13C direct detection experiments on the paramagnetic oxidized monomeric copper, zinc superoxide dismutase

In this report, the use of C-13 direct detection has been pursued in 2D experiments (C-13-C-13 COSY, C-13-C-13 COCAMQ, C-13-C-13 NOESY) to detect broad lines in nuclear magnetic resonance spectra of paramagnetic metalloproteins. The sample is a monomeric oxidized copper, zinc superoxide dismutase. Thanks to direct detection probeheads, cryogenic technology, and implementation of C-13 band-selective homodecoupling, many broadened signals were detected. Proton signals for the same residues escaped detection in H-1 and H-1-N-15 HSQC experiments because of the broadening. Only the C-13 signals which experience large contact coupling escaped detection, i.e., the C-13 nuclei of the metal coordinated histidines. Otherwise, nuclei as close to copper(II) as 4 Angstrom can be detected. Paramagnetic-based restraints can in principle be used for solution structure determination of paramagnetic metalloproteins and in copper(II) proteins in particular. The present study is significant also for the study of large diamagnetic proteins for which proton relaxation makes proton-based spectroscopy not adequate.