Molecular Platforms in Metal-Organic Frameworks for the Chalcogen Atom Study of Enzyme-Mimicking Radical Scavenging

The overproduction of reactive oxygen and nitrogen species (RONS) is tightly related to various diseases. Previous radical scavenging agents usually focus on biomaterials or noble metals, suffering from problems of stability and cost. Few nonmetallic agents have been reported for the RONS scavenging with enzyme-like activity. Herein, a series of three isoreticular metal-organic frameworks (MOFs) are specifically designed based on the UiO-68-type structure, in which a molecular platform derived from a rigid N-heterocycle has been attached to the backbone of MOFs by mimicking the structure of a Se-based antioxidative enzyme termed as selenoneine. Precise manipulation of three chalcogen atoms (O, S, and Se) has been successfully achieved in three similar N-heterocycles, that is, phenoxazine (PXZ), phenothiazine (PTZ), and phenoselenazine (PSZ). Two MOFs in this series exhibited excellent elimination efficiencies in radical solutions of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) and 2,2 '-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), that is, 80% clearance of PTIO radical in 2 h by UiO-68-PTZ and nearly 100% clearance of ABTS<middle dot>+ in 5 min by UiO-68-PSZ. The scavenging mechanisms of PTIO and ABTS<middle dot>+ have been further revealed and demonstrated by experiments and theoretical calculations, in which the S site and Se site can selectively scavenge the radicals through binding interaction with the PTIO radical or electron transfer with ABTS<middle dot>+.