Conformation of a low methoxyl citrus pectin in aqueous solution

Conformation of a low methoxyl citrus pectin in aqueous solution

Food Hydrocolloids vol. I no.5/6 pp.569-570, 1987 Conformation of a low methoxyl citrus pectin in aqueous solution M.A.V.Axe1os, J.Lefebvre and J.F.T...

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Food Hydrocolloids vol. I no.5/6 pp.569-570, 1987

Conformation of a low methoxyl citrus pectin in aqueous solution M.A.V.Axe1os, J.Lefebvre and J.F.Thibault l

Laboratoire de Physicochimie des Macromolecules and JLaboratoire de Biochimie et Technologie des Glucides, INRA rue de la Geraudiere 44072, Nantes Cedex 03, France Abstract. Viscosimetric measurements, small angle neutron scattering, light scattering and photon correlation spectroscopy have been performed on a 38% esterified citrus pectin in O.I M NaCI pH = 7 aqueous solution. The citrus pectin can be visualizedas a semiflexible moleculewith a persistence length of 10monomers and a ratio of contour length on persistence length of 200. In dilute solutions the pectin behaves as a nondraining coil with radius of gyration of 320 A. The overlap concentration c* was found to be such that c* [lj] = 0.8 and the intrinsic viscosity showed a 0.62 power dependence on the molecular weight.

The citrus pectin studied here was supplied by the Copenhagen Pectin Factory. After purification (1) and alkali de-esterification (2), chemical analysis (2) indicated a 78 % content in anhydrogalacturonic acid, a degree of esterification of 38 % and a neutral sugar content of 4 %. The rhamnose represented less than 1 % molar ratio. All the pectin samples were dissolved in 0.1 M NaCI and the pH was adjusted to 7 with 0.1 M NaOH. Viscosimetric measurements were carried out on a low shear co-axial viscometer, in a concentration range of 0.6- 24 gil. Intrinsic viscosity [rll and the Huggins coefficient Kh were measured at low concentration. Static light scattering and photon correlation spectroscopy intensities measured by means of a Malvern autocorrelator used in conjunction with a helium -neon ion laser provided determination of the single chain molecular weight, M" and hydrodynamic radius Ri; Small angle neutron scattering experiments were performed at CEN-Saclay, Laboratoire Leon Brillouin. From the scattering curves the persistence length L p , the contour length L and the mass of a monomer of the pectin chain could be calculated. The experimental results are listed in Table I. From neutron scattering data a mol. wt per monomer of 182 and a contour length of 2000 A were found in agreement with the expected values. Table I.

[lj](g/l) 0.377

0041

3.8 X 105

275

45

The high value of the ratio LI Lp indicates that the chain behaves like a flexible coil. The good agreement between the different values of the radius of gyration Rg , estimated from Lp (Rg = 367 A) and the one calculated from [1)] (R g = 320 A) (3), indicates that the pectin, under the given conditions, can be described as a non-draining flexible coil. The fact that light scattering results lead to larger structural values is ascribed to a small number of aggregates in the solution. © IRL Press Limited, Oxford, England

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M.A.V.Axelos, J.Lefebvre and J.F.Thibault

Using the value of Rg , the overlap concentration has been evaluated according to Graessley (4), and from the data of the relative viscosity versus clc* (5) we found the following relationship between [77] and M; [1]] == M,0.62. This exponent confirms the flexible coil behavior of the pectin chain. Conclusion The consistency between structural parameters and hydrodynamic data allow a reliable description of the pectin chain. The investigation ofthe pectin conformation in aqueous solutions is in progress. Using sharp molecular weight fractions of pectin it will be possible to find more precise correlation between the different structural parameters. Thus the coil expansion could be related to the degree of esterification and the ionic strength. References I. Jones,J.K.N. and Stoodley,R.J. (1965) In Whistler,R.L. (ed.), Methods ill Carbohydrate Chemistry.

Academic Press, New York, Vol. Y, pp. 36-38. 2. Rouau,X. and Thibault,J.F. (1984) Carbohydr. Polymers, 4,111-115. 3. Aharoni,S.M. (1987) Macromolecules, 20, 877-884. 4. Graessley,W.W. (1980) Polymer, 21, 258-262. 5. Lefebvre,J. (1982) Rheologica Acta, 21, 620-625.

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