Phototriggering Electron Flow through Re^I‐modified Pseudomonas aeruginosa Azurins

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 (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.

Relativistic effects in spectroscopy and photophysics of heavy-metal complexes illustrated by spin–orbit calculations of [Re(imidazole)(CO)3(phen)]+

“…Nevertheless, spin-free CASSCF/CASPT2-type [5][6][7][8] and (TD)DFT [8][9][10] techniques are still instrumental in interpreting photophysical behavior and spectra. DFT techniques with hybrid functionals are successfully used to visualize singlet and triplet CT states and reveal their characters by displaying differences of electron density distribution upon excitation and/or excited-state spin-density distributions [9][10][11][12][13]. In such studies, the UKS technique is employed to optimize the lowest triplet-state structures, which are then used to calculate excited-state IR and UV-vis absorption spectra [9,11,14,15].…”

“…DFT techniques with hybrid functionals are successfully used to visualize singlet and triplet CT states and reveal their characters by displaying differences of electron density distribution upon excitation and/or excited-state spin-density distributions [9][10][11][12][13]. In such studies, the UKS technique is employed to optimize the lowest triplet-state structures, which are then used to calculate excited-state IR and UV-vis absorption spectra [9,11,14,15]. Spin-free excited-state structures can also be optimized using TDDFT.…”

“…It will be demonstrated that the SO model accounts very well for absorption spectra, emission properties, and excited-state dynamics, some of its conclusions being qualitatively different from the spinfree approach. [Re(imH)(CO) 3 (phen)] + represents a broad class of Re I tricarbonyl-diimine complexes [21][22][23], which show very rich photophysics and photochemistry [10,14,15,24] and engage in a range of photonic applications such as photosensitizers and phototriggers of electron-transfer reactions [11,[25][26][27][28][29], photocatalysts of CO 2 reduction [30][31][32], phosphorescent labels and probes of biomolecules [33][34][35][36], sensors [37,38], molecular switches [39][40][41][42] and OLED emitters [43], or probes of ps-ns dynamics of solvents, proteins or supramolecular hosts [11,24,50,51,[63][64][65]. The chosen example [Re(imH)(CO) 3 (phen)] + [44] not only epitomizes the salient features of Re I carbonyl-diimine photophysics but also has a prominent position amongst Re-based photosensitizers because of its ability to trigger photoinduced electron transfer and relaxation dynamics in Re-labeled proteins [11,25,[44]…”

“…Mixing of MLCT and L → N ∧ N LLCT characters is especially strong in complexes containing donating ligand L (halides, NCS − , amides, phosphides, alkoxides, thiolates, imidazole, etc.) [9][10][11][12][13]21,22,60,61]. For example, the low-lying excited-states of [Re(I)(CO) 3 (bpy)] contain about 50% of I → bpy LLCT and 50% Re(CO) 3 → bpy MLCT characters [10] and are best viewed as delocalized Re(I)(CO) 3 → bpy CT. Mixing between metal d and ligand (L) p orbitals is well described (sometimes even exaggerated) by DFT with hybrid functionals [8,9,13,62], which calculates the lowest two MLCT/LLCT transitions as occurring from occupied (d-p) Kohn-Sham molecular orbitals, which are -antibonding with respect to the M-L bond, to a *(N ∧ N) orbital.…”

“…Experimentally, TRIR spectroscopy shows strong effects of the molecular environment on the amount of the MLCT contribution [15,51,[63][64][65][66]. Spin-free DFT and TDDFT calculations have been performed on [Re(X)(CO) 3 (N ∧ N)] (X = halide) [9,10,14,15,61], [Re(Etpy)(CO) 3 (bpy)] + in MeCN [10,63], [Re(Et-imH)(CO) 3 (phen)] + in MeCN [51], Re(histidine)(CO) 3 (phen) + in an azurin fragment in water [11], and the series [Re(imH)(CO) 3 (N ∧ N)] + (N ∧ N = bpy, phen, dmp) in DMF [14]. The lowest a 1 A → a 1 A MLCT transition due to HOMO → LUMO excitation has a very small oscillator strength because of a poor orbital overlap and is not supposed to contribute significantly to the absorption spectra.…”