The role of conformation on the thermodynamics and rheology of aqueous solutions of carbohydrate polymers

Joumul of Food Engineering 22 ( 1994) 27-42 6 1994 Elsevier Science Limited Printed in Great Britain. Ail rights reserved 0240-8774/941$7.00 ELSEViER

The Role of Conformation on the Thermodynamics and Rheology of Aqueous Solutions of Carbohydrate Polymers Attilio Ces&ro Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, Trieste, Italy and Laboratory of Technological Biopolymers, POLY bibs Research Centre. Area di Ricerca, Trieste, Italy

ABSTRACT A molecular description of the conformationa~ features of carbohydrate polymers and of their interactions with ~~olvent~water~ and with ions is given. The thermodynamics of solvat~on and of ion binding is illustrated in order to character&e the energetics of the fo~ation of the ordered conformations which can be induced by decreasing the temperature and/or screening the charge repulsion on the chain. It is shown that the persistence of weak interactions over an extended length of the chain results in the peculiar rheological properties of these carbohydrate systems.

INTRODUCTION Interactions between carbohydrates (either oligomers or polymers) and solvent, ions and other molecular species are the basis of many biologically and technologically relevant phenomena. Still a coherent frame for all these interconnections is elusive due not only to the different approaches used, but also to the incompleteness and the uncertainty of fundamental physico-chemical data (Cesaro, 1986; Franks, 1987; Angyal, 1989; Goldberg & Tewari, 1989). The properties of carbohydrate polymers are used in foods to control the rheology of the aqueous phase, as thickeners, dispersing and/or gelling agents, as well as systems with intermediate properties (Blanshard & Mitchell, 1978). Solubi~ty and properties are a function of the polymer chemical struc27

ture, which, in turn, determines secondary and higher levels of structure. A knowledge of polysaccharide structures up to three dimensional molecular shape and of its interaction with water and other ingredients is, therefore, essential for the understanding, controlling and upgrading of their properties in food applications. In the following, a series of sequential aspects will be summarised bearing in mind the relationships of macroscopic (e.g. rheological) properties with the co~fo~a~on and the~odyna~c properties of food polysaccharides in dilute and semidilute aqueous solution, which are finally understood. The aim is to review some of the basic concepts and related experimental results in order to understand the delicate balance of the factors which determine the properties of the aqueous solutions of carbohydrate polymers. Particular attention is paid to the solvation of the conformational isomers (Urbani & Cesaro, 1991), to the changes of enthalpy and volume which accompany ion binding (Cesaro et al, 1988) and gel formation (Paofetti et af, 1986), and to the key role of the conformation in determining the rheological properties (CeGro et al., 1992~) All the examples reported show dramatic variations in the physicochemical properties which are the basis of the technological importance of polysaccharides.

2 PRIMARY

AND SECONDARY STRUCTURES POLYSACCHARIDES

OF FOOD

The problems peculiar to the study of polysaccharides are mainly related to the various levels of conformation which may be contempora~ly present in solution, with respect to those derived from diffraction studies on oriented fibres. In the solid state a number of factors (e.g. the orientation degree, the presence of water molecules, the constraints) are taken into account to be consistent with the conformation theoretically evaluated from the minimisation of the internal energy and with the symmetry and stereochemical requirements for the efficient packing of chains. As an example of this variety, some amylose polymorphs are shown in Fig. 1. There is experimental evidence for the existence in solution of statistical conformations based on the whole energy surface, as well as - under given circumst~ces - for ordering processes promoted by decreasing temperature or, for ionic polysaccharides, by screening electrostatic interactions (Cesaro et aZ., 1992~). However, it is difficult to take properly into account on theoretical grounds all the enthalpic and entropic contributions and to predict correctly the thermodynamic stability of one of these conformations. For

Fig. 1. Molecular shapes of amylose helical polymorphs (French, 1979) and of a representative conformation generated by the Monte Carlo method (only the virtual bonds are shown for the latter).

example, it has been reported (Talashek & Brant, 1987) that the significant differences in the properties within the gellan family camot arise from intramolecular van der Waals interactions alone, which leads one to conclude that specific intramolecular interactions and/or interchain interactions must be responsible for the observed behaviour. Still, the

