Abstract
The change in the dipole moments (| |) and the average change in the polarizability (〈Δα〉) upon excitation for the Cys(S)⇒Cu(II) ligand-to-metal charge-transfer (LMCT) transitions in two species of azurin, a type I blue copper protein from Alcaligenes denitrificans (AD) and Pseudomonas aeruginosa (PA), were determined using Stark (electroabsorption) spectroscopy. Measurements at 77 K in a glycerol-water glassy matrix yield a value for | | of between 1.3 and 2.0 D. This value of | | is consistent with the highly covalent nature of the SCu bond in the ground state and is in agreement with the predictions of previous electronic structure calculations. The polarizability of the excited state was smaller than the ground state by about 10−20 A3. This negative value of 〈Δα〉, which is somewhat unusual, is interpreted in the context of the two-state model. Values for the electron-transfer matrix element (Hab) and the effective charge-transfer distance (Rab) derived from our measurements are also reported.
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