
setup was experimentally found to reach thermal
equili brium within 10 min.
The relative viscosity of a given so lut ion qr¢l (de-
fined as the ratio between the solution viscosity and
the solvent viscosity
q / q s )
was determined by measure-
ment of the relative efflux times in the capillary (Mays
and Hadjchristidis, 1991; Van Krevelen and Hoftyzer,
1976). Viscosity values were based on at least 2-3
efflux time readings taken for any given sample in-
serted into the viscometer. Variation between con-
secutive readings was lower than 1.5% and typically
around 0.5%. At least two independent viscosity de-
termi natio ns were performed for each concentration.
Four experimental points were used for extrapolation
to obtain the intrinsic viscosity [~/]. The intrin sic
viscosity is usually obtain ed from either the extrapola-
tion of In
? ] r e l / C
(Kraemer relation)
o r ? ] r e l -
1 ) / c
(Huggins relation) to zero polymer conce ntrat ion, c.
However, (~/rel -- 1 ) / C - - r h p / C where ~/sp s the spe-
cific viscosity r hp = q - q , ) / q s ) of flexible polyelec-
trolyte solutions in pure water exhibits a unique de-
pendence on concentration, i.e. it diverges rapidly
with dilut ion. This effect, due to polymer chain expan-
sion, makes it extremely inaccurate to extrapolate
q s p / C to infinite dilution. Several equations have been
proposed in the literature to describe the concentra-
tion dependence of the viscosity of polyelectrolyte
solutions and to satisfactorily handle the extrapola-
tion of the experimental data (Fuoss and Strauss,
1948; Fuoss, 1951; Liberti and Stivala, 1966; Yuan
e t a l . , 1972). However, some quest ions have been
raised concerning the validity of these methods and
their ability to determine intrinsic viscosities with
precision (Yuan e t a l . , 1972). Furthermo re, it has been
shown that for flexible polyelectrolytes the apparent
divergence in r lsp /C as c ~ 0 is actually a ma ximum
followed by a decrease in q s p / C at very low c values.
The maxi mum is attributed to configurational cha-
nges due to dissolved gases and other impurities (Co-
hen and Priel, 1988). Consequently, in the present
study viscosity data were extrapolated to zero concen-
tration by the combined standard Huggins and
Kraemer treatments. The choice of these latter
methods was also made because they appear to be
more widely used in the literature and thus facilitate
comparison of [q] obtained in the present study with
similar ones for other biopolyelectrolytes and with [q]
values for salt solutions which do not present any
extrapolation difficulty.
R S U L T S
A typical set of data for the viscosity of PspP
solutions as a function of polymer concentration is
depicted in Fig. 1. The data for solutions in water and
in four different aqueous NaC1 solut ions at 25 °C are
plotted in terms of r h p / C vs c. The reported (Cohen
and Priel, 1988; Yamanak a e t a l . , 1990) divergence of
r h p / C as e -~ 0 in water is not observed in the concen-
trati on range examined here. The effects of salt con-
centration, type of counterions, and temperature on
the intrinsic viscosity of the PspP are summarized in
Table 1. The reported values are averages of at least
two ind epende nt determina tions of [~/] with the vari-
ance always below 5% of the reported mean.
The intrins ic viscosity in NaCI solution s at 25°C is
plotted in Fig. 2 against I, the ionic strength of the salt
solution, I = ~ c i Z ~ ) / 2 p , where c i is the ion molar
concentration, Zi the nu mber of charges on the ion
and p the solvent density. The plot shows an initial
sharp decrease of [~/] as the amount of salt is
increased at low NaC1 concent rat ion s (I < 0.025),
followed by a very moderate decrease in [q] at higher
NaC1 concentrations.
The effect of the cation used to prepare the salt
solutions is shown in Fig. 3 (here [q] is plotted against
1 - ° 5 for reasons discussed in the following section).
