Materials and Methods Figs. S1 to S3 Table S1 References S1 SUPPORTING MATERIAL Materials and Methods Preparation of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I Mutant azurins were expressed and Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I was prepared using previously published protocols (S1,S2). Crystal Structure of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II Crystals of Re(4,7-dimethyl-1,10-phenanthroline)(CO) 3 (H 124){T 124 H|K 122 W|H 83 Q}(Cu II)azurin (Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II ; space group I222, cell dimensions 63.22 × 69.08 × 68.94 Å 3 ; α = β = γ = 90.00°, one molecule per asymmetric unit) grew from 4 μL drops made from equal volumes of 30 mg/mL Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II in 25 mM HEPES pH 7.5 and reservoir by vapor diffusion. The drops were equilibrated against 500 μL of reservoir

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 (M = ZnII CuII, CuI; Az = azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants together with excited-state DFT/TDDFT 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 is followed by ~110 fs intersystem crossing and ~600 ps structural relaxation to a 3CT state whose 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 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, competitively fast (~30 ns over 1.12 nm) CuI→W•+ ET producing ReI(dmp•−)(W122)AzCuII. The overall photoinduced CuI→Re(dmp) ET through ReI(dmp)(W122)AzCuI occurs over a 2 nm distance in <50 ns after excitation, the intervening fast 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, kinetics data do not indicate any significant interference from intermolecular ET steps. The ground-state dmp-indole ππ 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.