30

Attilio Cesliro

main source of interactions determining the secondary structure resides in the non-bonding forces occurring with the rotation about the glycosidic linkages ($J and 9, in addition the o angle in the case of 1-6 linkages). The energy of each confo~ational state of a dimeric saccharide can be easily calculated with several degrees of approximation. Confining all units in the same conformational state will stretch the polysaccharide into a helical shape which, for the minimum energy state, is only a function of the glycosidic type of linkage (Brant, 1976; Rees, 1977). Both the extension and direction of the chain segments dynamically change in solution, where it has already been mentioned that the degree of persistence of conformational order can only be partially retained. From the practical point of view, the persistence of conformational order (or disorder) determines the macroscopic properties of our interest (e.g. rheology). This situation is substantially different to the retention of a regular globular structure of proteins when they are dissolved in a nondenaturing solvent. Conformational disorder and solubility are ensured almost exclusively by the thermal fluctuations about the rotational angles. In the dissolved or amorphous state a chain is therefore described instantaneously by a sequence of conformational states (equivalent to angles) which contribute to the statistical average of the macroscopic properties. Provided that appropriate potential functions are available, the solution of this theoretical problem is based on the Monte Carlo method. Results have now been obtained on homopolysaccharides showing ‘snapshots’ (Fig. 1) of more or less coiled chain trajectories (Jordan et al., 1978; Brant, 1982). Whenever ionic groups are present (almost always carboxylate and/or sulphate groups), then charge repulsion affects both local conformation and overall chain dimensions. The ionic strength either modulates these charge repulsions or can even fully screen the fixed charges through counterion condensation or specific site-binding.

3 THE~ODYNA~rCS

OF SACCHARIDE CONFORMATION

SOLVATION AND

3.1 Theoretical approach to evaluating solvation effects ‘Solvation’ or ‘solvent effect’, are terms used not only to refer to, but also to explain, the complex phenomena which originate from solute-solvent interactions, although accurate theoretical results have been obtained for small size solutes and non-polar solvents. The computational complexi~

Thermodynamics and rheology of carbohydrate polymers

31

of intra- and inter-molecular interactions has hampered a widespread use of theoretical formalisms (see, for example, French & Brady, 1990) to macromolecular solutes and has often led to either very geometrical or merely statistical pictures of solute-solvent interaction being preferred, with a consequent loss of reality. In a binary dilute solution containing solvent and polymer the minimisation of the free energy occurs not only by replacing some of the interactions between chain segments on one hand and those between the pure solvent molecules on the other hand with those between solute and solvent, but also with a net entropic change. In very dilute solution and a ‘good solvent’ it is likely that only solute-solvent and solvent-solvent interactions prevail. I-lowever, in a ‘bad solvent’ (and in general in more concentrated solutions) persistence of segmental interactions among polysaccharide chains is the major contribution in the peculiar macroscopic properties of the system. The evaluation of the entropic contributions arising from the configurational nature of the chain molecule appears in theory accessible (Flory, 1969). The contribution of enthalpy change of mixing (A,,H) of non-ionic polysa~charides and water cannot be predicted, not even in sign. In addition, the scarcity of literature data does not allow any empirical rationalisation, although in most cases the contributions are significantly smaller than the related monomers. The general inference of all the above aspects is that the average dimensions of a chain, experimentally obtained with light scattering or viscometric measurements, depend upon solvent interactions and behaviour, in addition to the intrinsic features of the polysaccharide (chemical nature and linkage of monomers, conformational equilibria, etc.). Recent theoretical results (Urbani & Cesaro, 1991) show that changes in the population of the conformational energy levels of the polysaccharidic chains justify the experimental data of different unperturbed dimensions, such as those observed for amylose in binary water/ dimethyl sulphoxide mixtures (Jordan & Brant, 1980), while experimental data and plausible argumentation have been reported on the fairly i~sensitivi~ of the unperturbed dimensions of non-polar polymers to the solvent media (Flory, 1969). 3.2 Effect of the solvent on the conformational energy surface In order to understand the molecular aspects of these relationships, attention is focused on the perturbation due to the solvent on the conformational potential surface of a disaccha~de (maltose). The cont~butions to the solvation energy contain terms for the formation of a cavity in the solvent (G,,,), for electrostatic (G,,) and dispersion (Gdisp) interactions