As clearly observed in this figure the type of cation
and not only its valence are of importance. Sodium
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Dilute solution viscosity of red
5 0 . . . . t . . . . i . . . .
0
v
40
i1
~ 0 M
N a I
i
• _e . . . . . • . . . .
~ 6 ~ 2 5 e 4 M
~ ~ 1 - . 2 5 e M
•
3 0
~ - ~ ~ -
v . v . . . . v i f
J
0 . 0 0 0 . 0 1 0 . 0 2
P o l y s a c c h a r i d e c o n c e n t r a t i o n g / d l )
0.03
Fig. 1. Specific viscosity of Porphyrid ium sp. polymer solu-
tions at 25°C. Solvents: (e ) bidistilled water, (11)
6.25 x 10- 5 M, ( ) 1.25 x 10 -4 M, (T) 0.01 M, and (4,) 0.1 M
NaC1 aqueous solutions. Each data point represents the
average of at least two independent determinations. The
lines represent the best fit linear regression.
salt is less effective in reducing molecular dimensi ons
than potassium. N o significant difference is observed
between intrinsic viscosity values for Ca ++ and
Mg + +, the two divalent c ations used in this study,
and at identical ionic strength [r/] is considerably
smaller for divalent cations than for mono vale nt ones.
Figur e 4 s hows the effect of pH on the [-~/] of Psp P
solutions prepared in water and in the presence of
microalga exopolysaccharide 1489
0.5 M aqueous NaC1. For all test solutions in water,
there was a decrease in the [r/] values after the first
adjustment to acidic or basic pH. These lower Jr/]
values were very similar for all pH values. It can also
be seen from Fig. 4 that in the prese nce of external salt
the l-r/] of test solutions is unaffected by the pH
adjustments remaining essentially constant in the
2.0-11.0 pH range. This demonstrates once again the
dimensio nal stabilization effect of salt solutions.
DISCUSSION
Ef f e c t o f i o ni c s t re n g th
A decrease in intrinsic viscosity is observed when
the ionic strength increases (see Table 1). For a salt-
free solution, electrostatic interactions due to the
charges on the polymer favour a stretched chain con-
formation as a result of long-range electrostatic ef-
fects. This behavi our results in higher intrinsic viscos-
ity. Addition of a simple electrolyte screens these
intermolecular electrostatic interactions and causes
the polyme r to assu me a more flexible configurat ion
(wormlike chain), resulting in reduced intrinsic viscos-
ity (Robinson e t a l . 1991; Smidsrod, 1970; Smidsrod
and Haug, 1971; Tinland and Rinaudo, 1989; Shatwell
et al. 1990; see Figs 1 and 3). Thus, recorded differ-
ences in viscosity between solutions of two different
batches of Ps pP in water can stem from differences in
the amounts of salts present as impurities in the two
samples (due for example, to ineffective dialysis pro-
cedure), and need not necessarily be related to differ-
ences in molecular characteristics such as molecular
weight and its distribution or branching mode.
E f f e c t o f t h e n a t u r e o f c o u n t e ri o n
The fact that the different cations gave different [r/]
values (Table 1 and Fig. 3) suggests that the effect of
the counterion on Psp P is specific and dependent on
the type of counterion. The KC1 salt (compared to
NaC1) was mo re effective in redu cing [r/] at very low
Table 1. Intrins ic viscosity [-~/] (dl g- 1) of Porphyrid ium sp polysaccharide as a function of temperature and salt concentra-
tion
Salt concentrat ion (M)
Temp.
