Correlated distributions in g and A tensors at a biologically active low-symmetry cupric site

Frozen-solution EPR spectra from the cupric site in the protein azurin were measured at 6, 9, and 35 GHz, and subjected to a model-independent analysis. Anomalies in the splittings and shapes of the hyperfine peaks of the low-field quartet are faithfully simulated when a correlated distribution in spin-Hamiltonian parameters is employed, with Δ${\mathit{A}}_{11}$/Δ${\mathit{g}}_{11}$=-0.08 ${\mathrm{cm}}^{\mathrm{-}1}$ and ${\mathrm{\sigma }}_{\mathit{g}}$=0.009. A hybrid-orbital ‘‘united-atom’’ model is used as the basis for a perturbation calculation of the spin-Hamiltonian parameters. With a simple assumption relating the spreads in the hybridization parameters, these spreads and Δ${\mathit{A}}_{11}$/Δ${\mathit{g}}_{11}$ are computed from the model. Equipartition is applied to estimate force constants for the protein modes responsible for the distribution in site geometry. With hybridization parameters in ranges constrained to produce agreement between the calculated and observed magnetic-resonance properties, a force constant at the upper end of those for bending modes is obtained for tetrahedral displacement, and a stiffer one, consistent with stretch, is found for displacement around the symmetry axis.