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

15N-1H 1J couplings were measured at 500 MHz and 800 MHz for 15N enriched oxidized cytochrome b562 from E. coli. The magnetic field dependence of 70 1J 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. Ctherz. 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.