(°C) Salt type 0 6.25 × 10 -5 1.25× 10 -4 0.01 0.0 25 0.05 0.1 0.5 1.0
25 NaC1 42.7 35.0 33.2 28.7 24.4 24.6 24.8 23.2 22.8
CaC12 28.3 25.4 23.8 23.2 20.3 - -
MgClz'6HzO 27.1 26.6 24.3 20.0 19.7 - -
KC1 32.0 31.0 25.3 24.0 - - - -
45 36.0
55 35.5
65 NaCI 36.5 26.7 - - - - 23.7 22.8 22.3
CaC12 23.7 - - - - 23.4 - - - -
MgCI2 •6H20 24.7 -- -- 21.1 -- --
75 NaC1 34.6 26.7 - - - - 23.4 22.3 22.5
CaC12 22.9 - - - - 24.0 - - - -
24.5 -- -- 20.3 - - --
85 NaC1 32.4 26.6 - - - - 22.0 20.7 21.5
CaCIz 25.8 23.0 - - - -
MgClz • 6HzO 23.5 22.3 - - - -
Note: The [q] values given represent the average of at least two independent determinations.
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1490
E . E T E S H O L A
et a l
A
0
0
9
4 5
I = 0
o l
: 3 5 ,
3 0
2 5
2 0
) . 0 0
4 0
3 5
3 0
2 5
2 O
1 . e - 5
i r ' ' ' ' 1 ' ' . . . . 1 ' ' ' ' . . . . I
- . . ° . ° .
_ . . .
- . . &
i i q i i i L i l
1 . e - 4
- - - . . • . . . .
• - - • - . . . • . •
° ° .
i ~ 1 1 I L l i i i J l l l
1 . e - 3 1 . e - 2
, , I , , L , I i ~ , ~ I
0 . 2 5 0 . 5 0 0 . 7 5
I o n i c S t r e n g t h ( N a C I )
1 . 0 0
Fig. 2. Intr insic viscosity of
Porphyridium
sp. polysaccharide as a fu nct ion of the ionic s t rength in aq ueous
NaC I a t 25°C. The inser t i s an enlargem ent of the low ionic s t rength range .
A
i,.,
0
0
._~
.o
C
° ~
C
3 5
3 0
2 5
2 0
i i t I i i ~ i i i I
I ' X ,
° i i i i i i i • • . . . . . . . . .
i i v
i i i
°
. • ° ' •
I I I I
1
i i i i ~ i i i I
.
.
• , D
N a C I . - KCI
. - '
. ' . -
• - • - • . * '
•
• o .
. - . . .
C a C I 2
M g C I 2
[ I I L I I I I I
1 0 1 0 0
[ - 0 . S
Fig. 3. In trins ic viscosity of
Porphyridium
sp. polymer for several m ono valen t and diva lent ca t ions . The
lines represent the best fit l inear regression of the data.
a n d m o d e r a t e l y l o w s a l t c o n c e n t r a t i o n ( < 1 × 1 0 - 2 M ) .
T h e d i v a l e n t c a t i o n C a + + r e d u c e d Jr /] e v e n f u r t h e r i n
c o m p a r i s o n w i t h t h e m o n o v a l e n t c a t i o n N a + . A s t h e
h y d r a t i o n l e ve l o f K + i s h i g h e r t h a n t h a t o f N a + th e
v a l u e o f [ q ] i s l o w e r f o r t h e f o r m e r b e c a u s e t h e h y -
d r a t e d i o n s o f K + a r e m o r e t i g h t ly b o u n d ( P a s i k a ,
1 97 7) . T h e d i v a l e n t c a t i o n s , w h i c h g i v e e v e n l o w e r J r/ ]
v a lu e s, m o s t p r o b a b l y f o r m i o n p a i r s to a m u c h
g r e a te r e x t e n t t h a n d o t h e m o n o v a l e n t c a ti o n s w i t h
t h e c a r b o x y l a n d t h e h a l f e s t er s u l f a te g r o u p s o f P s p P .
