Spectroscopic and Electronic Structure Studies of Phenolate Cu(II) Complexes: Phenolate Ring Orientation and Activation Related to Cofactor Biogenesis

Abstract

A combination of spectroscopies and DFT calculations have been used to define the electronic structures of two crystallographically defined Cu II -phenolate complexes. These complexes differ in the orientation of the phenolate ring which results in different bonding interactions of the phenolate donor orbitals with the Cu II , which are reflected in the very different spectroscopic properties of the two complexes. These differences in electronic structures lead to significant differences in DFT calculated reactivities with oxygen. These calculations suggest that oxygen activation via a Cu I phenoxyl ligand-to-metal charge transfer (LMCT) complex is highly endergonic (> 50 kcal/mol), hence an unlikely pathway. Rather, the two electron oxidation of the phenolate forming a bridging Cu II peroxoquinone complex is more favorable (11.3 kcal/mol). The role of the oxidized metal in mediating this two electron oxidation of the coordinated phenolate and its relevance to the biogenesis of the covalently bound topa quinone in amine oxidase are discussed.

Copper Active Sites in Biology

“…1239 These models closely represent the tetrahedral [Cu(His) 3 Tyr] + structure that has been proposed for the preprocessed site based on X-ray crystallography and for the 350 nm intermediate, but do not display reactivity with O 2 (perhaps due to sterics). However, the Cu-phenolate bond lengths in these models are significantly shorter than what has been observed in the preprocessed enzyme 1195 (L1 Cu-O 1.731 Å, L3 Cu-O 1.837 Å), confirming that the Cu-Tyr interaction in the preprocessed enzyme is not a tyrosinate-copper bond.…”

“…1239 These calculations show only a small amount of radical character on the phenolate ring (14%-9% spin density) relative to 70% in the calculations of Prabhakar and Siegbahn. This is not due to large differences in the structures of the models relative to the protein active site, but instead reflects the difference in the choice of functional (BP86 with 38% HF exchange for Ghosh et al and B3LYP for Prabhakar and Siegbahn).…”

“…Thus in the absorption spectrum of the L1 complex (Figure 222 left top) the most intense transition is band 6, the oop phenolate to Cu CT, while in the absorption spectrum of the L3 complex (Figure 222 right top) the most intense transition is the ip phenolate to Cu CT, band 8. 1239 …”

Valence tautomerism in synthetic models of cytochrome P450

“…This phenomenon has remained an enigma ever since the initial reports of synthetic thiolate-bound ferric porphyrin complexes by Holm and coworkers (7) and Collman and Sorrell (8) four decades ago. Similar sensitivity to O 2 by a formally oxidized metal center is exhibited by the active sites of ammine oxidase and intradiol dioxygenases (9,10). The sensitivity to O 2 is significantly reduced in thiolate-bound ferric porphyrins in aqueous environments.…”

Synthesis, structure elucidation, biological screening, molecular modeling and DNA binding of some Cu(II) chelates incorporating imines derived from amino acids

“…The charge transfer bands being more sensitive to solvent changes than bands resulting from local transitions. A broad band from 507 to 630 nm, which indicating that band could be mainly attributed to d → d transition in an octahedral structure of the prepared complexes [47,48] Thermogravimetric analysis (TGA)…”