Attiio Cesdro

32

between solute and solvent. Figures 2(a), 2(b) and 2(c) show these contributions given by the solvents dioxane, DMSO and water, respectively, on a section of the maltose4conformation map (Q, = - 30”). The cavity term in each solvent is a strict function of the size of the molecule which is determined by the relative orientation of the two glucose residues. The variation of the dipole moment, p, for the maltose dimer as a function of the rotational angle r#~is revealed by the change in the electrostatic contribution (Figs 2(a)-2(c)}. The third term, due to dispersion forces, gives a significant favourable contribution to the absolute value of the solvation energy; however, it noticeably shows a very low co~ormation~ dependence. The perturbation thus introduced by the explicit account of the solvent results in the modification of the iso-energetic levels. Even if the overall conformational Ramachandran curves (hard repulsions) are not modified by the presence of the solvent, important changes occur in the profiles at low energy. For example, the effect of water on the probability associated with the various conformers of maltose results in the sharpening of a peak probability conformation at q&9 = - lo”, - 20”. This ‘pre-

-180

0

l&O 9 rdw

) of the pe~urbation due to (a) the solvent, (b) DhkO, (c) Fig. 2. Energy profiles (-water on a section of the conformational map of maitose. The relative changes of energy are also plotted for the contributions of cavity (---- ), electrostatic (-- -) and dispersion (...) terms.

Thermodynamics and rheology of carbohydrate polymers

33

ferred’ co~ormation would favour a left helical twist with about six monomers per turn and a pitch of O-175 nm/monomer. The major conclusions of this kind of approach reside in the differences in the overall chain dimensions evaluated under conditions defined unperturbed or O-conditions, i.e. with compensation between long-range chain-chain and solute-solvent interactions. In other words, the results here discussed indicate that if a polymer exists in an unperturbed state in different solvents or in the amorphous state, then chain dimensions will differ in the different conditions; this may be interpreted as being due to changes in the persistence length in different Oconditions.

4 INDUCED

CONFORMATIONS

THROUGH

ION BINDING

4.1 Higher levels of structure and physical gels Having understood the influence of the solvent on the local conformation, let us now consider the experimental conditions in which the formation of physical gels typically takes place in most of the polysaccbaridic systems. As a general rule, by decreasing the temperature or screening fixed charges one may facilitate local ordering of linear polysaccharidic chains with a consequential clustering of ordered chains, interacting side-by-side through intermolecular bonds and/or salt bridges. The latter is the driving force for the well known phenomenon of getation of carrageen~s with alkaline cations and of alginates and pectins with calcium (and other) divalent cations (Fig. 3). Most striking of all is that gela-

ii 80

‘3

e 80

a

ap

0

1

2

3

4

5 C,mM

6

7

Fig. 3. Percentage of gel precipitated from a solution of pectate (open symbols) and alginate (full symbols) with increasing concentration of divalent salts Cu (0, a), Ca (0, l ) at 25°C. Polymer concentration CP = lo-” eq/litre.

tion can occur at concentrations of polymer of even less than 1 gflitre, and that despite the excess of water (e.g. up to 99*9%), ‘self standing’ gel can be obtained. In some cases (e.g. in the agar and carrageenan family) gel formation is thermo-reversible due to the temperature balance of the entropic and enthalpic terms. A high salt concentration shifts the temperature-induced gel -+sol transition of carrageenan to a higher temperature. A noticeable hysteresis is often observed not only with heating and cooling cycles, peculiar to phase transitions involving polymeric systems under metastable conditions, but also in the isothermal ion-induced ordering (Paoletti et aL, 1985a). In fact, nucleation, which may kinetically control the gelation, has to be associated with the induction of locally ordered structures. 4.2 PoIyelectrolytic background for ion binding at equilibrium Alg~tes and pectins are well known for their gelling properties under conditions in which ordered chain a~regation (c~sta~ite-like) is favoured by the screening of the charged groups. I3ue to their polyelectrolytic character, the ‘binding’ of counterions is a peculiarity stemming from the intrinsic charge density of the polymers and of the structures induced, making it difficult to factorise the contribution of specificity (if any) from the raw binding data. From a thermodynamic

0

0.1

0,2

0.3 CT/%

Fig. 4. pectate

Binding isotherms (from equilibrium dialysis experiments at 25”C} of Ca’+ to (0) and alginate (polymer concentration=@4x lO_’ I, 0.93~ lo-’ LI and 1.4 x 10 - z 0 equiv./litre).