S m i d s r o d ( 1 9 7 0 ) s u g g e s t e d t h a t M g + ÷ i o n s , w h Jc ~
g a v e l o w e r I -r /] t h a n N a ÷ i o n s p r o b a b l y f o r m i o n p a i r s
w i t h t h e c a r b o x y l g r o u p s o f a lg i n a t e . A l t e r n a ti v e l y ,
t h e g r e a t e r e f fe c ti v en e s s o f t h e d i v a l e n t c a t i o n s m a y b e
d u e t o m o r e e f fe c ti v e s h i e l d i n g o f t h e P . s p p o l y i o n
c h a r g e s b y b i v a l e n t i o n s ( S c h n e i d e r a n d D o t y , 1 95 4) .
Determinat ion of the f lexibi l i ty parameter B
F o r m a n y y e a r s i t h a s b e e n k n o w n t h a t , fo r f le x i b le
p o l y e l e c t r o l y t e s , i n t r i n s i c v i s c o s i t y v a r ie s w i t h 1 - 0 . 5
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5
' ' ' l ' ' ' I
4
. _ _ >
8
= o
u
3
Dilute solut ion viscosi ty of red micro alga exopolysaccharide
, I , , , I ~ , ,
8 1 0 1 2
2 0 , , , I , = , I , , , I ,
0 2 4 6
p H
Fig. 4. Va riatio n of the Psp P intrinsic viscosity as a function
of the pH in water (0 ) and in 0.5 M NaC1 solut ion (11).
1491
( w i t h I t h e i o n i c s t r e n g t h ) , a n d t h a t t h e s l o p e o f t h i s
p l o t , S , is a n i n d i c a t o r o f c h a i n f l e x ib i l i ty ( S h a t w e l l
e t a l .
1990).
S m i d s r o d a n d H a u g (1 97 1) d e v e l o p e d a s i m p l e
s e m i - e m p i r i c a l m e t h o d b a s e d o n t h i s o b s e r v a t i o n .
T h i s i s t h e s o - c a l l e d B v a l u e m e t h o d , in w h i c h t h e
p a r a m e t e r B i s g i v e n b y :
B s / l - r / ] o . 1 ) 1 .3 1 )
w h e r e [ r/ ]o .1 i s t h e i n t r i n s i c v i s c o s i t y i n 0 .1 M N a C 1 ,
a n d t h e p e r s i s t e n c e l e n g t h a i s :
a = 0 .2 6 /B ( 2)
( f o r [ r / l in d l g - 1 a n d a i n n m ) . T h e c o n s t a n t ( 0 .2 6 ) w a s
o r i g i n a l l y f o u n d b y c o n s t r u c t i n g a c a l i b r a t i o n p l o t f o r
a n u m b e r o f s e m i - fl e x i b l e b i o p l o y m e r s . T h e p e r s i s t -
e n c e l e n g t h i s a m e a s u r e o f t h e l e n g t h o v e r w h i c h t h e
c h a i n ' p e r s i s t s' i n t h e d i r e c t i o n o f th e f ir s t b o n d o f t h e
c h a i n . F o r s t if f w o r m l i k e c o i l s t h e K u h n s t e p l e n g t h , l,
i s g i v e n b y ( A n t h o n s e n e t a l . 1993)
l = 2a. (3)
F o l l o w i n g t h is t y p e o f a n a l y s is , t h e i n t r i n s i c v i s c o s it y
d a t a f o r P s p P i n N a C 1 s o l u t i o n s a r e p l o t t e d i n F i g . 5
a s f u n c t io n o f I - ° ' s . F o r c o m p a r i s o n p u r p o s e s d a t a
f o r s e v e r a l o t h e r c o m m o n p o l y s a c c h a r i d e s a r e a l s o
s h o w n i n t h e s a m e f i g u r e . T h e l i n e a r d e p e n d e n c e o f
I-r /] o n i - o . s i n t h e s a l t c o n c e n t r a t i o n r a n g e o f
0 . 0 1 -1 . 0 M N a C 1 i s c l e a r l y o b s e r v e d . B o t h q u a l i t a t -
i v e ly a n d q u a n t i t a t iv e l y t h e b e h a v i o r o f P s p P i s i n
o
>
.2
t - .
