Journal of Applied Phycology 10: 315–322, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 315 Cell wall polysaccharides from Gelidium species: physico-chemical studies using MRI techniques Erminio Murano 1,2,∗ , Vladim´ ır Jell´ uš 3 , Alessandro Piras 4 & Renato Toffanin 1 1 POLY-bi´ os Research Centre and 2 POLY-tech Scrl, Area Science Park, Padriciano 99, I-34012 Trieste, Italy 3 Institute of Measurement Science, Slovak Academy of Sciences, D´ ubravsk´ a cesta 9, Sk-842 19 Bratislava, Slo- vakia 4 Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy ( ∗ Author for correspondence) Received 1 December 1997; revised 2 April 1998; accepted 4 April 1998 Key words: Gelidium crinale, Gelidium floridanum, Gelidium pusillum, Gelidium serrulatum, agar, gel strength, magnetic resonance imaging (MRI), magnetic resonance microscopy (MRM), relaxation, magnetisation transfer Abstract Magnetic resonance imaging (MRI) has already been successively used to investigate polysaccharide matrices. In particular, MRI at microscopic resolution (MR microscopy) is now one of the most powerful techniques for studying the physical properties of natural hydrogels. To contribute to a better understanding of the correlation between chemical and physical properties of agar gels, we report here the measurement of the water magnetic parameters for agar gels extracted from different species of Gelidium: T 1 and T 2 relaxation times, magnetisation transfer (M s /M 0 ) and diffusion (D) were measured to evaluate their use for studying the gel characteristics. MR microscopic images were acquired at 7.05 Tesla using various pulse sequences. The results obtained confirmed the possibility to use quantitative MRI for the characterisation of physical parameters correlated with the type of agar chemical structure. In particular, T 2 data obtained for gels at different concentrations indicate that this magnetic parameter is very sensitive to the agar concentration and hence particularly useful for the gel strength determination. Introduction The genus Gelidium is an excellent source of agar and a major part of the world’s annual exploitation of agarophytes (Jensen, 1993; Armisen, 1995). Agar polymers constituting the cell wall matrix of Gelidium have achieved great importance because of their in- dispensable ion-independent thermoreversible gelling properties in many applications in food, medicine and biotechnology (Armisen & Galatas, 1987; Renn, 1990). The basic structure of agar consists of strictly alternating 3-O-linked β -D-galactopyranose and 4-O- linked 3,6-anhydro-α-L-galactopyranose. Such struc- tural regularity may be altered in a number of ways by the presence of sulphate, methoxyl and pyruvate groups (Craigie, 1990). Agar and agarose, the fraction with the greatest gelling ability, have been studied by means of optical activity (Morris et al., 1986), X-ray diffraction (Arnott et al., 1974) light and neutron scat- tering (Guenet et al., 1993), spectrophotometry (Vento et al., 1979), rheology (Watase & Nishinari, 1983) and nuclear magnetic resonance (NMR) spectroscopy (Usov, 1984; Lahaye et al., 1989; Gordon-Mills et al., 1990). Investigation of the gelling mechanism in such polysaccharide solutions and gels have shown that NMR is an effective tool for studying these biopoly- mers (Rees, 1969; Gamini et al., 1997). In particular, proton NMR relaxation of water can be used as a probe of the dynamic properties of polysaccharide systems (Hills et al., 1991). In the past, magnetic resonance imaging (MRI) has been used to map the spatial distribution of struc- ja550.tex; 17/10/1998; 10:06; p.1 Article: JA550 Pips nr. 168093 (japhkap:bio2fam) v.1.1