Phototriggering Electron Flow through ReI‐modified Pseudomonas aeruginosa Azurins

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Abstract
The [ReI(CO)3(4,7‐dimethyl‐1,10‐phenanthroline)(histidine‐124)(tryptophan‐122)] complex, denoted [ReI(dmp)(W122)], of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) CuI center in the protein. Analysis of time‐resolved (ps–μs) IR spectroscopic and kinetics data collected on [ReI(dmp)(W122)AzM] (in which M=ZnII, CuII, CuI; Az=azurin) and position‐122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants, together with excited‐state DFT/time‐dependent (TD)DFT calculations and X‐ray structural characterization, reveal the character, energetics, and dynamics of the relevant electronic states of the [ReI(dmp)(W122)] unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re–azurins. Optical population of [ReI(imidazole‐H124)(CO)3]→dmp 1CT states (CT=charge transfer) is followed by around 110 fs intersystem crossing and about 600 ps structural relaxation to a 3CT state. The IR spectrum indicates a mixed ReI(CO)3,A→dmp/π→π*(dmp) character for aromatic amino acids A122 (A=W, Y, F) and ReI(CO)3→dmp metal–ligand charge transfer (MLCT) for [ReI(dmp)(K122)AzCuII]. In a few ns, the 3CT state of [ReI(dmp)(W122)AzM] establishes an equilibrium with the [ReI(dmp.−)(W122.+)AzM] charge‐separated state, 3CS, whereas the 3CT state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, 3CS is populated by fs‐ and ps‐W(indole)→ReII ET from 1CT and the initially “hot” 3CT states, respectively. The 3CS state undergoes a tens‐of‐ns dmp.−→W122.+ ET recombination leading to the ground state or, in the case of the CuI azurin, a competitively fast (≈30 ns over 1.12 nm) CuI→W.+ ET, to give [ReI(dmp.−)(W122)AzCuII]. The overall photoinduced CuI→Re(dmp) ET through [ReI(dmp)(W122)AzCuI] occurs over a 2 nm distance in 3CT–3CS equilibrium being the principal accelerating factor. No reaction was observed for the three Y, F, and K122 analogues. Although the presence of [Re(dmp)(W122)AzCuII] oligomers in solution was documented by mass spectrometry and phosphorescence anisotropy, the kinetics data do not indicate any significant interference from the intermolecular ET steps. The ground‐state dmp–indole π–π interaction together with well‐matched W/W.+ and excited‐state [ReII(CO)3(dmp.−)]/[ReI(CO)3(dmp.−)] potentials that result in very rapid electron interchange and 3CT–3CS energetic proximity, are the main factors responsible for the unique ET behavior of [ReI(dmp)(W122)]‐containing azurins.

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