Vectorial photoinduced electron-transfer in tailored redox-active proteins and supramolecular nanoparticle arrays

Abstract

Vectorial electron transfer (ET) is a central feature of many biological transformations such as the photosynthetic apparatus or the biocatalytic oxygen assimilation. The present essay summarizes recent studies directed to the chemical modification of biomaterials to yield new photoactive materials that mimic natural photosynthesis to the extent that photoinduced directional ET leads to charge separation of the photogenerated redox products. Specifically, the reconstitution of proteins with photosensitizer–electron-acceptor units is addressed. One example involves the reconstitution of apo-myoglobin with a bis-bipyridinium Zn(II)-protoporphyrin dyad. Vectorial photoinduced ET initiated by the photoactive protein in the presence of the secondary electron acceptor Ru(NH 3) 6 3+ leads to substantial stabilization of the redox products. A second example includes the reconstitution of apo-β-hemoglobin with Co(II)-protoporphyrin IX as catalytic redox center and the chemical functionalization of the cysteine-93 residue with the eosin chromophore. The photogenerated redox species Eo − β-HbCo(I) are stabilized against back ET, k b=370 s −1. The modified protein acts as a photoenzyme for the photoinduced hydrogenation of acetylene to ethylene. A different approach to accomplish photoinduced vectorial ET involves the organization of layered Au-nanoparticle arrays crosslinked by the oligocationic bis-bipyridinium-Zn(II)-protoporphyrin IX ( 1), acting as a photosensitizer–electron-acceptor dyad. Photoinduced ET assembly leads to the transport of electrons through the conductive Au-nanoparticle array and to the generation of a photocurrent. A further approach to stimulate photoactivated vectorial ET involves the use of a photoisomerizable monolayer associated with an electrode as a command interface for the photochemical activation and deactivation of the electrical contact between cytochrome c (Cyt. c) and the electrode. A photoisomerizable monolayer consisting of pyridine and nitrospiropyran units is used to control the electrical contact between Cyt. c and the electrode. The electrically contacted Cyt. c activates the biocatalyzed reduction of O 2 by cytochrome oxidase (COx). The integrated system, consisting of the photoisomerizable monolayer-functionalized electrode and the Cyt. c/COx/O 2 components, provides a system for the amplified electrical transduction of photonic signals recorded by the monolayer interface. The system duplicates functions of the vision process.