NMR determination of the magnetic susceptibility anisotropy of cytochrome c ' of Rhodobacter capsulatus by (1)J(HN) dipolar coupling constants measurement: Characterization of its monomeric state in solution

(1)J(NH) splittings have been measured at 600 and 400 MHz on the paramagnet ic (S = 2 and S = 5/2) and on the diamagnetic state (S = 0) of cytochrome c ' of Rhodobacter capsulatus. Residual dipolar splittings and the magnetic s usceptibility tensor have been then derived using two methods. The first me thod equaled for a paramagnetic species the dipolar splitting to the differ ence of the measured (1)J(NH) splittings between the paramagnetic and diama gnetic forms at one field. The second method consisted in deriving the tota l magnetic tensor of the paramagnetic forms from the splittings at two fiel ds with a post-correction for the diamagnetic contribution. Both methods ar e shown to produce equivalent results for the determination of the suscepti bility magnetic tensor. Comparison with the magnetic tensor obtained with a previous study based on the dipolar shifts leads to the evidence that, unl ike most other cytochromes c', Rb. capsulatus cytochrome c' of strain M1131 is monomeric in solution. This result is discussed in view of the cytochro me c' function. Our study confirms that dipolar splittings sure a powerful tool for structure determination, providing insight not only into local str uctural details but also into the global fold of molecules.