Iota karagenan jellerinde şişme kinetiği: Floresans çalışma.

Carrageenans are water soluble polysaccharides extracted from red algae. Their basic structure consists of a repeating unit of 3,6- anhydro--Dgalactose and  -D-galactose. They are differentiated by the number and the position of ester sulphate groups and the amount of 3,6-anhydro-D-galactose which they contain. They come in three major types designated by means of Greek letters as kappa (-), iota (-) and lambda (-). They are well known for their gel forming properties and are used extensively in food and pharmaceutical industry as gelling or thickening agents. Kappa carrageenan and iotacarrageenan undergo a temperature dependent coil (disordered state) to helix (ordered state) transition in aqueous solution. -carrageenan usually forms firm, brittle gels and it is sensitive to potassium ions. -carrageenan usually generates soft, elastic gels and it is sensitive to calcium ions. Iota carrageenan is the most highly sulphated of the helix-forming polysaccharides and is also the most fully characterized in the solid state and in solution (Morris etal., 1980). It is well known that iota carrageenan forms a three dimensional network constituted of polysaccharide chains structured as double helices. This gelation is induced by cooling heated suspensions of the polysaccharide in water under appropriate salt conditions resulting in a coil-helix conformational transition (Morris etal., 1980; Tako etal., 1987). It has been known that in solution, iota carrageenan can reversibly transform from an ordered to a disordered conformation. Naturally at high ionic strength and low temperature iota carrageenan forms an ordered state. On heating, the helices dissolve and the ι- carrageenan forms a random coil conformation (Rees etal., 1982). Intermolecular double helix formation should result in a doubling in the observed molecular weight of the ι-carrageenan, which has been observed in many groups (Austen etal., 1985; Morris etal., 1980). However some authors have proposed monomolecular single-helix formations (Norton etal., 1983). The swelling shrinking and drying kinetics of physical gels are important in many technological applications. Especially in pharmaceutical industries, in designing controlled release of drugs and in using cosmetic ingredients, understanding the kinetics of gels is highly desirable. The knowledge of the gel kinetics is an important requirement for producing storable foods in agricultural industry and developing artificial organs in medical applications. Many different experimental techniques have been used to study the kinetics of swelling and shrinking of chemical and physical gels such as neutron scattering (Bastide etal., 1984), quasielastic lightscattering (Peters and Candau, 1988), macroscopic experiments (Zrinyi etal., 1993), in situ interferometric measurements (Wu and Yan, 1987) and photon transmission techniques (Kara and Pekcan ,2000). A different method was employed namely the steady state fluorescence technique, to study the swelling of iota carrageenan gels. The fluorescence technique is efficiently used for analyzing the structural properties of gels. This technique ensures the fluorescence molecules added in a structure to be considerably sensitive to the structural properties of a system and provide information about the medium in which they are located. In this work, we studied the swelling process of - carrageenan gels at various temperatures by using steady state fluorescence technique. The fluorescence intensity measurements were carried out with Perkin-Elmer LS-50 model spectrometer equipped with a temperature controller .Pyranine was embedded in -carrageenan and used as a fluorescence probe. -carrageenan gels were completely dried and then swelled in water vapor. The swelling experiments were performed at 30, 40, 50 and 60oC. At each temperature, the disc-shaped gel samples were placed in the wallof a 1x1x4.5cm3 quartz cell saturatedwith vapor for the swelling experiments. Scattered light intensities, Isc and fluorescence intensities, I were monitored during the swelling of - carrageenan gels. The scattered light intensity, Isc decreased, however the fluorescence intensity increased exponentially as swelling time is increased. The increase in I was modeled using Li-Tanaka equation from which selling time constants, c and cooperative diffusion coefficients, Dc were determined. It was observed that swelling time constants, c decreased and cooperative diffusion coefficients, Dc increased as the swelling temperature was increased from 30 to 60oC. The swelling energies, E were obtained by using Arrhenius relation and found to be 63.1kJmol-1. [ABSTRACT FROM AUTHOR]