t -
1 6 0
1 5 0
1 4 0
5
4
3
2
1
i i n } i i i i I i i n I
r h a m s a n
¢
[ ] ~
xanthan
[ ]
_ - - ~
welan
P s p
~ . . ~ - , a l g i n a t e
h l i l
K - c a r r a g e e n a n
I i ~ i i [ ~ i i I
5 1 0 1 5
..o.s
Fig. 5. Va riatio n of intrinisc viscosity with ionic strength, I ,
for
P o r p h y r i d i u m
sp. polysa char ide (O) , rham san (V) , xan-
than (D) , welan ( i) , a lginate (A) , and x-carrageenan (O) .
Data for a lginate , welan and rhamsan were taken f rom
Robinson e t a l . (1991), for x-carageenan from Slootmaekers
e t a l . (1988) and for xan than f rom Shatwell e t a l . (1990):
a c c o r d w i t h t h a t o b s e r v e d f o r o t h e r p o l y s a c c h a r i d e s .
A l i n e a r d e p e n d e n c e o f [r /] o n 1 - 0 . 5 i s a ls o o b s e r v e d
f o r a l l o t h e r s a l t s e x a m i n e d h e r e ( F i g . 3 ) .
A t h i g h i o n i c s t re n g t h , c h a r g e s c r e e n i n g r e d u c e s t h e
i n fl u e nc e o f c o u l o m b i c i n t e ra c t i o n s o n p o l y m e r c o n -
f o r m a t i o n , w h e r e a s a t l o w i o n i c s t r e n g t h t h e s e i n t e r a c -
t i o n s b e c o m e i m p o r t a n t a n d d o a ff ec t c o n f o r m a t i o n .
T h i s f ac t m a y b e d e d u c e d f ro m t h e n o n l i n e a r i t y o b -
s e r v e d in t h e p l o t i n F i g . 6, w h i c h i n c l u d e s d a t a o n t h e
e ff e ct o f N a C 1 a t v e r y l o w i o n i c s t r e n g t h a n d o v e r
a c o n s i d e r a b l y b r o a d e r c o n c e n t r a t i o n r a n g e t h a n t h a t
s h o w n i n F i g . 5 . T h e d e c r e a s e i n [ ~ /] w i t h i n c r e a s i n g
i o n i c s t r e n g t h i m p l i e s t h e e x i s t e n c e o f a t t ra c t i v e i n t e r -
a c t i o n s b e t w e e n c h a i n e l e m e n t s , p o s s i b l y o f e l e c t r o s -
t a t i c o r i g i n . T h e s l o p e c h a n g e o b s e r v e d i n t h e p l o t
a r o u n d I ~ 0.0 1 M m a y h i n t a t a t r a n s i t i o n i n t h e
P . s p p o l y i o n c o n f o r m a t i o n , m o s t l i k e l y r e f le c t in g
a c o n t r a c t i o n a n d c o n f o r m a t i o n a l o r d e r i n g ( w i th i n -
c r e a s i n g i o n i c s tr e n g t h ) o f th e p o l y i o n c h a i n f r o m
a h i g h l y s t r e t c h e d c o n f o r m a t i o n t o a s t if f b u t n e v e r -
t h e l e ss w o r m l i k e c h a i n . S i m i l a r t r a n s i t i o n s h a v e b e e n
r e p o r t e d i n t h e l i t e r a t u r e ( H o l z w a r t h , 1 98 1; S l o o t -
m a e k e r s
e t a l .
1988).
