Abstract
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.
Original language | English (US) |
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Pages (from-to) | 227-240 |
Number of pages | 14 |
Journal | Journal of Magnetic Resonance (1969) |
Volume | 91 |
Issue number | 2 |
DOIs | |
State | Published - Feb 1 1991 |
Funding
This work has been supported by the USDA (87~CRCR-1-2430 BMH; 87-CRCR-I-2429 JTB), the NSF (DMB 8907559 BMH), and the NIH (GM40067 LM, and also HL13531 BMH). Its inception owes a great debt to the insights and encouragement of Professor W. H. Orme-Johnson. We acknowledge the invaluable technical efforts of Clark Davoust.