Formation of incubated marine mucilages investigated by FTIR and UV–VIS spectroscopy and supported by two-dimensional correlation analysis

Abstract

This study describes some specific aspects of the mechanisms of chemical reactions involved in the formation of marine mucilage aggregates obtained from several living organisms incubated in laboratory under static conditions in absence of water and air circulation. After appearance, the synthetic mucilages were sampled daily during the evolution of the aggregation process and for each sample infrared (FTIR) and ultraviolet visible (UV–VIS) spectra were collected. Then the spectra were examined by two-dimensional correlation analysis (2DCORR), a powerful tool for the study of the dynamic structural and compositional changes of a molecular system caused by any possible external perturbation. This approach showed specific peculiarities concerning the role played by each class of biomolecules and metals present in the aggregation of organic matter leading to mucilages. In more detail, synchronous 2DCORR applied to FTIR spectra showed that the hydrogen bond interactions played mainly by hydroxyl groups of carbohydrates and proteins, were a constant factor present in all the steps of the aggregation process, even more significant than polymerization reactions involving mono and oligosaccharides, aminoacids and oligopeptides. Disrelation 2DCORR spectra suggested that lipids such as fatty and ester fatty acids played a different role during aggregation. This different role of lipids may consist of bridging small aggregates by means of micelle formation (i.e., the so called cross-linking role) whereas their interaction with metals was negligible. The role played by metals in the aggregation of mucilages was well described by the joint use of FTIR and UV–VIS spectra for two-dimensional correlation analysis which in this case, is called heterogeneous two-dimensional correlation analysis (2HDCORR), ([Noda, I., Ozaki, Y. Two-Dimensional Correlation Spectroscopy, Application in vibrational and optical spectroscopy, J. Wiley & Sons, Chichester UK, 2004.]). This approach showed that in presence of high concentrations of bivalent cations (Cu, Pb) and As in seawater, negatively charged polar groups of carbohydrates and proteins interact with metals to form aggregates whereas lipids do not interact with metals; in addition, interactions of lipids with polar groups of carbohydrates and proteins resulted modified or reduced. The use of two-dimensional correlation analysis for the elaboration of FTIR and UV–VIS spectra of synthetic mucilages support the evidence concerning the different roles played by carbohydrates proteins, lipids and metals during the aggregation of natural soil organic matter (SOM) and mucilages.