Spectroscopic studies of stellacyanin, plastocyanin, and azurin. Electronic structure of the blue copper sites

Abstract

Low-temperature absorption and room temperature circular dichroism and magnetic circular dichroism spectral studies of the blue copper proteins Rhus vernicifera stellacyanin, bean plastocyanin, and Pseudomonas aeruginosa azurin have been made. Low-energy bands attributable to d-d transitions in a flattened tetrahedral (£>2<r) copper(11) center are observed in the near-infrared CD spectra at 5000, 9150, and 1 1 200 cm-1 in plastocyanin, 5250, 8100, and 10 500 cm-1 in stellacyanin, and 5800 and 10 200 cm-1 in azurin. Bands in the near-infrared MCD spectra are at 10 600, 8800. and 10 500 cm-1 in plastocyanin, stellacyanin, and azurin, respectively. The low energies of these bands rule out tetragonal sixor five-coordinate as well as square-planar four-coordinate structures, and trigonal four or five coordination is inconsistent with the blue copper EPR parameters. The band positions accord well with ligand field calculations based on a tetrahedral structure that is distorted approximately 6°toward a square plane (2B2 (dA2_,2) ground state), the predicted transitions being 2B2 -2E, 2Bi -2B], and 2B2 -* 2A¡, in order of increasing energy. EPR parameters for the three blue proteins calculated using this ligand field model are in reasonable agreement with the observed values. Ligand field stabilization energy contributions to the reduction potentials have been calculated: azurin, plastocyanin, and stellacyanin are destabilized by 250, 345, and 340 mV. respectively, relative to the aquo Cu(ll) ion. Based on a comparison of absorption and CD intensities, the characteristic bands at about 13 000, 16 000, and 22 000 cm-1 in the blue proteins are assigned to the ligand to metal charge transfer transitions tS(Cvs) -* dA2_|2, ffS(Cys) -* dA2_,.2, and irN(His) -*• dA2_,2, respectively, in a flattened tetrahedral Cu(ll) unit.

Electrochemical titrations and reaction time courses monitored in situ by magnetic circular dichroism spectroscopy

“…One-electron reduction to the Cu(I) (d 10 ) precludes charge transfer and bleaches the color, abolishing both CD and MCD. Ambient temperature CD and MCD of Paz, recorded across the visible region (not shown), were consistent with previous reports for blue copper centers in that the MCD is significantly lower in intensity than the natural CD [41]. The CD results from chirality of ligand conformation at Cu(II), but the orbital angular momentum required for MCD is almost completely quenched.…”

Optical investigation of the electron transfer protein azurin–gold nanoparticle system

“…3 ligand-to-metal-charge transfer (LMCT) absorption band located at 628 nm [23,24]. Interestingly, it has been shown that an electron transfer well mimicking the thermal one can be induced by light irradiating Az in its LMCT band [25], similarly to other blue copper proteins [26][27][28], pointing out an intriguing interplay between Az optical and electrical properties.…”

The effect of pH and complexation on redox reactions between RS• radicals and flavins

“…In metal-containing enzymes, such as xanthine oxidase (1) and copper proteins (2), thiol groups frequently serve as binding ligands, while in other systems they may be involved as separate entities in the electron transport pathway. For example, in glutathione reductase, lipoamide dehydrogenase and thioredoxin reductase, for which the substrates themselves are sulphur-containing species, the electron transfer pathway in the direction of substrate oxidation is considered to be (3)(4)(5): Here the disulphide (S-S) is covalently bound to the protein, while the flavin (Fl) is strongly but non-covalently bound.…”