The molybdenum-iron (MoFe) protein of nitrogenase from Clostridium pasteurianum forms monoclinic crystals in space group P21. The unit cell comprises two molecules, each of which possesses a molecular twofold symmetry axis and binds two chemically identical molybdenum-iron cofactors (M centers). Thus, there are four magnetically distinct M centers within the unit cell related by two different symmetry operations: a molecular twofold axis of symmetry in the bc ∗ plane relates the two EPR centers within a molecule; a crystallographic twofold screw axis of symmetry along the b axis of the unit cell relates the two different molecules in a unit cell. Single-crystal EPR studies at X-band (9.5 GHz) and Q-band (35 GHz) microwave frequencies have been employed to determine g tensors in the crystallographic frame for the four magnetically distinct M centers. Determination of these tensors has been achieved by a novel procedure that relies heavily on the symmetry relations between sites; it rests primarily on measurements that only involve rotation of the magnetic field parallel to a single plane of the crystal and uses g values from frozen-solution samples instead of additional rotations. Single-crystal 1H ENDOR spectra of the M centers are presented.
