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The fumarate reductase of Escherichia coli and other bacteria is a membrane-bound enzyme consisting of four subunits. A soluble periplasmic 64 kDa tetrahaem flavocytochrome c3 from Shewanella frigidimarina NCIMB400 which possesses a catalytic fumarate reductase activity has been crystallized. The crystals belong to space group P212121 with unit-cell parameters a = 72.4, b = 110.1, c = 230.2 A. Assuming a molecular dimer in the asymmetric unit, the crystals contain 65% solvent and, when cryocooled to 100 K, the crystals diffract to at least 3.0 A resolution. The crystals, however, display an inherent lack of isomorphism and the plausibility of a MAD phasing experiment has therefore been investigated by measuring the iron K absorption edge from a single crystal.

3-Hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone (1) exists mainly in the quinoid form with a small contribution from the aromatic zwitterion form, while 3-hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone hydrochloride (2) and 1-ethyl-3-hydroxy-2-methyl-4(1H)-pyridinone hydrochloride hydrate (3) are observed to occur in the aromatic form with a minor contribution from the quinoid resonance form. Different substituents at the ring N position do not have significant geometric effects in either the neutral molecules, as a group, or in the hydrochloride salts.

The structural results clearly indicate that 3-[1-(ethylamino)ethylidene]-6-methyl-3H-pyran-2,4-dione exists as a keto-enamine tautomer in the solid state. The H atom bonded to N(9) refines with a normal temperature factor and the bond distance of the keto group [C(4)--O(13)] of 1.263 (1) A is elongated due to resonance. This resonance is also indicated by an averaging of the single bonds [C(2)--C(3), C(3)--C(4), C(4)--C(5) and C(8)--N(9)] and double bonds [C(3)--C(8) and C(4)--O(13)] in the crystal structure and by the fact that not only is the pyran ring planar [r.m.s. deviation 0.0212 (9) A] but also there is planarity for the whole molecule [r.m.s. deviation 0.074 (1) A]. A strong intramolecular hydrogen bond is found between the 4-oxo group and the amine H atom.