Majda Sekkal

A spectroscopic investigation of the carrageenans and agar in the 1500-100 cm-1 spectral range

Abstract Fourier transform infrared (FTIR) and Raman spectra in the 1600-100 cm−1 range have been employed in a structural analysis of biopolymers of the polygalactane type. In spite of the complexity of the spectra in this region, precise assignments have been made, first on the basis of previously calculated frequencies of the basic unit ( d -galactose) and secondly by comparing the spectra of kappa-, iota-, and lambda-carrageenans, as well as agar and appropriate disaccharides. The results of this work provide confirmation of previous assignments of IR absorptions at 930, 820, 805 and 845 cm −1 and the interpretation of several other bands, notably those due to bending vibrations of the glycosidic linkages and those between 1040 and 1010 cm−1. The latter are associated with the OSO symmetrical stretch. Assignments are also presented for the 700-100 cm−1 region, on which there are no previous reports. The present analysis may provide a basis for further studies of the conformational changes accompanying gelation, a process which is different from one polygalactane to another.

A vibrational molecular force field of model compounds with biological interest. III: Harmonic dynamics of α : and β-D-galactose in the crystalline state

Abstract The infrared spectra of α- and β- d -galactose were recorded, both in the mid-IR range (4000-500 cm −1 ) and in the far-IR (500-50 cm −1 ). The Raman spectra were also obtained. These spectra constitute the basis of a crystalline-state force field established for these two molecules through a normal coordinate analysis. A modified Urey—Bradley—Shimanouchi force field was combined with an intermolecular potential energy function which includes van der Waals interactions, electrostatic terms and an explicit hydrogen bond function. The force constants were varied, so as to obtain an agreement between the observed vibrational frequencies and the calculated ones of α- d -galactose. The force field obtained was then applied to α- d -galactose O- d 5 and β- d -galactose, in order to test its transferability. The computed potential energy distribution was found to be compatible with previous assignments for d -glucose, particularly for the modes involving C6 and COH groups. For β- d -galactose the same force field was used with changing the force constants due to the C1 and C6 groups.

Force-field and vibrational spectra of oligosaccharides with different glycosidic linkages—Part I. Trehalose dihydrate, sophorose monohydrate and laminaribiose

Abstract The vibrational spectra of the disaccharides, trehalose dihydrate, sophorose monohydrate and laminaribiose, have been recorded in the crystalline state in the 4000-100 cm−1 spectral region for the IR spectra and in the 4000-20 cm−1 spectral range for the Raman spectra. These three disaccharides exhibit the same monosaccharide composition (i.e. glucose residue), but differ in the position and configuration of the glycosidic linkage (α, 1-1; β, 1-2 and β, 1-3 for trehalose, sophorose and laminaribiose, respectively). Most of these spectra have not yet been reported, particularly in the low frequency range. They constitute the basis of theoretical calculations of normal modes of vibration. Normal coordinate analysis has been made in the crystalline state using a modified Urey-Bradley-Shimanouchi intramolecular potential energy combined with a specific intermolecular potential energy function. The force field parameters are transformed from initial works on both anomers of glucose. The vibrational assignments of the observed bands are made on the basis of the potential energy distributions. It appears that the greatest part of the vibrational modes is very highly coupled vibrations. The calculated vibrational frequencies agree very well with the observed frequencies in the whole spectra, particularly in the “fingerprint” regions and in the low frequency range. The bands observed at 733, 773 and 755 cm−1 for trehalose dihydrate, sophorose monohydrate and laminaribiose, respectively, are calculated at 728, 772 and 755 cm−1 and are due to bending modes of heavy atoms involved in the corresponding glycosidic linkage C1O1C′x. Moreover, some known characteristic structural regions may be divided into different parts that have a specific significance. Ale standard deviation between calculated and observed frequencies below 1500 cm−1 leads to values of 3.0, 3.7 and 4.2 cm−1 for the three disaccharides, respectively.

Force field and vibrational spectra of oligosaccharides with different glycosidic linkages—Part II. Maltose monohydrate, cellobiose and gentiobiose

Abstract Complete Raman and IR spectra of maltose monohydrate, cellobiose and gentiobiose have been recorded in the crystalline state. These three disaccharides present the same monosaccharide composition of the glucose molecule and the remaining studied position (1–4 and 1–6) of the glycosidic linkage. Moreover, maltose and cellobiose present the different configurations of the glycosidic linkage α, 1–4 and β, 1–4, respectively. These data will constitute the support for theoretical calculations of normal modes of vibration. The assignments of the calculated bands of vibration will be made on the basis of the potential energy distributions using a modified Urey—Bradley—Shimanouchi intramolecular potential energy combined with a specific intermolecular potential energy function. The calculations show that using a correct initial force field, it is possible to reproduce correctly the density of observed vibrational states for large molecules such as disaccharides. The standard deviation between calculated and observed frequencies, below 1500 cm −1 , leads to values of 4.7, 4.2 and 4.6 cm − for maltose monohydrate, cellobiose and gentiobiose, respectively. Our previous investigations on trehalose dihydrate, sophorose monohydrate and laminaribiose are confirmed in this study and complete the previous assignments for the whole set of disaccharides.