W e w i l l e x a m i n e th e P s p P J r/] d a t a w i t h i n t h e
c o n c e n t r a t i o n r a n g e 0 . 0 1 -1 . 0 M N a C I i n g r e a t e r d e ta i l
( F i g . 5 ). A p p l y i n g e q s (1 ) - (3 ) t o t h e d a t a p r e s e n t e d i n
F ig . 5 , w e obt a in B = 0 .0074 f or the s ti f fnes s pa r am ete r ,
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1492
4 5
4
~ ~ 35
0
u
.~_
~ 3 0
° -
2 5
. . . . I I l l l l l l l l l l l l l l l l I
1 = 0
2
~ 1 ~ I ~ 1 ~ l l ~ I
0 2 5 5 0 7 5 1 0 0 1 2 5
I - .0 .5
Fig. 6 . The depende nce of the in t r ins ic v i scos i ty of P o r -
p h y r i d i u m s p . po l ym e r on t he c onc e n t r a t i on o f a dde d s od i um
chloride at 25°C.
a = 3 5 n m f o r t h e p e r s is t e n c e le n g t h a n d 7 0 n m f o r t h e
K u h n s te p l e n g t h fo r P s p P . T h e s e v a l u es a r e r e p o r t e d
i n T a b l e 2 i n c o m p a r i s o n w i t h o t h e r b i o p o l y e l e c -
t r o ly t e s . T h e v a l u e o f 0 . 0 07 4 e s t i m a t e d f o r B , w h i c h i s
a m e a s u r e o f c h a i n f l e x i b i li t y ( h i g h e r B v a l u e c o r r e s -
p o n d s t o a m o r e f l e x i b l e c h a i n ) , is c l o s e t o v a l u e s
E ETESHOLAe t a l .
o b t a i n e d f o r ri g i d h e l ic e s s u c h a s x a n t h a n a n d D N A ,
b u t l es s t h a n t h o s e o b t a i n e d ( S m i d s r ~ d a n d H a u g ,
1 97 1) f o r c a r b o x y l a t e d p o l y s a c c h a r i d e s w i t h c h a r a c -
t e r is t ic a l l y m o r e f l ex i b le b a c k b o n e s , s u c h a s a l g i n a t e
a n d c a r b o x y m e t h y l c e l l u l o s e ( A x e l o s a n d T h i b a u l t ,
1 9 9 1 ; S l o t m a e k e r s e t a l . 1 98 8; s e e T a b l e 2 ) . T h e r e f o r e
t h e m o d e s t c h a n g e i n [ r/ ] a s a f u n c t i o n o f i o n i c
s t r e n g t h ( F i g . 5 ) m a y b e t a k e n t o a t t e s t t o t h e s t i ff n e s s
o f t h e P s p P c h a i n s t r u c t u r e . A p o s s ib l e e x p l a n a t i o n
f o r t h is p r o p e r t y i s t h a t t h e P . s p b i o p o l y m e r h a s
a c o n f i g u r a t i o n w h i c h m a i n t a i n s t h e c h a i n m o l e c u l e i n
a s ti ff f o r m s i m i l a r t o t h e s i t u a t i o n r e p o r t e d f o r D N A
( C o x , 1 96 0). W e n o t e t h a t t h e q u e s t i o n o f w h e t h e r t h e
P s p P m o l e c u l e is a s i n g le o r a m u l t i p l e s t r a n d e d h e l ix
i n a q u e o u s s o l u t io n m a y n o t h a v e b e e n a n s w e r e d b y
t h e e x p e r i m e n t s r e p o r t e d h e r e . H o w e v e r , p r e l i m i n a r y
X - r a y d i f f r a c t i o n s t u d i e s ( E t e s h o l a
e t a l .
s u b m i t t e d )
i n d i c a t e t h e p o s s i b i l i t y o f a t w o - f o l d h e l i c a l s tr u c t u r e .
I n t e r m s o f p r o p e r t y - s t r u c t u r e r e l a t io n s h i p s , t h e lo w
e x p a n s i o n c o e f f i c ie n t o b s e r v e d f o r t h e P s p P c h a i n ( t h e
l i n e a r i t y o b s e r v e d i n F i g . 5 ) c o u l d a l s o b e d u e t o
a c o m p l e x b r a n c h i n g m o d e ( P a i n t e r , 1 9 8 3 ; F l a i b a n i
e t a l .
