Calculation of Resonance Raman Spectra and Excited State Properties for Blue Copper Protein Model Complexes

Abstract

Laccase enzymes have been investigated for their potential as eco-friendly biofuel cell cathode catalysts. In these applications, electrons transfer from the electrode directly to the so-called Type 1 (T1) redox site within the enzyme. Over the years, resonance Raman (RR) spectroscopy has been utilized to probe the nature of ligand interactions with the Cu2+ center at the T1 site of the resting, oxidized form of laccases and related blue copper proteins. Spectral interpretation has been guided by parallel studies of small molecule T1 site mimics. Reported herein are results of time-dependent density functional theory (TDDFT) calculations performed on a series of experimentally well-studied T1 site mimics based on Cu2+ thiolate complexes stabilized by either a tridentate tris(pyrazolyl)hydroborate ligand or a bidentate β-diketiminate ligand. Vertical excitation energies were computed for the first 10 doublet states of each complex. The electronic absorption and RR spectra derived show excellent agreement with the experiment. RR spectra were dominated by a strong Cu–S stretching transition near 400 cm–1, which is sensitive to molecular conformation within the thiolate ligand. The results provide a foundation to build from in advancing models of laccase catalytic active sites in support of sustainable technologies.