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.