ANALYSIS OF THE HYPERFINE-SHIFTED N-15 RESONANCES OF THE OXIDIZED FORM OF ANABAENA-7120 HETEROCYST FERREDOXIN

Hyperfine-shifted nitrogen signals have been detected in one-dimensional N-15 NMR spectra of oxidized Anabaena 7120 heterocyst ferredoxin labeled uniformly with N-15. Several of these have been classified by amino acid type by reference to results from selective N-15-labeling studies. Remarkable agreement is seen between a dipole-dipole analysis of the N-15 T-1 relaxation and the distances of several of the nitrogens from the irons of the cluster as derived from the X-ray structure of this protein [Jacobson, B. L., Chae, Y. K., Markley, J. L., Rayment, I., & Holden, H, M. (1993) Biochemistry 32, 6788-6793], The agreement is within experimental error for hyperfine-shifted nitrogens that are at least 4.2 A distant from either of the irons of the cluster; however, the simple model appears to fail for hyperfine-shifted nitrogens that are closer to the cluster. The failure of the model for short distances may stem either from a breakdown of the point-dipole approximation and/or from neglect of delocalization of unpaired electron density from the iron ions to other atoms. Even with the above limitations, dipolar analysis of N-15 relaxation should provide useful distance constraints for solution-state studies of iron-sulfur proteins.