Theoretical Study of the Electronic Spectra of Small Molecules That Incorporate Analogues of the Copper–Cysteine Bond

Abstract

The copper-sulphur bond which binds cysteinate to the metal centre is a key factor in the spectroscopy of blue copper proteins. We present theoretical calculations describing the electronically excited states of small molecules, including CuSH, CuSCH 3 , (CH 3 ) 2 SCuSH, (imidazole)-CuSH and (imidazole) 2 -CuSH, derived from the active site of blue copper proteins that contain the copper-sulphur bond in order to identify small molecular systems that have electronic structure that is analogous to the active site of the proteins. Both neutral and cationic forms are studied, since these represent the reduced and oxidised forms of the protein, respectively. For CuSH and CuSH + , excitation energies from time-dependent density functional theory with the B97-1 exchange-correlation functional agree well with the available experimental data and multireference configuration interaction calculations. For the positive ions, the singly occupied molecular orbital is formed from an antibonding combination of a 3d orbital on copper and a 3p p orbital on sulphur, which is analogous to the protein. This leads several of the molecules to have qualitatively similar electronic spectra to the proteins. For the neutral molecules, changes in the nature of the low lying virtual orbitals leads the predicted electronic spectra to vary substantially between the different molecules. In particular, addition of a ligand bonded directly to copper results in the low-lying excited states observed in CuSH and CuSCH 3 to be absent or shifted to higher energies.

Copper–Chalcogen Bonds in Olfaction: Accurate ab Initio Characterization of CuSH and CuOH

“…Figure , which displays the outermost molecular orbitals (MOs) of CuSH and CuOH, reveals that the singly occupied molecular orbitals (SOMO) are quite different, whereas both molecules exhibit similar LUMOs. For instance, the SOMO of CuSH is antibonding in nature and corresponds to the antibonding interaction between the 3d x 2 –y 2 orbital of Cu with the 3p π orbital of S, where the lone pairs of the sulfur atom forms a lateral bond with the π-orbitals of copper, in line with the analysis of Do and Besley . In contrast, the SOMO of CuOH corresponds to a σ-type bond between Cu and O.…”

“…For the Cu–S bond, copper hydrosulfide provides the simplest model of copper–cysteinate or copper–odorant interaction, CuSH being analogous to the reduced form of the protein. The relevance of CuSH as a building block of larger protein environments has been previously reported by Do et al, who studied the electronically excited states of several small-sized molecular analogues of the copper cysteine bond (e.g., CuSH, CuSCH 3 , ImidazyoleCuSH ...) in their neutral and ionic forms. The authors explored the similarities of the electronic structure of their model systems with that of the active site of the protein with the aim to provide a guide for experimental studies on these systems …”

A QM/MM study of the nature of the entatic state in plastocyanin

“…This change can be associated with changes in the structure of the active site, and other work has shown the excitation energy of the intense LMCT band correlate strongly with the copper‐cysteine bond length . The spectroscopy of the reduced form of the protein contains less information, thus there have been fewer studies that have focused on this form of the protein …”

“…[14] The spectroscopy of the reduced form of the protein contains less information, thus there have been fewer studies that have focused on this form of the protein. [26][27][28] The structure of the active site plays a key role in the function of the protein. One debated aspect of plastocyanin redox chemistry has been the entatic [29] (or reduced rack [30] ) state, whereby the structure of the active site represents a compromise between the structures favored by the oxidized and reduced forms of the protein.…”

Comparative analysis of the performance of commonly available density functionals in the determination of geometrical parameters for copper complexes

“…However, and particularly in the case of the “spectroscopically quiet” transition metals such as Cu(I), these methodologies are often not enough to fully characterize such complexes, and so, theoretical methods have arisen as an excellent complement to their characterization; in particular those based on Density Functional Theory (DFT). This is supported by the numerous theoretical studies that surfaced in the literature over the years …”