Thermoreversible gelation in aqueous binary solvents of chemically modified agarose

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Abstract

The thermoreversible gelation of chemically modified agarose has been studied in aqueous binary solvents (dimethyl sulfoxide and a series of formamide) by differential calorimetry, mechanical testing, and small-angle neutron scattering. The temperature–composition phase diagrams have been established. It is concluded that gelation is promoted by the formation of ternary complexes modified agarose/water/cosolvent, wherein the cosolvent mediates the interaction between chains through the formation of electrostatic interactions.

Introduction

Due to the paucity of the X-ray diffraction pattern on dried and stretched gels of agarose, the molecular structure is still a matter of debate [1], [2]. In particular, whether double or single helices are dealt with remains controversial although recent developments have suggested that single helices could be better candidates [2], [3], [4]. Similarly, the formation of complexes between agarose and the solvent is another question of interest as it has a direct bearing upon the gelation properties of this polymer [5]. Recent results by Ramzi et al. [6] have led these authors to consider the occurrence of ternary complexes when binary solvents, wherein one solvent is water, are used. From this work, these authors conclude that the formation of complexes does not arise from hydrogen bonding but from electrostatic interactions instead. The purpose of this paper is to test further this assumption by using chemically modified agarose samples where hydrogen atoms located on OH groups are replaced by CH3 groups [7] (see Fig. 1). This modification suppresses definitely the possibility for hydrogen bonding while it promotes the polarization of the covalent bonds thus leading to the appearance of fractional electric charges on different atoms.

A deeper knowledge of the gelation of these modified polymer is also of interest for understanding the gelling properties of the agarose extracted from seaweeds of various origin [8], [9], [10]. As a matter of fact, the methyl content varies from one species to another [11], and still remains puzzling despite some thorough investigations.

As in previous studies [6], the experiments have been carried out with dimethyl sulfoxide (DMSO) and a series of formamide (FOR).

Section snippets

Materials

The three chemically modified agarose samples used in this study are of differing degrees of modification: some hydrogens of the hydroxyl groups are replaced at random by CH3 groups. The three samples were kindly provided by Hispanagar (Burgos, Spain). Their molecular weights as estimated by viscometry measurements ([η]=0.07Mv0.72, from Ref. [12]) and their water content were as follows: for M1, Mv=9.87×104 and water content=12.8%; for M2, Mv=1.03×105 and water content=8%; and for M3, Mv=1.02×10

Phase behavior

The thermal behavior has been studied in the four binary mixtures water/cosolvent mentioned above. Special attention has been paid for the system agarose/water/DMSO for which kinetic effects have been assessed. In particular, various ageing times have been investigated (24 h and 1 week) to find out whether this had any effect on the amount of gelled agarose for the sample of least gelling capability, i.e. M3. As can be seen in Fig. 2, the gel melting enthalpy is little dependent upon ageing

Concluding remarks

The results presented in this paper clearly show the role of the cosolvent in the thermoreversible gelation of chemically modified agarose. This cosolvent allows the gelation of a part of the material that could not gel in pure water. As has been discussed, this effect is not due to the special interactions that may exist between water and the cosolvent. Again, the notion of polymer–solvent complex already considered for non-modified agarose is contemplated. A possible model is drawn in Fig. 10

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      Agarose, a neutral and linear polysaccharide generally extracted from red algae including Gracilaria and Gelidiella [1], is composed of repeating units consisted of alternating 1,3-linked β-d-galactose (G) and 1,4-linked α-l-3,6-anhydro-galactose (A) [2,3]. Strong hydrogen bonds, where random coils associate to form single [4,5] and double [6,7] helices, contribute to the formation of the high-mechanical-strength gel [8]. With its beneficial properities of bio-inertness, bio-compatibility, and high mechanical gel strength, agarose is widely applied in DNA electrophoresis, drug delivery, cell therapy, and molecular and tissue engineering [9,10].

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    1

    Permanent address: Faculté des Sciences, Université Mohamed 1er, Oujda 6000, Morocco.

    2

    Permanent address: Rhodia Recherche, 52 rue de la Haie Coq, F-93308 Aubervilliers, France.

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