113Cd-NMR and Fluorescence Studies of the Interactions between Cd(II) and Extracellular Organic Matter Released by Selenastrum capricornutum

113Cd NMR spectra were measured in solution for a series of adducts between the extracellular organic matter (EOM) of the green alga Selenastrum capricornutum and cadmium(II). From the results it appears that EOM forms complexes with Cd(II), which are in a fast exchange in NMR time scale. Thus the observed shift is the molar average of limit values for the exchanging free and bound cadmium species. Definite support for this dynamic stems from an extensive 113Cd NMR equilibrium analysis. Although in principle a multiple binding mode cannot be excluded, our 113Cd NMR findings and the consideration that carbohydrates are the prevailing constituents of algal releasing in stationary growth phase led us to suggest a carbohydrate type coordination. This hypothesis is also supported by NMR studies on model compounds. However, this is probably an oversimplified view of the binding which does not properly account for the fluorescence results. Fluorescence spectroscopy provided further evidence of the EOM binding process also allowing a discrimination between the fluorophoric groups involved. The addition of Cd(II) affected both fluorescence intensity and peak position in the "humic-like" band (between 340 and 400 nm Ex). Finally, the 113Cd NMR and synchronous fluorescence measurements showed a linear correlation between 113Cd chemical shifts and EOM concentration (as fluorescence intensity at 340 and 360 nm). NMR and fluorescence data suggest the existence of structurally different binding sites (carbohydrate-like and humic-like) which appear somehow related. Further, they definitely put in evidence the EOM-Cd interaction together with the specific organic components mostly involved.