Equilibrium and Nonequilibrium Association Processes of
κ-Carrageenan in Aqueous Salt Solutions
Karin Bongaerts and Harry Reynaers
Laboratory of Macromolecular Structural Chemistry, Department of Chemistry, K.U. Leuven,
Celestijnenlaan 200F, Heverlee, Belgium
Flavio Zanetti
POLY-tech S.C.r.l., AREA Science Park, Padriciano 99, I-34012 Trieste, Italy
Sergio Paoletti*
Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste,
via L. Giorgieri 1, I-34127 Trieste, Italy
Received July 31, 1998; Revised Manuscript Received December 1, 1998
ABSTRACT: A large number of light-scattering data indicate that the sulfated polysaccharide κ-carra-
geenan undergoes an intramolecular conformational transition under suitable conditions from a
semiflexible, disordered conformation to a rather rigid, helical one. By varying the parameters controlling
the conformational phase diagram (i.e., by increasing the concentration of polymer or that of the supporting
electrolyte, or both, or by decreasing temperature), a tertiary structure is manifested, through a progressive
increase of the weight-average molar mass (M h
w) as determined by light scattering. This can be very well
explained in terms of a reversible association of stretches of such intramolecular helical structures and
quantitatively described by a model of “open association” of two or more chains. The association of
κ-carrageenan was demonstrated to take place at around room temperature also in the presence of iodide
ions, as long as their concentration is g0.20 M. In this way the apparent inconsistency of a number of
published M h
w results can be reconciled. Additional light-scattering results indicated that the demonstrated
thermodynamic tendency to reversible interchain association may lead to an irreversible aggregation of
polymer chains if an improper solution preparation procedure is used.
Introduction
Carrageenans are water-soluble, sulfated polysaccha-
rides extracted from different species of marine red
algae of the class Rhodophyceae. One of their basic
properties is the ability to induce thickening or gelation
of solutions. This process appears to be thermoreversible
with or without hysteresis and has been the object of
numerous studies.
Supramolecular structure formation in aqueous solu-
tions of carrageenans is a complex process involving a
temperature-induced disorder-to-order conformational
transition and a chain-chain association superimposed
over polyelectrolyte effects due to their ionic nature.
Temperature, polymer concentration, ionic strength,
and the nature of co- and counterions markedly influ-
ence and modulate these mechanisms in a complicated
manner. In particular, the strong tendency to associa-
tion contributed to hamper the achievement of a fast
and universal consensus on the nature of the ordered
conformation involved in those processes.
The behavior of κ-carrageenan is no exception to this
rule. The primary structure of κ-carrageenan consists
ideally of a regular alternation of R(1-3)-D-galactose-
4-sulfate and -(1-4)-3,6-anhydro-D-galactose, although
other monosaccharides such as galactose, galactose-2-
sulfate, galactose-6-sulfate, and galactose-2,6-disulfate
may be present in minor amounts.
The scope of this paper, like that of the accompanying
one,
1
is confined to a detailed analysis of molar mass
determinations only, purposely combining a critical
assessment of all significant data from the literature
with new original data obtained to confirm previous
results or to support the critical evaluation of others.
This paper is devoted to the analysis of the intermo-
lecular association aspects, both when they represent
a true equilibrium state of the system and under
conditions of metastability. Use will be made of the well-
established formalism developed to analyze light-scat-
tering data of associating polymeric systems.
2
Experimental Section
Materials. The κ-carrageenan sample and the other chemi-
cals were the same as described in the preceding paper.
1
Also,
the low-angle laser light scattering (LALLS) setup and the
measuring protocol was the same as previously described.
1
The procedure for the preparation of solutions was the same
as in ref 1, which follows strictly the protocol of ref 3. When
explicitly desired, the protocol followed for the preparation of
the polymer solution, in the presence of NaCl and NaI, is the
one reported in ref 4.
The inverse reduced scattering intensity, (KC
p)/Rθ, de-
pendence on angle, θ, of the data of Figure 6b of ref 4 was
analyzed. Visual inspection of the plot revealed a significant
downward curvature. In fact, linear regression analysis gave
a comparatively high value of the sum of the squares of the
residuals,
2
:
2
) 2.58 × 10
-14
. A polynomial regression of
the data points significantly reduced
2
to 2.87 × 10
-15
.
Consequently, the limiting values of (KCp)/Rθ|θ ) 0° from such
a polynomial analysis were used for the calculation of M h w and
of the second virial coefficient, A2.
For the evaluation of the ionic strength, I, of the solutions
of ref 8, the contribution of the polyelectrolyte counterions has
been neglected for all values of the molarity of the supporting
* Corresponding author. Tel.: +39 040 676 36 92. Fax: +39
040 676 36 91. E-mail: paolese@bbcm.univ.trieste.it.
683 Macromolecules 1999, 32, 683-689
10.1021/ma981204r CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/23/1999