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SANS contrast in iota-carrageenan gels and solutions in D2O
Physica B 234-236 (1997)283-285
ELSEVIER
SANS contrast in iota-carrageenan gels and solutions in D20 N. Mischenko a'*, B. Deneff, K. Mortensen b, H.
Reynaers a
"Departament of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 20OF, B-3001 Heverlee, Belgium bRis¢ National Laboratory, DK-4000 Roskilde, Denmark
Abstract SANS of Na+-iota-carrageenan in D20/saline solutions was measured as a function of concentration, temperature and type of counterions (K + or Na ÷). High and low scattering-contrasted gels and solutions were detected. High contrast is caused by aggregation of low-hydrated chains at high concentration of carrageenan in the presence of gel-promoting counterions. The aggregates do not disappear at 60°C, although at this temperature the molecules should partialy lose their ordered conformation. The network formation, observed by SANS, does not happen in high-contrast system immediately after cooling and takes a few hours to be completed at 12°C. The minimum concentration sufficient for the high-contrast regime in 0.1 M KC1 lies between 15 and 25 mg/ml. Keywords: Small-angle neutron scattering; Electrolytes; Aggregates; Gels
1. Introduction
2. Results and discussions
The mechanism of gel formation in polyelectrolytic polysaccharides is a matter of debate for more than a decade. Several models have been proposed to account for this phenomena, involving (I) a conformational transition from a disordered to an ordered (helical) state on the level of individual [1] or intertwined [2] molecules or aggregated single- [3] or double-stranded [4, 3] helices and (II) further aggregation of the molecules or small aggregates in the ordered conformation. In the present work, we observe SANS of iota-carrageenan in D20/saline solutions under or close to gelling conditions. Sample preparation and experimental conditions for the measurements are described elsewhere [5, 6]. SANS measurementso(Riso) were carried out in a q range 0.003~). 13 A - 1
In spite of using D 2 0 as the solvent, a sufficient scattering contrast was only observed for a 25 mg/ml Na+-iota carrageenan in 0.1 M KC1 both at high and low temperatures (Fig. 1). SANS intensity of the carrageenan solutions of the same concentration but in 0.1 M NaC1 or even larger concentration (100 mg/ml) in salt-free D 2 0 , as well as of the solutions of lower (5 and 15 mg/ml) concentration of carrageenan in 0.1 M KC1 is more than one order of magnitude lower in the region 2rt/q ~ 150-200 A. This difference becomes smaller on going to larger q. Instead, the influence of the type of counterions (Na ÷ against K ÷) becomes more clear. The high level of SANS is ascribed to the existence of the aggregates of carrageenan molecules with low hydration resulting in the high contrast with respect to scattering density of D20. The aggregates formation depends on the carrageenan concentration and the presence of
* Corresponding author.
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S092 1-4526(96)00958- 1
o
284
N. Mischenko et al. / Physica B 234-236 (1997) 283-285
K+-ions. This finding also means that at the conditions normally used for small-angle scattering study [5-10] of polyelectrolytic polysaccharides (concentration of the polymer 1-20 mg/ml, concentration of added salt: ~<0.1 M) the hydration phenomena sufficiently decrease the scattering of the chains both in disordered and ordered conformations making the determination of the cross-sectional characteristic of the polymer extremely difficult. On the other hand, the aggregates are detected even at rather high temperature ( ~ 60°C) when the polymer helices could soften or partially get in the disordered conformation. This evidences that aggregation phenomena in presence of gel-promoting ions play important role even before the completion of the conformational transition I-3, 6, 10]. The stiffness of the helical units is increasing within the aggregates on reducing temperature. This might promote the penetration of water molecules inside the aggregates and, as the contrast of the polysaccharides is negative with respect to D20, results in SANS intensity reduction (Fig. 2). The fit of intensity is of exponential type, where the main change is going with the pre-exponential term depending on contrast and/or number of domains. Below 15°C the contrast and the number of aggregates is not changing any more. Instead, the formation of the spatial network is proceeding. This is represented by the increase of intensity in the smallest angles
o
backgr3/4.7A
0.1 KCI/d2o/Nb
1E-4
•
• • • = = = = mm
1E-5 _o
# • •
~..
=====
"'.... : ** ~"~'~.~
.
25 i 60"C .1NaCI
• •
25 i 56"C .1KCI 25 i 15"C .1KCI
o
5 i 220C .1KCI
==
x
15 i 600C .1KCI 15 i 22"C .1KCl
1E4 .
.
.
.
.
~
,o
t
O,
0.0t
0.01
0.1
log q Fig. 2. Differential SANS from 25 mg/ml Na + - i o t a carrageenan in 0.1 M KCI on reducing temperature from 55°C (open and filled circles) to 15°C (crossed circles). For 55 °C and 15°C the curves are extended to lowest q. Inset: time dependence of the intensity in low q region at 12°C.