Thermodynamics and rheology ofcarbohydrate polymers

35

point of view, the overall distribution of binding modes of ions by polyelectrolytes (i.e. non-loc~ised, localised and site-biding) must satisfy the electrostatic requirements given by the charge density of the polymer and depends on the very polyelectrolytic nature of the chain (waning, 1979). The deter~ation of charge density is not without problems, however, and for semi-flexible polyelectrolytes it has had to be redefined recently on a statistical basis (Cesaro et ai., 1989). The results of equilib~um dialysis of calcium ions can only discriminate the higher affinity of calcium with pectin with respect to that with alginate (Fig. 4f. However, the different behaviour shown in Fig. 4 is ascribed, at least as far as the extent of binding is concerned, to the different composition of the binding sites in polygalacturonate with respect to alginate, in which only the homooligomeric guluronate sequences are similarly effective (Kohn, 1975). It is not uncommon that the free-energy term suffers from such ‘simplicity’, in contrast with other thermodynamic functions, such as enthalpy. 4.3 Molecular thermodynamics of ion binding to alginates and pectins A schematic summary of the results of dilatometric and microcalorimetric experiments on the volume and enthalpy changes which take place upon mixing an ionic polysaccharide (a&mate and pectate) with salt is reported in Fig. 5 (Cesziro et aL, 1988). Leaving aside the description of the polyelectrolyte model and the theoretical aspects of the derivation of the thermodynamic functions associated with the ‘ion binding’, let us briefly resume that: - with the Debye-H~ckel modeliing of the ionic components (in dilute solution) a polyelectrolyte approach based on the ~ounterion condensation theory (waning, 1979) predicts a small positive volume change and a negative entropy change on mixing the polymer with a simple salt (Paoletti et af, 1985a); - under the same circumstances, a positive enthalpy change is also predicted for the merely electrostatic interaction of the point charges with the linear charged polyelectrolyte (Paoletti et al.,

1985b). The experimental results always show a positive volume change measured upon mixing polymer solutions with various salt solutions to reach the final salt-to-polymer ratio, R = Cion/Cpolymer.This is in agreement with the general observation that ion-ion interaction occurs with (at least) partial desolvation of the interacting groups. The absolute values and the shapes of the dilatometric curves (i.e. of A,,, V versus R j

Attilio Cesliro

36

0

0.4

0.8

1.2

R

Fig. 5. Schematic trend in the change of volume (upper) and enthalpy (lower) on mixing alginate or pectate with divalent cations (Cesaro et al., 1988). The volume change with pectate (region a) is in the order: Cu:+ > PB2+ > Zn2+ > Cd?+ > Ca?+, while with alginate Cuz + and Pb?+ fall in region a and the other cations fall in region b (with Cd?+ > Ca” > Zn? ‘). The enthalpy change (see text) reflects the mere electrostatic interaction (a) or the induced conformational change (b).

indicate that a strong desolvation seems to occur with divalent cations such as Pb and Cu and that the trend is more regular for pectate than for alginate. In both cases A V is in the order: Cu E Pb > Cd > Ca. Possible extended modifications in the chain conformation or intrachain interactions do not affect (or barely affect) the dilatometric measurements in the case of pectate (Cesaro et al., 1982) and Kcarrageenan (Paoletti et al., 1985b). In other words, large changes in the Amix V of ions with polysaccharides have been mainly, if not exclusively, associated with the interaction at a specific charged site. Scrutiny of the dependence of AH as a function of the degree of complexation (or of the ion-to-polymer molar ratio) can readily disclose a binding and/or state transition cooperativity. Claims have been made that the trend of the enthalpy change upon mixing with ions monitors the occurrence of a conformational transition (Paoletti et aL, 1985a; Cesaro, 1986). The key to disclosing this process lies in the comparison of the experimental data with the quantitative theoretical prediction of the enthalpy change for a mere mixing process (Fig. 5).

Thermodynamics

and rheofttgy ~~carb~hydrate polymers

37

Cations like Zn and Cd give an endothermic heat effect which can be accounted for by the sole electrostatic contribution calculated for the addition of divalent salt to the polyelectrolyte system. Similarly, a smooth positive enthalpy of an interaction is obtained upon mixing oligomannuronate with all divalent cations investigated. On the other hand, the enthalpies of mixing Ca and Pb with both alginate and pectate are anomalous with respect to the theoretical prediction (exothermic enthalpy change). Moreover, these enthalpy changes show a sigmoidal trend (i.e. cooperativity) which cannot be traced back to a Langmuir-ape binding. Only in the case of alginate polymers a positive AH is exhibited by Cu, in agreement with other reported data on the complex formation of Cu with carboxylate groups. From all the thermodynamic data a significant cooperativity of the ion-polymer interaction clearly appears in a few cases. In particular, both alginate and pectate systems appear to be cooperative with Ca and Pb. As a final remark, it is worthwhile remembering the stereochemical differences between galacturonate and guluronate, limited to the orientation of the hydroxyl group on carbon 3 on, otherwise, mirror image structures. The picture, however, emerges of a specific contribution of ‘the polymeric chain conformation’. Support for the involvement of the stereochemistry of the dimeric sugar moieties come from the conformational calculations and chiro-optical data on the complex of Pb ions with al&rate and pectate (Fig. 6).

