Behaviour Of Amphiphilic Polysaccharides in Aqueous Medium

www.medigraphic.org.mx junio, 2008 Rinaudo, M.: Behaviour of amphiphilic polysaccharides in aqueous medium 35 ARTÍCULO ORIGINAL D.R. © TIP Revista Especializada en Ciencias Químico-Biológicas, 11(1):35-40, 2008 BEHA VIOUR EHAVIOUR OF AMPHIPHILIC POL YSACCHARIDES POLYSACCHARIDES IN AQUEOUS MEDIUM Marguerite Rinaudo Centre de Recherches sur les Macromolécules Végétales (CERMAV-CNRS) affiliated with Joseph Fourier University, BP53, 38041 Grenoble cedex 9 (France). E-mail: A BSTRACT This paper describes the rheological and thermal behaviour of alkylated chitosan and hyaluronan derivatives and methylcellulose, Their behaviour is characterized by the formation of hydrophobic domains which may be considered as crosslinked points. Then, depending on the thermodynamic conditions (temperature, salt concentration), an aqueous solution transforms to a 3D-physical network formed on the basis of hydrophobic cooperative interactions. Key Words: Alkylated chitosan, alkylated hyaluronan, amphiphilic polysaccharides, methylcellulose, rheological behaviour. RESUMEN Este trabajo describe el comportamiento reológico y térmico de quitosano y hialuronano alquilados y de metilcelulosa. Dicho comportamiento está caracterizado por la formación de dominios hidrofóbicos que pueden considerarse como puntos de entrecruzamiento. Por tanto, en dependencia de las condiciones termodinámicas (temperatura, salinidad), las disoluciones acuosas pueden transformarse en una red física tridimensional basada en interacciones cooperativas de carácter hidrofóbico. Palabras Clave: Quitosano alquilado, hialuronano alquilado, polisacáridos anfifílicos, metilcelulosa, comportamiento reológico. I NTRODUCTION I n this paper, different amphiphilic polysaccharides based on chitin, cellulose and hyaluronan are described and few of their properties are given. First, chitin is a poly (β(1-4)-N-acetyl-D-glucosamine); it is a natural polysaccharide of major importance, first identified in 1884 (Figure 1a). This biopolymer is synthesized by an enormous number of living organisms (schrimps, crabs…).1 As many polysaccharides, chitin has some hydrophobicity and in aqueous NaOH it can form a lower critical solution temperature (LCST) solution with a critical temperature around 30°C.2 This characteristic temperature is very important to characterize the influence of temperature on the behaviour of a polymer in solution: at a temperature lower than the LCST, the polymer is soluble and one gets one phase; over the LCST, two phases Nota: Artículo recibido el 02 de mayo de 2008 y aceptado el 27 de mayo de 2008. separate corresponding to a concentrated and a diluted solutions at equilibrium. After deacetylation in alkaline conditions, one gets chitosan which becomes soluble in aqueous acid solution due to the protonation of the –NH2 group at C-2 position of the D-glucosamine. The different chitosans are characterized by their average number of acetyl groups per monomeric unit (degree of acetylation,DA)(Figure 1a). Using the reductive amination, a series of amphiphilic chitosan derivatives were produced from aldhehydes with different chain lengths (Cn from 3 to 14) at controlled DS (usually lower than 10% to maintain water solubility in acidic conditions)3,4. Alkylated chitosans with good solubility in acidic conditions (pH < 6) have a number of very interesting properties. Hyaluronic acid (HA), a linear polysaccharide composed of repeating disaccharide units of N-acetyl-D-glucosamine and Dglucuronic acid (Figure 1b), is the only non sulfated glycosaminoglycan found in the extracellular matrix. HA is an 36 TIP Rev.Esp.Cienc.Quím.Biol. Vol. 11, No. 1 (a) OR OR RO O O RO OR O OR O n Figure 2. Chemical structure of cellulosic backbone. R=H cellulose; R=H or CH 3 in methylcellulose. (b) Figure 1.Chemical structure of the repeat unit of chitin (when DA>0.9) and chitosan (when DA<0.5) (a) and hyaluronan (b). attractive polymer for the development of new biocompatible materials with many applications in viscosupplementation, tissue engineering, and drug delivery5,6. This biopolymer has been the subject of various chemical modifications leading for example to chemically cross-linked materials with successful application in the treatment of osteoarthritis and HA-drug conjugates having controlled-release and cell-targeted properties7, 8. We have recently prepared new water-soluble alkylated derivatives of hyaluronan (HA-C10).9,10 The third polymer examined is the methylcellulose; this derivative of cellulose is a LCST polymer (the LCST is located at 29±2°C) which gives a strong physical gel on heating over 50°C 11. It is often used in food applications. In this paper, it is intended to summarize the behaviour of these three systems we have previously studied and which are based on polysaccharide derivatives. We intend to show the original properties that such amphiphilic copolymer –like systems which can be considered as smart materials being thermosensitive, salt and /or pH-sensitive. EXPERIMENTAL Methylcellulose used is a commercial sample (Methocel A4M Premium from Dow Chemical; the average degree of substitution was 1.8) (Figure 2). All the solutions tested were prepared by direct dissolution at ambient temperature in 0.1M NaCl solution and fixed at 6g/L. Alkyl-chitosans were prepared by Rinaudo et al. 4; they are dissolved in acidic conditions (acetic acid 0.3M) in the presence or the absence of an external salt (sodium acetate). Alkyl-hyaluronan under its sodium salt form is dissolved in water; NaCl is added when necessary to get a 0.15 M NaCl salt concentration, reflecting the biological conditions. HA is produced from a bacterial strain by ARD Cy (Pomacle, France). The dynamic rheological measurements were performed on a AR 1000 from TA Instruments using a plate-cone geometry; the cone has 4 cm as diameter and 3 ° 59’angle. The temperature was controlled by the Pelletier plane in the range of 20 or 37°C up to 65 or 70°C with a controlled rate of temperature variation (3°/min for methylcellulose). The strain percent was selected to be in the linear domain. A film of silicone oil was carefully disposes on the examined solution such as to prevent its evaporation. DSC experiments were performed on methylcellulose with a microcalorimeter DSCIII (SETARAM, France) in the same range of temperature as adopted for rheological measurements and with the same solutions but with a rate of temperature change of 0.5 °/min for heating and cooling cycles. R ESULTS AND DISCUSSION A- Alkylated chitosans Alkyl-chitosans were prepared by reductive amination on the free –NH2 in C-2 position of the D-glucosamine units with a degree of substitution around 0.05 to preserve the water solubility in acidic conditions. They were described previously in our laboratory3,4. There are two important effects of the grafting of alkyl-chains on chitosan as soon as the alkyl-chain length is large enough (carbon chain n = 10 or better 12): (...truncated)