The electronic properties of a model active site for blue copper proteins as probed by Stark spectroscopy

A complex that mimics many of the properties of the blue copper protein center that was synthesized by Holland and Tolman¹ is studied using Stark spectroscopy to determine the values of two electronic properties, the change in the dipole moments (|Δ→Μ|) and the average change in the polarizability (〈Δα〉) for excitation into the ligand-to-metal charge transfer (LMCT) band. Measurements at 77 K in methylcyclohexane yield a value for |Δ→μ| of between 1.3 and 1.9 D for the lowest energy LMCT band, which has been assigned as a predominantly thiolate (Sπ) → Cu(d_x2-y2) transition.² The value of |Δ→μ| is remarkably similar to that we have measured earlier for the type-1 blue copper protein azurin in a glycerol-water glass. The sensitivity of Stark spectroscopy to charge-transfer transitions is utilized to identify and, in some cases, to confirm the assignment previously made via magnetic circular dichroism (MCD)² of several higher-energy charge-transfer bands. Values for the electron-transfer matrix element (H_ab) and the effective charge-transfer distance (R_ab) derived from our measurements on this complex are also reported. These parameters are likewise found to be quite similar to those previously determined for azurin.