1989) . Y ua n
e t a l .
( 1 9 7 2 ) h a v e r e p o r t e d t h a t
b r a n c h i n g c a n c u r t a i l t h e a b i l it y o f a p o l y i o n t o
e x p a n d .
T h e p e r s i s t e n c e l e n g t h w a s e s t i m a t e d a s a l r e a d y
i n d i c a t e d a b o v e . I n a d d i t i o n , t h e K u h n s t e p l e n g t h
w a s e s t im a t e d ( d a ta n o t s h o w n ) b y i n t e r p o l a ti o n f r o m
F i g u r e 1 0 i n t h e p a p e r b y S m i d s r o d a n d C h r i s t e n s e n
(1 99 1), w h i c h m e n t i o n s a n e m p i r i c a l c o r r e l a t i o n b e -
t w e e n B a n d t h e K u h n l e n g t h . T h i s r e l a ti o n s h i p a l s o
r e v e a l s t h a t t h e P s p P h a s a c h a i n s t if fn e s s c o m p a r a b l e
t o t h at o f D N A a n d x a n t h a n . T h u s , f r o m t h e v a ri o u s
s t i f f n e s s i n d i c e s r e p o r t e d i n T a b l e 2 , i t m a y b e c o n -
c l u d e d t h a t t h e o v e r a ll m o l e c u l a r c o n f o r m a t i o n a n d
d i m e n s i o n s o f t h e P . s p b i o p o l y m e r c h a i n s a s r e f le c t e d
b y t h e i n t r i n s i c v i s c o s i t y b e a r s o m e s i m i l a r i t i e s t o
Table 2. St iffness indices for several biopolyelectrolytes
Polyelec t rolyte B a (nm) l (nm)
P o r p h y r i d i u m sp. 0.0074 35.1 70.2
R ha m s a n* 0 .003 88 - -
Xa ntha n 0.00525 t 50* 100
Xanthan* 7 * (coil)
40** (helix)
Xan than ps . PXO61~ 42.2 84.4
Xa nth an ps. 556 ~ 39.2 78.4
DN A* 0.0055 45 90
Pect in (C72)~ 0.017 15.3 30.6
Alginates-gu luronate- r ich * 0 .031 7 .8 1
-man nuron ate- r ich* 0 .040 6 .5 - -
x-carrageenan~ 6.8 t*
3.7**
C a r boxym e t hy l c e l lu l o s e* 0.065 4.1 - -
* R ob i ns on e t a l . 1991.
t Tinlan d and Rinaud o, 1989.
S l oo t m a e ke r s e t a l . 1988.
~Shatwell e t a l . 1990.
~Axelos and Thibaul t , 1991.
** By vi scomet ry .
t t By l ight scat tering.
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Dilute solut ion vi scos i ty of red microalga exopolysacchar ide
t h o s e o f D N A a n d x a n t h a n . F r o m t h e a b o v e r e su l ts
a n d o b s e r v a t i o n s , i t i s h y p o t h e s i z e d t h a t t h e P . s p
p o l y s a c c h a r i d e c h a i n m o l e c u l e s a d o p t r e l a t iv e l y s t if f
c o n f o r m a t i o n i n d il u t e s o l u t io n s o v e r a r e l a t iv e l y w i d e
s a l t c o n c e n t r a t i o n r a n g e .
E f f e c t o f p H a n d i o n i c s t re n g t h
T h e i n t r in s i c v i s c o s i t y d e c r e a s e a s a r e s u l t o f p H
c h a n g e s ( F i g . 4 ) c a n b e r e g a r d e d a t m o s t a s m o d e r a t e ,
a n d s o m e o f th e d e c r e a s e , e s p e c ia l l y u n d e r a c i d i c
c o n d i t i o n s , i s n e g l ig i b l e . T h e P s p P s e e m s t o b e s o m e -
w h a t m o r e s t a b l e u n d e r a c i d i c t h a n b a s i c c o n d i t i o n s .
