The auto-orientation in high magnetic fields of oxidized cytochrome b562 as source of constraints for solution structure determination

N-15-H-1 (1)J couplings were measured at 500 MHz and 800 MHz for N-15 enriched oxidized cytochrome b(562) from E. coli. The magnetic field dependence of 70 (1)J values, which could be measured without signal overlap, shows that there is a molecular magnetic anisotropy which provides partial molecular orientation in the magnetic field and, consequently, residual dipolar couplings (rdc). The rdc were used as further constraints to improve the existing structure [Arnesano et al. (1999) Biochemistry, 38, 8657-8670] with a protocol which uses the rhombic anisotropy [Banci et al. (1998) J. Am. Chem. Soc., 120, 12903-12909]. The overall large molecular magnetic anisotropy has been found to be determined by both the low spin iron (III) and the four helix bundle structure magnetic susceptibility anisotropy contributions.