Construction of Artificial Photosynthetic Reaction Centers on a Protein Surface: Vectorial, Multistep, and Proton-Coupled Electron Transfer for Long-Lived Charge Separation

Abstract

Artificial photosynthetic reaction centers have been constructed on a protein surface by cofactor reconstitution, which mimic the function of photosynthetic organisms to convert light energy to chemical potential in the form of long-lived charge-separated states. They feature a ruthenium tris(2,2‘-bipyridine) moiety as the sensitizer, which is mechanically linked (i.e., in catenane-type) with a cyclobis(paraquat-p-phenylene) unit (BXV4+, acceptor) and covalently linked with a protoheme or Zn−protoporphyrin (donor) located in the myoglobin pocket. Their cofactors 1 and 2, which are tris(heteroleptic) Ru−bipyridine complexes, were synthesized by sequential coordination of the two different functionalized bipyridine ligands with a readily obtainable precursor [Ru(4,4‘-dimethyl-2,2‘-bipyridine)Cl3]n followed by metal insertion; this represents a new efficient synthetic method for tris(heteroleptic) Ru(II) complexes of bidentate polypyridine ligands. Reconstitution of apo-myoglobin (Mb) with 1 and 2 affords the two Mb-based artificial triads, Mb(FeIIIOH2)−Ru2+−BXV4+ and Mb(Zn)−Ru2+−BXV4+. Laser flash photolysis of the Ru(bpy)3 moiety of Mb(FeIIIOH2)−Ru2+−BXV4+ in an aqueous solution yields an initial charge-separated state, Mb(FeIIIOH2)−Ru3+−BXV3+•, via noncovalent electron transfer, followed by dark electron transfer to generate an intermediate consisting of porphyrin cation radical, Mb(FeIII•OH2)−Ru2+−BXV3+•. Mb(FeIII•OH2)−Ru2+−BXV3+• thus generated is subsequently converted, via a proton-coupled process and with a quantum yield of 0.005, into the final charge-separated state, Mb(FeIVO)−Ru2+−BXV3+•, which bears an energy more than 1 eV above the ground state and a lifetime (τ > 2 ms) comparable to that of natural photosynthetic reaction center. Photoexcitation of Mb(Zn)−Ru2+−BXV4+ also gives rise to a vectorial two-step electron-transfer relay with the intermediate CS state, Mb(Zn)−Ru3+−BXV3+•, for the main pathway leading to the final CS state, Mb(Zn+)−Ru2+−BXV3+•, in a yield of 0.08. Although the driving forces for the recombination of Mb(FeIVO)−Ru2+−BXV3+• and Mb(Zn+)−Ru2+−BXV3+• are similar (ΔG ≈ 1.30 eV), the recombination rate of the former is at least 102−103-fold slower than that of the latter. By analogy with a related system reported previously, it was considered that back ET from BXV3+• to Mb(FeIVO) might be coupled to the protonation of Mb(FeIVO) and governed by the slow interconversion between the metal−oxo form and the proton-activated species, rendering the CS state Mb(FeIVO)−Ru2+−BXV3+• specially long-lived. Control experiments clearly demonstrated that partial incorporation of the triads into the protein matrix plays a crucial role in regulating the electron-transfer pathway and stabilizing the charge separation state.