Crystal structure dependent in vitro antioxidant activity of biocompatible calcium gallate MOFs

Two novel 3-D coordination polymers, denoted MIL-155 and MIL-156 (MIL stands for Materials Institute Lavoisier), built up from calcium and the naturally occurring gallic acid (H₄gal), have been hydrothermally synthesized and their crystal structures were determined by single-crystal X-ray diffraction. These solids are based on different inorganic subunits: infinite chains of edge-sharing dimers of CaO₇ polyhedra linked through partially deprotonated gallate ligands (H₂gal^2-) for MIL-155 or [Ca₂(H₂O)(H₂gal)₂]·2H₂O, and ribbon-like inorganic subunits containing both eight-fold or six-fold coordinated Ca^II ions linked through fully deprotonated gallate ligands (gal^4-) for MIL-156 or [Ca₃K₂(H₂O)₂(gal)₂]·nH₂O (n∼ 5). Both solids contain small channels filled with water molecules, with, however no accessible porosity towards N₂ at 77 K. MIL-155 and MIL-156 were proven to be biocompatible, as evidenced by in vitro assays (viability and cell proliferation/death balance). While the high chemical stability of MIL-156 makes it almost bioinert, the progressive degradation of MIL-155 leads to an important protective antioxidant effect, associated with the release of the bioactive gallate ligand.