Molecular structures in biopolymers sols and gels

The short range molecular structure of ogarose in the sol state and r-carrageenans in the gel state have been investigated by small-angle neutron scattering (0.1< q(nm'l) < 2.5). Agarose in the sol state possesses a rod-like conformation (minimum persistence length of 14 nm) with a mass per unit length consistent with a single helm The existence of a double helix in the gel state is accordingly questioned. Agarose in sols from DMSOWATER blends still exhibits the same rigidity yet the mass per unit length varies with solvent composition. It is surmised that this arises from strong complexation of agarose by DMSO which has consequences on the gelation propensity. Results obtained with K-carrageenans gels are accounted for by an array of rigid rods consisting of two populations of differing cross-sectional radius. INTRODUCTION The present knowledge of the molecular structure of polysaccharides either in the sol state or in the gel state has been gained from polarimetry [1,2] together with X-ray diffraction [3,4]. Polarimetry experiments led to the conclusion that the chains had to behave as random flexible coils in the sol state. During the gelation process chains were thought to rigidify thanks to the formation of double helices that eventually aggregate to produce the final gel state. Strong evidence exists for iotacarrageenan to support the concept of double helices. Yet, in the case of agarose, the paucity of features in the X-ray diffraction pattern has led to a controversy [5] . Recently, Foord and Atkins [6] have suggested that the diffraction pattern can be as well interpreted with single helices. Clearly, additional data are necessary to cast some light on to the molecular structure of these polysaccharides. This is the aim of this note to report on data mainly gained from the sol state that may have a direct bearing on the structure in the gel state. EXPERIMENTAL Data were taken at LLB on PAXE small-angle camera in a q-domain ranging from q = 0.1 nm-' to 2.5 nm-'. To obtain the normalized intensity, IN(@, data were corrected for transmission, thickness and calibrated with an incoherent standard (cis-decalin). The blank sample contained the solvent plus an equivalent amount of protonated water so as to mimic the incoherent signal of the polysaccharide. The absolute intensity of the latter, IA(q), then reads: in which C is the polysaccharide concentration, S(q) its scattering function. K is the calibration tr constant whlch reads: Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993819 100 JOURNAL DE PHYSIQUE IV in which ap and aD are the scattering amplitudes and the molar volumes of the polysaccharide and of the solvent, respectively, 6& and Tdec the thickness and the transmission of the cis-decalin calibration sample, NA the Avogadro's number, g(b) a correction factor, dependent upon the camera and upon the wavelength distribution, which has been presently determined by means of Cotton's method 171, and the molecular weight of the polysaccharide residue. In what follows r 3 l i k e chains or arrays of rod-like structures are dealt with for which the intensity in the present q-range approximates to [8]: in which wi, ri, pi are the weight fraction, the cross-section radius and the mass per unit length, respectively, of the ith rod-like structure. RESULTS AND DISCUSSION