Phototriggering Electron Flow through ReI-modified Pseudomonas aeruginosa Azurins
The [Re-I(CO)(3)(4,7-dimethyl-1,10-phenanthroline)(histidine-124)(tryptophan-122)] complex, denoted [Re-I(dmp)(W122)], of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) Cu-I center in the protein. Analysis of time-resolved (ps-ms) IR spectroscopic and kinetics data collected on [Re-I(dmp)(W122) AzM] (in which M=Zn-II, Cu-II, Cu-I; Az=azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants, together with excited-state DFT/time-dependent (TD)DFT calculations and Xray structural characterization, reveal the character, energetics, and dynamics of the relevant electronic states of the [Re-I(dmp)(W122)] unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re-azurins. Optical population of [Re-I(imidazole-H124)(CO)(3)]-> dmp (CT)-C-1 states (CT=charge transfer) is followed by around 110 fs intersystem crossing and about 600 ps structural relaxation to a (CT)-C-3 state. The IR spectrum indicates a mixed Re-I(CO)(3), A -> dmp/pi ->pi* (dmp) character for aromatic amino acids A122 (A=W, Y, F) and Re-I(CO)(3)-> dmp metal-ligand charge transfer (MLCT) for [Re-I(dmp)(K122)AzCu(II)]. In a few ns, the (CT)-C-3 state of [Re-I(dmp)(W122)AzM] establishes an equilibrium with the [Re-I(dmp(center dot)-)(W122(center dot+))AzM] charge-separated state, (CS)-C-3, whereas the (CT)-C-3 state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, (CS)-C-3 is populated by fs-and ps-W(indole)-> Re-II ET from (CT)-C-1 and the initially "hot" (CT)-C-3 states, respectively. The (CS)-C-3 state undergoes a tensof-ns dmpC(center dot-)-> W122(center dot+) ET recombination leading to the ground state or, in the case of the Cu-I azurin, a competitively fast (approximate to 30 ns over 1.12 nm) Cu-I -> W center dot+ ET, to give [Re-I (dmp(center dot-))-(W122)AzCu(II)]. The overall photoinduced Cu-I -> Re(dmp) ET through [Re-I(dmp)(W122)AzCu(I)] occurs over a 2 nm distance in <50 ns after excitation, with the intervening fast (CT)-C-3-(CS)-C-3 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)AzCu(II)] 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 pi-pi interaction together with well-matched W/W center dot+ and excited-state [Re-II(CO)(3)(dmp(center dot-))]/[Re-I(CO)(3)(dmp(center dot-))] potentials that result in very rapid electron interchange and (CT)-C-3-(CS)-C-3 energetic proximity, are the main factors responsible for the unique ET behavior of [Re-I(dmp)(W122)]-containing azurins.