5 EFFECT

OF THE CONFORMATION PROPERTIES

ON THE RHEOLOGICAL

5.1 Rheological properties of polymers Polymeric solutions show a Newtons behaviour, that is a viscosity independent of the shear rate, only in the limits of low concentration, low molecular weight or low shear rate. Non-Newtonian rheological behaviour of polymeric solutions is commonly referred to as pseudoplastic; the viscosity is a decreasing function of shear rate. Moreover, the pseudoplastic behaviour may show the presence of a yield point, above which the solution flows, or a typical time-dependence (thixotropy). Flow-curve diagrams report the shear rate dependence of viscosity in a double logarithm. The profile of the flow curves and the position of the three portions of the curves (the upper plateau, the power-law regime and the lower plateau) depend on the concentration, molecular weight and conformational rigidity of the polysaccharidic chains. The variety of both type of residues and linkages causes the polysaccharide chains to

38

A ttilio Cesriro

-4m

2 D A 0

Fig. 6. Circular dichroic bands of the Pb?’ cation complexed by polygalacturonate ) and alginate (- ----), at about half saturation (equiv./equiv.). In the insert (are shown the two quasi-mirror images of the di-galacturonate and the diguluronate structures.

have a very wide range of chain stiffness with characteristic ratio values ranging from 3 to 400. As a result, a wide range of viscoelastic behaviours may then be expected depending on both the rigidity and conformational topology of the chains in solution. For non-gelling systems two limiting viscoelastic behaviours in semidilute solution can be traced through comparative analysis of the available experimental data. The behaviour of linear flexible polysaccharides in good solvent resembles that of synthetic polymers, like polystyrene in toluene. The viscoelastic spectrum at low frequencies shows a predominantly liquid-like behaviour with the loss modulus, G”, greater than the storage modulus, G’. The dependence on frequency, o, (G” 0: o and G’ CC02) is that expected for a liquid system. At a higher frequency a region appears in which G” < G’ so that the curves cross each other; the cross-over frequency (for G” = G’) decreases by increasing the concentration or molecular weight. The complex viscosity, r]*, which is almost frequency independent at low frequencies, shows a power law decrease as the frequency increases. The flow curves shown by the polymer coil solution in semidilute regime are characterised by the Newtonian plateau at low y and by the ‘shear thinning’ behaviour in the region of high shear rates.

Entangled networks formed by linear flexible polysaccharides have been observed to be more strain ~dependent than those formed by stiff wormlike (or more rigid rod) molecules. Moreover the recovery of nonlinear properties after steady shearing flow (transient experiments) has been observed to be dependent upon the chain ~exib~ity. In particular, the time scale for the re-establishment of entangled networks is several orders of rna~i~de slower for rigid molecules than for flexible molecules (Clark & Ross-Murphy, 198 7). A remarkable deviation from the above described behaviour is found for ~olysaccharide chains with moderate ~exibility (i.e. persistence length L p> 30 run) in the same concentration regime and under conditions promoting an ordered conformation in aqueous solution. In particular, the viscoelastic spectrum resembles that of a typical gel system with both G” and G’ almost frequency independent and with G’ > G” in the whole range of frequencies made accessible with conventions instruments. The flow curves give an indication of the existence of an apparent yield stress, at low shear rate, without evidence of a Newtonian plateau. Moreover, the remarkable shear thinning behaviour can be quantified in terms of the higher slope in the power-law region. The published data regarding the normal stresses as well as the elongatio~l viscosity make it possible to quote the remarkable differences between the behaviour of flexible polysaccharides and that of stiff chains. For stiff polysaccha~de systems the superposition of the 7 versus y and q * versus co curves (Corx-Merz rule) fails and q* appears higher than q in the range of y and u) investigated. On the other hand, for random coil flexible polysaccharides the Corn-Merz supe~osition is obeyed with the exception of a very low concentration regime. It seems clear the the rheological properties reflect the different supramolecular structure adopted by polysaccharide chains, either flexible or stiff, in the semidilute regime. The two limiting viscoelastic behaviours mentioned above suggest that the ~nd~ent~ point to be clarified is whether the supramolecular structure in semidilute solution is directly correlated with the molec~ar co~ormation adopted by the polymer chains in dilute solutions. In other words, can the polysaccharide conformational state be predicted on the basis of the rheological properties of aqueous se~d~ute solutions? This possible struc~re-rheolo~ correlation would greatly assist in the effort to upgrade polysaccharide properties and will be examined for a mixture of two ~lysaccha~des produced naturally by a soil microorganism. The presence of these blended polysaccharides in the culture broths of bacteria focuses attention on the effect of the two components on the rhe~~logicalproperties of the broths.