T h e d e p e n d e n c e o f t h e i n tr i n s i c v is c o s i t y o f P s p P
s o l u t i o n s o n t h e p H i n t h e p r e s e n c e o f e x t e rn a l s a l t is
c h a r a c t e r i s t i c f o r a p o l y e l e c t r o l y t e s o l u ti o n . T h e l a r g e
c o n f o r m a t i o n a l c h a n g e s d u e t o s t r o n g s c r e e n i n g o f
c o u l o m b i c i n t e r a c t i o n s b y t h e s al t m a s k s t h e s m a l l
c h a n g e s d u e t o t h e p H e f fe c ts d i s c u s s e d a b o v e . S i m i l a r
o b s e r v a t i o n s h a v e b e e n m a d e f o r t h e p o l y e l e c t r o l y t e s
N a - x a n t h a n a n d N a - c a r b o x y m e t h y l - c e l lu l o s e ( R i n a u d o
a nd Mi l a s , 1978) .
CONCLUSION
T h i s p a p e r r e p r e s e n t s t h e f i r s t a t t e m p t t o s t u d y
c h a i n f l e x i b il i ty a n d c o n f o r m a t i o n a l f e a t u r e s f o r P o r -
p h y r i d i u m s p . p o l y s a c c h a r i d e i n d il u t e a q u e o u s a n d
s a l t s o l u t i o n s b y i n t r i n s i c v i s c o m e t r y .
P o l y e l e c t r o l y t i c b e h a v i o u r i s c o n f i r m e d b y t h e d e -
c r e a s e o f I -q ] w i t h t h e a d d i t i o n o f n e u t r a l s a l t. C o m -
b i n e d d a t a f o r v e r y lo w t o m o d e r a t e i o n i c s t r e n g t h
i n d i c a t e a c h a n g e i n s l o p e i n t h e p l o t o f E ~ /] v s 1 - ° 5 ,
w h i c h m a y h i n t a t a t ra n s i t i o n i n P s p P c h a i n c o n -
f i r m a t i o n f r o m a h i g h l y s t r e tc h e d t o a s ti ff , w o r m l i k e
c h a in . H o w e v e r , n o c l e ar o r d e ~ d i s o r d e r c o n f o r m a -
t i o n a l t r a n s i t i o n a s a r e s u lt o f i o n i c s t re n g t h c h a n g e s
w a s d e t e c t e d . T h e s t if fn e s s p a r a m e t e r w a s d e d u c e d
f r o m t h e d e p e n d e n c e o f [ q ] o n i o n i c s t re n g t h ; f r o m t h e
v a l u e o b t a i n e d i t i s c o n c l u d e d t h a t t h e s t if fn e s s o f
P s p P c h a i n s is i n t h e s a m e r a n g e a s x a n t h a n a n d
D N A . D u e t o t h e st if f c o n f i g u r a t i o n a d o p t e d b y t h e
c h a i n m o l e c u l e s i n s o l u t i o n , a r e l a t i v e l y lo w s e n s i ti v i ty
t o i n c r e a s i n g i o n i c s t r e n g t h i s o b s e r v e d . T h e l o w t h e r -
m a l e x p a n s i o n c o e f f ic i e n t d i s p l a y e d m a y b e r e l a t e d t o
t h e d e t a il e d p r i m a r y a n d c h e m i c a l s tr u c t u r e o f t h e
p o l y m e r . T h e p o l y m e r s h o w e d s p e c if i ci ty t o c o u n t e r -
i o n t y p e a n d v a l e n c y .
A c k n o w l e d g e m e n t - - M G a c know l e dge s t he s uppor t o f the
Is rae l Science Foundat ion adminis tered by the I s rae l Acad-
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