DIRECT STRUCTURAL IDENTIFICATION OF POLYSACCHARIDES FROM RED ALGAE BYFTIR MICROSPECTROMETRY .1. LOCALIZATION OF AGAR IN GRACILARIA-VERRUCOSA SECTIONS

Unlike carrageenans, agars have not been studied very extensively by i nfrared spectroscopy, in so far as the structures of this kind of poly galactanes are not as well defined as carrageenans. However, in a prev ious work we have carried out a vibrational analysis of both carrageen ans and agars and some important assignments of the main absorptions h ave been made. Consequently, the present work has been undertaken in o rder to identify agars without any extraction directly in various seaw eeds using the infrared microspectrometry method. The main advantage o f this method is that the sample consists only of a dehydrated algal s ection. The red algae Gracilaria verrucosa has been the subject of the present study. In the first place, spectra of extracted agars were re corded, as they can help us to confirm the nature of the compound iden tified by this technique. In a second stage, spectra of different part s of the sections have been carried out. The comparison between the re sulting spectra with those of the extracted polysaccharides, has demon strated, firstly that the best results are obtained from the cortical area, because, as expected, the agar is mainly located in the cell wal l of this area of the algae. Indeed, the feature bands of agars are al l observed, especially the intense ones between 1000 and 1100 cm-1 and the more characteristic absorptions in the wavenumbers range below 10 00 cm-1 so as the ones at 988, 965, 930, 890, 870, 771 and 741 cm-1. S econdly, it may be also identified in smaller amounts in the medullar area, the cells are greater than in the cortical area and the cytoplas m is preponderent. However, in the latter case a coexisting polysaccha ride, present in a considerable quantity and called floridean starch ( Its structure is not very well known, as it varies from one algae to a nother), masks the spectra of agar, as its spectrum is very similar to those of polygalactanes.