Pulse Electron Double Resonance Detected Multinuclear NMR Spectra of Distant and Low Sensitivity Nuclei and Its Application to the Structure of Mn(II) Centers in Organisms

The ability to characterize the structure of metal centers beyond their primary ligands is important to understanding their chemistry. High-magnetic-field pulsed electron double resonance detected NMR (ELDOR-NMR) is shown to be a very sensitive approach to measuring the multinuclear NMR spectra of the nuclei surrounding Mn(II) ions. Resolved spectra of intact organisms with resonances arising from ⁵⁵Mn, ³¹P, ¹H, ³⁹K, ³⁵Cl, ²³Na, and ¹⁴N nuclei surrounding Mn²⁺ centers were obtained. Naturally abundant cellular ¹³C could be routinely measured as well. The amplitudes of the ¹⁴N and ²H ELDOR-NMR spectra were found to be linearly dependent on the number of nuclei in the ligand sphere. The evolution of the Mn(II) ELDOR-NMR spectra as a function of excitation time was found to be best described by a saturation phenomenon rather than a coherently driven process. Mn(II) ELDOR-NMR revealed details about not only the immediate ligands to the Mn(II) ions but also more distant nuclei, providing a view of their extended structures. This will be important for understanding the speciation and chemistry of the manganese complexes as well as other metals found in organisms.