Photoinduced electron transfer in eosin-modified Co(II)-Protoporphyrin IX reconstituted myoglobin and α- or β-Hemoglobin subunits: Photocatalytic transformations by the reconstituted photoenzymes

A series of hemo-protein-derived photocatalysts, prepared by reconstitution of the respective apo-proteins with Co(II)-protoporphyrin IX and chemical modification of the protein with the eosin chromophore, is presented. Apo-myoglobin, Apo-Mb, was reconstituted with Co(II)-protoporphyrin IX and further modified with eosin-isothiocyanate (3) to yield the photocatalyst Eo^2--Mb-Co(II). The protein is loaded by two eosin chromophore units. Photoexcitation of Eo^2--Mb-Co(II) yields the electron transfer species Eo^.--Mb-Co(I) formed by direct oxidative quenching of ^TEo^2--Mb-Co(II), k_q = 5.2 × 10⁴ s^-1, and via an indirect path where self-quenching of the eosin-chromophore units yields the intermediate redox products (Eo^.3-+Eo^.-)-Mb-Co(II) that, in the presence of Na₂-EDTA, generate the Eo^.--Mb-Co(I) in a secondary dark electron transfer, k_r = 330 s^-1. The reconstituted protein Eo^2--Mb-Co(II) reveals photocatalytic features and its steady-state illumination in the presence of Na₂EDTA yields hydrogen evolution, φ = 2 × 10^-4, or photohydrogenation of acetylene to ethylene, φ = 1 × 10^-2. The reconstituted photocatalyst Eo^2--Mb-Co(II) reveals enzyme-like behavior. Photohydrogenation of acetylenedicarboxylic acid (6) by Eo^2--Mb-Co(II) in the presence of Na₂EDTA reveals stereospecificity and formation of maleic acid as the hydrogenation product and kinetics that follow the Michaelis-Menten model, K_m = 4 mM, V_max = 0.6 μM·min^-1. Similarly, the α- and β-subunits of hemoglobin, Hb, were reconstituted with Co(II)-protoporphyrin IX to yield α-Hb-Co(II) and β-Hb-Co(II). The β-Hb-Co(II) was specifically modified at cysteine 93 residue by eosin maleimide (4) to form Eo^2--β-Hb-Co(II). The α-Hb-Co(II) was modified at a single, unknown, lysine residue by eosin maleimide (4) to generate Eo^2--α-Hb-Co(II). Only oxidative quenching proceeds in Eo^2--β-Hb-Co(II) and Eo^2--α-Hb-Co(II) to yield the redox photoproducts, Eo^.--β-Hb-Co(I) and Eo^.--α-Hb-Co(I), k_q^β = 1.8 × 10³ s^-1 and k_q^α = 6.5 × 10³ s^-1, respectively. The back electron-transfer rates of the redox species are k_b^β = 0.37 × 10³ s^-1 and k_b^α = 3.4 × 10³ s^-1, respectively. The site-specific modification at cysteine 93 residue of β-Hb by the chromophore and the known X-ray structure of β-Hb which defines the electron transfer distance in Eo^2--β-Hb, d = 12.87 Å, enabled the analysis of the experimental electron-transfer rate constants according to Marcus theory: λ = 1.1 eV; β = 1.35 Å^-1. The reorganization energy, λ, associated with the electron transfer in Eo^2--α-Hb-Co(II) is similar, λ = 1.15 eV. Application of the β-value extracted for the Eo^2--β-Hb-Co(II) system to the Eo^2--α-Hb-Co(II) assembly enabled estimation of the electron-transfer distance in the latter system, d = 11.2 Å, and elucidation of the lysine-90 residue as the modification site by the eosin chromophore. The two reconstituted proteins, Eo^2--α-Hb-Co(II) and Eo^2--β-Hb-Co(II), reveal photocatalytic properties. Their steady-state irradiation in the presence of the Na₂EDTA resulted in photohydrogenation of acetylene to ethylene, φ^α = 0.02; φ^β = 0.004. The photogenerated redox species of the reconstituted proteins Eo^.--Mb-Co(I) and Eo^.--α/β-Hb-Co(I) reveal pronounced stabilities against back electron transfer. This was attributed to spatial separation of the redox species by the rigid protein assemblies. The stability of the redox photoproducts Eo^.--Mb-Co(I) enabled tailoring a cyclic photosynthetic assembly where the semisynthetic photoenzyme Eo^2--Mb-Co(II) as a reductive biocatalyst is coupled to lactate dehydrogenase, LDH, an oxidative biocatalyst, using N-methylferrocene caproic acid (5) as diffusional electron mediator. Steady-state irradiation of an assembly composed of Eo^2--Mb-Co(II), LDH, and 5 in the presence of acetylene and lactic acid yields the cyclic photoinduced hydrogenation of acetylene by lactic acid to yield ethylene and pyruvic acid, φ = 2 × 10^-3. The system mimics artificially the functions of the photosynthetic bacteria chloroflexus.