Direct structural identification of polysaccharides from red algae by FTIR microspectrometry I: Localization of agar inGracilaria verrucosa sections

Unlike carrageenans, agars have not been studied very extensively by infrared spectroscopy, in so far as the structures of this kind of polygalactanes are not as well defined as carrageenans. However, in a previous work we have carried out a vibrational analysis of both carrageenans and agars and some important assignments of the main absorptions have been made. Consequently, the present work has been undertaken in order to identify agars without any extraction directly in various seaweeds using the infrared microspectrometry method. The main advantage of this method is that the sample consists only of a dehydrated algal section. The red algaeGradlaria verrucosa has been the subject of the present study. In the first place, spectra of extracted agars were recorded, as they can help us to confirm the nature of the compound identified by this technique. In a second stage, spectra of different parts of the sections have been carried out. The comparison between the resulting spectra with those of the extracted polysaccharides, has demonstrated, firstly that the best results are obtained from the cortical area, because, as expected, the agar is mainly located in the cell wall of this area of the algae. Indeed, the feature bands of agars are all observed, especially the intense ones between 1000 and 1100 cm−1 and the more characteristic absorptions in the wavenumbers range below 1000 cm−1 so as the ones at 988, 965, 930, 890, 870, 771 and 741 cm−1. Secondly, it may be also identified in smaller amounts in the medullar area, the cells are greater than in the cortical area and the cytoplasm is preponderent. However, in the latter case a coexisting polysaccharide, present in a considerable quantity and called floridean starch (Its structure is not very well known, as it varies from one algae to another), masks the spectra of agar, as its spectrum is very similar to those of polygalactanes.

The use of FTIR microspectrometry as a new tool for the identification in situ of polygalactanes in red seaweeds

FTIR microspectrometry method has been applied on sections of different red algae, their cell wall is mainly constituted by polygalactanes such as agar and carrageenans. The former has previously been identified in the red algae Gracilaria verrucosa, whereas in Dilsea carnosa, Chondrus crispus and Hypnea musciformis, different kinds of carrageenans were present. The comparison of the obtained spectra with those previously reported for the extracts from the same seaweeds allowed the confirmation of the nature of each carrageenan. For example, in the obtained infrared spectrum from the cell wall of Chondrus crispus, an intense band at about 1250cm−1 indicates the presence of considerable amounts of sulfate groups, likewise the band at 845 cm−1 which is characteristic of the C-O-S vibration demonstrates that the half ester-sulfate group is substituted on the C4·of the 1,3 linked galactopyranosyl. Thus it was concluded that κ-carrageenan is the main constituent of this algae.

Direct structural identification of polysaccharides from red algae by FTIR microspectrometry II: Identification of the constituents ofGracilaria verrucosa

The use of the infrared microspectrometry analytical technique as a new tool for the identification of the polysaccharides contained in the red algaeGracilaria verrucosa has demonstrated that in addition to agar spectra, features of the other coexisting constituents can also be obtained. Indeed, the infrared spectra recorded previously, all exhibit two important bands at about 1645 and 1530cm−1. These two bands were not present in the infrared spectra of the extracted agars and they are expected to be due to the amide I and amide II protein vibrations. In order to confirm this supposition, we have applied some enzymatic treatments, firstly on the whole algae and secondly on the ground algae (the algae has been previously depigmented and then dehydrated). Agarase, xylanase and cellulase were successively carried out on the algae. The last resulting spectrum, i.e. the spectrum obtained from the fraction which has undergone the three treatments, has been identified to be characteristic of proteins. This spectrum contained, both the amide I and II vibrations and in addition, weak absorption at 1230 cm−1 due probably to the amide III, was observed. Additional weak bands in the 1400–1300 cm−1 due to the different skeletal modes of the proteins were also present in this spectrum.The infrared spectra also revealed that the use of the enzymatic treatments on the ground algae is more efficient than when it is carried out on the whole algae.

Vibrational analyses of neoagarose and neocarrabiose oligomers

The structural characterization of five oligomers from the carrageenan family and three oligomers of agarose, has been the aim of the present work. The compounds were chosen so as to study principally the effect of the substitution by a sulphate group on the two main vibrations due to the two glycosidic linkages (the (alpha) 1,3 and the (beta) -1,4). The C-O stretching modes of the two quoted glycosidic linkages have previously been identified in the spectral region between 1120 and 1160 cm-1, whereas the C-O-C bending mode has been demonstrated to occur at about 730 cm-1.

Dynamic study on the mechanism of gelation of carrageenans by ATR-FTIR spectroscopy at variable temperature

The high gelling ability of polygalactanes is not only due to their tertiary structure in double helices but also to the formation of aggregates. This step, which has been demonstrated to be very important in the gelation process, depends on the adjunction of cations in aqueous solutions of polysaccharides. However, the gel strength varies with the nature and the concentrations of the cations. In this work, we study solutions of kappa and iota carrageenans in the presence of Li+, Na+, K+ by ATR-FTIR spectroscopy between the room temperature and 80 degree(s)C. Differences due to the interactions of cations with polysaccharides are interpreted from the variations of the sulphate groups and the glycosidic linkages vibrations.

Direct structural characterization of agar on red algae by FTIR microspectrometry

FTIR microspectrometry is used to control the quality of agar directly in algae microtom cuts. The gelling ability which defines the quality of this polysaccharide depends on the degree of the galactose unit. FTIR microspectrometry is a powerful tool which responds to our needs because we must analyze small samples (diameter of about 10 micrometers ) coming directly from algae or little quantities of extracted agar. However, the presence of components other than agar in the algae forces us to include other chemical and theoretical methods to achieve our purpose.