(Fig. 2, inset). Simultaneously, the middle part of the curve tends to follow a q-1 asymptotic behaviour while its beginning can be approximated by q-~, 1 < ~ < 1.5. The formation of the network is certainly promoted by the stiffening and elongating of the aggregates.
Acknowledgements The support from EC Large-Scale Facilities program (Risoe) and from INTAS (project 93-645-ext) is gratefully acknowledged. N.M. is grateful to KUL for the research fellowship.
References
o £~6o'c-~KC,
** ~ = , .
.
,0111:1 :oooo
.
j
0.t
0,01 log q
Fig. 1. Experimental small-angle neutron scattering (including background, O) from Na + -iota carrageenan in 0.1 M NaCI ( + ) and 0.1 M KC1 (all the rest) at different temperatures and carrageenan concentrations.
[1] O. Smidsr~d, I.L. Andresen, H. Grasdalen, B. Larsen and T. Painter, Carbohydrate Res. 80 (1980) 11. [2] D.A. Rees, Polysaccharide Shapes (Chapman & Hall, London, 1977). [3] S. Paoletti, O. Smidsred and H. Grasdalen, Biopolymer 23 (1984) 1771. I-4] E.R. Morris, D.A. Rees and G. Robinson, J. Mol. Biol. 138 (1980) 349. [5] D. Slootmaekers, C. De Jonghe, H. Reynaers, F.A. Varkevisser and C.J. Bloys van Treslong, Int. J. Biol. Macromol. 10 (1988) 160.
N. Mischenko et al. / Physica B 234-236 (1997) 283-285
[6] B. Denef, N. Mischenko, M.H. Koch and H. Reynaers, Int. J. Biol. Macromol. 18 (1996) 151. [-7] K. Kajiwara, in: Polymer Gels, ed. D. DeRossi, (Plenum Press, New York, 1991) 3. [8] M. Milas, P. Lindner, M. Rinaudo and R. Borsali, R. Macromolecules 29 (1996) 473.
285
[9] T. Turquois, C. Rochas, C., F.-R. Taravel, J.L. Doublier and M. Axelos, Biopolymers 36 (1995) 559. [10] N. Mischenko, B. Denef, M.H. Koch and H. Reynaers, Int. J. Biol. Macromol. 19 (1996) 185.
ELSEVIER
SANS contrast in iota-carrageenan gels and solutions in D20 N. Mischenko a'*, B. Deneff, K. Mortensen b, H.
Reynaers a
"Departament of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 20OF, B-3001 Heverlee, Belgium bRis¢ National Laboratory, DK-4000 Roskilde, Denmark
Abstract SANS of Na+-iota-carrageenan in D20/saline solutions was measured as a function of concentration, temperature and type of counterions (K + or Na ÷). High and low scattering-contrasted gels and solutions were detected. High contrast is caused by aggregation of low-hydrated chains at high concentration of carrageenan in the presence of gel-promoting counterions. The aggregates do not disappear at 60°C, although at this temperature the molecules should partialy lose their ordered conformation. The network formation, observed by SANS, does not happen in high-contrast system immediately after cooling and takes a few hours to be completed at 12°C. The minimum concentration sufficient for the high-contrast regime in 0.1 M KC1 lies between 15 and 25 mg/ml. Keywords: Small-angle neutron scattering; Electrolytes; Aggregates; Gels
1. Introduction
2. Results and discussions
The mechanism of gel formation in polyelectrolytic polysaccharides is a matter of debate for more than a decade. Several models have been proposed to account for this phenomena, involving (I) a conformational transition from a disordered to an ordered (helical) state on the level of individual [1] or intertwined [2] molecules or aggregated single- [3] or double-stranded [4, 3] helices and (II) further aggregation of the molecules or small aggregates in the ordered conformation. In the present work, we observe SANS of iota-carrageenan in D20/saline solutions under or close to gelling conditions. Sample preparation and experimental conditions for the measurements are described elsewhere [5, 6]. SANS measurementso(Riso) were carried out in a q range 0.003~). 13 A - 1
In spite of using D 2 0 as the solvent, a sufficient scattering contrast was only observed for a 25 mg/ml Na+-iota carrageenan in 0.1 M KC1 both at high and low temperatures (Fig. 1). SANS intensity of the carrageenan solutions of the same concentration but in 0.1 M NaC1 or even larger concentration (100 mg/ml) in salt-free D 2 0 , as well as of the solutions of lower (5 and 15 mg/ml) concentration of carrageenan in 0.1 M KC1 is more than one order of magnitude lower in the region 2rt/q ~ 150-200 A. This difference becomes smaller on going to larger q. Instead, the influence of the type of counterions (Na ÷ against K ÷) becomes more clear. The high level of SANS is ascribed to the existence of the aggregates of carrageenan molecules with low hydration resulting in the high contrast with respect to scattering density of D20. The aggregates formation depends on the carrageenan concentration and the presence of
* Corresponding author.
