Complexation of europium with chondroitin sulfate

Glycosaminoglycans (GAGs) are linear polysaccharides and highly negatively charged. GAGs are part of proteoglycans which are major components of the extracellular matrix. They are involved in binding cations (such as sodium, potassium and calcium) and water, and also regulating the movement of molecules through the matrix. Individual functions of proteoglycans can be attributed to either the protein core or the attached GAG chain. The GAG family consists of heparin/heparan sulfate, chondroitin sulfate (CS) and dermatan sulfate (DS). CS is composed of the disaccharide unit N-acetylgalactosamine (D-GalNAc) and D-glucuronic acid which can be sulfated at the C4 and C6 of GalNAc (CS4S and CS6S). DS is defined by presence of L-iduronic acid residues and is always sulfated at C4 at the GalNAc. The ability of the lanthanide ions, like Europium (Eu), which show luminescence properties, allowed studying the binding behavior of GAGs.
The behavior of the complex formation of Eu3+/GAGs was analyzed under physiological conditions by several experimental methods such as time-resolved laser-induced fluorescence spectroscopy (TRLFS) and infrared spectroscopy (ATR-FT-IR), supplemented by theoretical calculations of the possible structures and resulting spectra.
All three GAGs (CS4S, CS6S and DS) caused an increase in luminescence intensity of the hypersensitive 7F2 emission band of Eu3+ due to complex formation, which was more pronounced for CS4S and DS compared to CS6S. The luminescence lifetimes increased with CS4S and DS up to 200-300 µs, corresponding to 2-4 remaining H2O molecules in the first coordination shell of Eu3+. With CS6S, the luminescence lifetime was even more prolonged up to ~650 µs (~1 remaining H2O).
FT-IR showed that the binding of GAGs to Eu3+ occurs not only via the carboxyl groups but also via the sulfate groups.
Even though the coordination behavior of GAGs towards Eu3+ is in general quite similar, particular differences could be identified: GAGs with C4 sulfation seem to be stronger ligands, whereas C6 sulfation seems to be sterically more ambitious since it can replace more H2O molecules from the first spherical coordination shell of Eu3+ than C4 sulfation.