Thiolate Spin Population of Type I Copper in Azurin Derived from ³³S Hyperfine Coupling

The electron transfer mediating properties of type I copper proteins stem from the intricate ligand coordination sphere of the Cu ion in their active site. These redox properties are in part due to unusual cysteine thiol coordination, which forms a highly covalent copper-sulfur (Cu-S) bond. The structure and electronic properties of type I copper have been the subject of many experimental and theoretical studies. The measurement of spin delocalization of the Cu(II) unpaired electron to neighboring ligands provides an elegant experimental way to probe the fine details of the electronic structure of type I copper. To date, the crucial parameter of electron delocalization to the sulfur atom of the cysteine ligand has not been directly determined experimentally. We have prepared ³³S-enriched azurin and carried out W-band (95 GHz) electron paramagnetic resonance (EPR) and electron-electron double resonance detected NMR (EDNMR) measurements and, for the first time, recorded the ³³S nuclear frequencies, from which the hyperfine coupling and the spin population on the sulfur of the thiolate ligand were derived. The overlapping ³³S and ¹⁴N EDNMR signals were resolved using a recently introduced two-dimensional correlation technique, 2D-EDNMR. The ³³S hyperfine tensor was determined by simulations of the EDNMR spectra using ³³S hyperfine and quadrupolar tensors predicted by QM/MM DFT calculations as starting points for a manual spectral fit procedure. To reach a reasonable agreement with the experimental spectra, the ³³S hyperfine principal value, A_z, and one of the corresponding Euler angles had to be modified. The final values obtained gave an experimentally determined sulfur spin population of 29.8 ± 0.7%, significantly improving the wide range of 29-62% reported in the literature. Our direct, experimentally derived value now provides an important constraint for further theoretical work aimed at unravelling the unique electronic properties of this site.