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S092 1-4526(96)00958- 1
o
284
N. Mischenko et al. / Physica B 234-236 (1997) 283-285
K+-ions. This finding also means that at the conditions normally used for small-angle scattering study [5-10] of polyelectrolytic polysaccharides (concentration of the polymer 1-20 mg/ml, concentration of added salt: ~<0.1 M) the hydration phenomena sufficiently decrease the scattering of the chains both in disordered and ordered conformations making the determination of the cross-sectional characteristic of the polymer extremely difficult. On the other hand, the aggregates are detected even at rather high temperature ( ~ 60°C) when the polymer helices could soften or partially get in the disordered conformation. This evidences that aggregation phenomena in presence of gel-promoting ions play important role even before the completion of the conformational transition I-3, 6, 10]. The stiffness of the helical units is increasing within the aggregates on reducing temperature. This might promote the penetration of water molecules inside the aggregates and, as the contrast of the polysaccharides is negative with respect to D20, results in SANS intensity reduction (Fig. 2). The fit of intensity is of exponential type, where the main change is going with the pre-exponential term depending on contrast and/or number of domains. Below 15°C the contrast and the number of aggregates is not changing any more. Instead, the formation of the spatial network is proceeding. This is represented by the increase of intensity in the smallest angles
o
backgr3/4.7A
0.1 KCI/d2o/Nb
1E-4
•
• • • = = = = mm
1E-5 _o
# • •
~..
=====
"'.... : ** ~"~'~.~
.
25 i 60"C .1NaCI
• •
25 i 56"C .1KCI 25 i 15"C .1KCI
o
5 i 220C .1KCI
==
x
15 i 600C .1KCI 15 i 22"C .1KCl
1E4 .
.
.
.
.
~
,o
t
O,
0.0t
0.01
0.1
log q Fig. 2. Differential SANS from 25 mg/ml Na + - i o t a carrageenan in 0.1 M KCI on reducing temperature from 55°C (open and filled circles) to 15°C (crossed circles). For 55 °C and 15°C the curves are extended to lowest q. Inset: time dependence of the intensity in low q region at 12°C.
(Fig. 2, inset). Simultaneously, the middle part of the curve tends to follow a q-1 asymptotic behaviour while its beginning can be approximated by q-~, 1 < ~ < 1.5. The formation of the network is certainly promoted by the stiffening and elongating of the aggregates.
Acknowledgements The support from EC Large-Scale Facilities program (Risoe) and from INTAS (project 93-645-ext) is gratefully acknowledged. N.M. is grateful to KUL for the research fellowship.
References
o £~6o'c-~KC,
** ~ = , .
.
,0111:1 :oooo
.
j
0.t
0,01 log q
Fig. 1. Experimental small-angle neutron scattering (including background, O) from Na + -iota carrageenan in 0.1 M NaCI ( + ) and 0.1 M KC1 (all the rest) at different temperatures and carrageenan concentrations.
[1] O. Smidsr~d, I.L. Andresen, H. Grasdalen, B. Larsen and T. Painter, Carbohydrate Res. 80 (1980) 11. [2] D.A. Rees, Polysaccharide Shapes (Chapman & Hall, London, 1977). [3] S. Paoletti, O. Smidsred and H. Grasdalen, Biopolymer 23 (1984) 1771. I-4] E.R. Morris, D.A. Rees and G. Robinson, J. Mol. Biol. 138 (1980) 349. [5] D. Slootmaekers, C. De Jonghe, H. Reynaers, F.A. Varkevisser and C.J. Bloys van Treslong, Int. J. Biol. Macromol. 10 (1988) 160.
N. Mischenko et al. / Physica B 234-236 (1997) 283-285
[6] B. Denef, N. Mischenko, M.H. Koch and H. Reynaers, Int. J. Biol. Macromol. 18 (1996) 151. [-7] K. Kajiwara, in: Polymer Gels, ed. D. DeRossi, (Plenum Press, New York, 1991) 3. [8] M. Milas, P. Lindner, M. Rinaudo and R. Borsali, R. Macromolecules 29 (1996) 473.
285
[9] T. Turquois, C. Rochas, C., F.-R. Taravel, J.L. Doublier and M. Axelos, Biopolymers 36 (1995) 559. [10] N. Mischenko, B. Denef, M.H. Koch and H. Reynaers, Int. J. Biol. Macromol. 19 (1996) 185.