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 (H2gal2−) for MIL-155 or [Ca2(H2O)(H2gal)2]·2H2O, and ribbon-like inorganic subunits containing both eight-fold or six-fold coordinated CaII ions linked through fully deprotonated gallate ligands (gal4−) 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.