40

Artilio Cesdro

One of the polysaccharides (succinoglycan) has a backbone with four sugars bearing, at position 6 of one glucose residue, a side-chain (composed of four sugars) with a pyruvic ketal linked at position O-4,0-6 on the terminal side-chain glucose. In addition, a succinyl substituent and an 0-acetyl group are present, with a content which may change upon biosynthesis. Several experimental findings consistency show the occurrence of a temperature induced conformational transition on succinoglycan in aqueous solution. The order-to-order transition is detected by a sharp decrease in relative viscosity and in optical activity and appears thermally reversible. The mid-point transition temperature, T,, depends on the salt concentration. The other exopolysaccharide {g~actoglycan) is composed of a simple disaccharide repeat unit, having galactose and glucose with acetyl and pyruvyl substituents (the molar ratio of the four groups is 1 :l: 1 :l ). This structure is undoubtedly more simple and suitable for both molecular modelling and quantitative structure-property correlations. Much of the work on the solution properties and conformation of galactoglucan (Ces&ro et d, 1992b) has constantly given results which can only be interpreted in terms of a randomly disordered chain conformation. The behaviour of the polymer does not show abrupt changes, typical of con-

40

60

Succinoglycan

60 weight

100 fraction,

%

Fig. 7. Storage modulus, G’ at three different frequency values (100 rad/s upper curve, 10 rad/s and 1 rad/ lower curve), as a function of the percentage fraction of succinoglycan produced in the culture broth (total polymer concentration 1% w/v in aqueous 0.1 M NaCI).

Thermodynamics

and rheology of carbohydrate polymers

41

formational transitions, as a function of pH, temperature or ionic strength. The experimental viscoelastic results on the native mixtures of the two polysaccharides, succinoglycan and galactoglycan, show the shear rate dependent behaviour of the solutions always shear-thinning (Navarini et al., 1992), however, the profiles of the flow curves and the storage modulus, G’, change dramatically with the composition of the mixture (Fig. 7). The upper Newtonian plateau is evident for galactoglucan-rich systems and both the zero shear-rate viscosity and the shear rate value, at which the onset of shear thinning behaviour occurs, could be evaluated. The rheological behaviour of succinoglycan-rich samples is again shear thinning, however the steady shear viscosity is strongly shear rate dependent, and exhibits approximate power law behaviour with y over the whole range of shear rate investigated with no evidence of either upper or low Newtonian plateau. From the viscoelastic properties and from their comparison with those observed for xanthan it is suggested that a viscoelastic behaviour close to that of a typical gel-like system originates from the presence of the ordered conformation.

6 CONCLUSIONS The experimental and theoretical data presented in this paper suggest that the conformations of the polysaccharides are responsible for the behaviour of their solution properties. Although the analysis has been limited to dilute and semidilute solutions, the results show the important role of the interactions with the solvent and with other solutes (and ions) in the modification of the conformational state of the polymers. It is claimed, therefore, that a knowledge of all these phenomena at molecular level is a prerequisite for an understanding of the functional properties of polysaccharides in food application.

ACKNOWLEDGEMENTS The author wishes to dedicate this paper to the memory of A. Ciana, who has contributed to the results here quoted. Financial support from the Italian National Research Council (CNR) and the Ministry of Scientific and Technological Research (MURST ), and the technical assistance of Mr J. Cumani are gratefully acknowledged.

42

Attilio Cesaro

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Paoletti, S., Cesaro, A., Delben, F. & Ciana, A. ( 1986). In Recent Advances in the Chemists and junction of Pectins, eds M. L. Fishman & J. J. Jen. American Chemical Society Symposium Series No. 3 1, Chap. 7, pp 73-87. Rees, D. A. ( 1977). Po~sa~cha~de Shapes. Chapman & Hall, London, UK. Talashek, T. & Brant, D. A. (1987). Carbohydr. Res., 160,303. Urbani, R. & Cesaro. A. (1991). Polymer, 32,3013.