Structure and Reversible Pyran Formation in Molybdenum Pyranopterin Dithiolene Models of the Molybdenum Cofactor

The syntheses and X-ray structures of two molybdenum pyrano­pterin dithiolene complexes in biologically relevant Mo­(4+) and Mo­(5+) states are reported. Crystallography reveals that these complexes possess a pyran ring formed through a spontaneous cyclization reaction of a dithiolene side-chain hydroxyl group at a CN bond of the pterin. NMR data on the Mo­(4+) complex suggest that a reversible pyran ring cyclization occurs in solution. These results provide experimental evidence that the pyrano­pterin dithiolene ligand in molybdenum and tungsten enzymes could participate in catalysis through dynamic processes modulated by the protein.