Two Tryptophans Are Better Than One in Accelerating Electron Flow through a Protein

Abstract

We have constructed and structurally characterized a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122, indole separation 3.6–4.1 Å) are inserted between the CuI center and a Re photosensitizer coordinated to the imidazole of H126 (ReI(H126)(CO)3(4,7-dimethyl-1,10-phenanthroline)+). CuI oxidation by the photoexcited Re label (*Re) 22.9 Å away proceeds with a ∼70 ns time constant, similar to that of a single-tryptophan mutant (∼40 ns) with a 19.4 Å Re–Cu distance. Time-resolved spectroscopy (luminescence, visible and IR absorption) revealed two rapid reversible electron transfer steps, W124 → *Re (400–475 ps, K1 ≅ 3.5–4) and W122 → W124•+ (7–9 ns, K2 ≅ 0.55–0.75), followed by a rate-determining (70–90 ns) CuI oxidation by W122•+ ca. 11 Å away. The photocycle is completed by 120 μs recombination. No photochemical CuI oxidation was observed in Re126FWCuI, whereas in Re126WFCuI, the photocycle is restricted to the ReH126W124 unit and CuI remains isolated. QM/MM/MD simulations of Re126WWCuI indicate that indole solvation changes through the hopping process and W124 → *Re electron transfer is accompanied by water fluctuations that tighten W124 solvation. Our finding that multistep tunneling (hopping) confers a ∼9000-fold advantage over single-step tunneling in the double-tryptophan protein supports the proposal that hole-hopping through tryptophan/tyrosine chains protects enzymes from oxidative damage.

Competing ultrafast photoinduced electron transfer and intersystem crossing of [Re(CO)$$_3$$(Dmp)(His124)(Trp122)]$$^+$$ in Pseudomonas aeruginosa azurin: a nonadiabatic dynamics study

“…These results confirm the assignment of the sub-ps time constants of the related experiments [9,13] and constitute the first computational evidence for the ultrafast formation of the chargeseparated states in Re-sensitized azurin. Similar competitive intersystem crossing and electron transfer phenomena have been observed in other protein-coupled transition metal complexes [56][57][58], for example in Re(CO) 3 diimine complexes anchored to different locations in azurins [8,12], Re(CO) 3 diimine complexes in azurins with two mediating tryptophan moieties [59], or Ru complexes in azurins [60]. The study of these competing photoinduced processes is critical in understanding and utilizing electron transfer through proteins [57,58].…”

Hole Hopping Across a Protein–Protein Interface

“…Shortest atom-atom intramolecular distances between redox-active sites. 2 b An additional close contact (3.9 Å) exists between the W124 indole ring and C(CH 3 -dmp). c Closest distance between the indole ring and C(CH 3 -dmp) = 3.5 Å.…”

Influence of Hetero‐Biaryl Ligands on the Photo‐Electrochemical Properties of [ReINCS(N∩N)(CO)3]‐Type Photosensitizers

“…Whereas pypm (13) and pypz (14) are already known and accessible via published procedures, a synthetic strategy for the synthesis of the asymmetric pyridyl-diazines pypdz (12) and pypym (15) based on a Suzuki-Miyaura coupling procedure [49] is presented. In the next step, the four new asymmetric [Re I Br(CO) 3 pydz] complexes (22)(23)(24)(25) were obtained. Applying a modified procedure of Takeda and co-workers [16] to the symmetric (22)(23)(24)(25) and asymmetric bromide complexes (26)(27)(28)(29) gave access to eight new isothiocyanate complexes 32-39 from the corresponding bromide derivatives (22)(23)(24)(25)(26)(27)(28)(29) avoiding the use of silver reagents.…”

“…It allows the fine-tuning of absorption and emission properties by subtle structural variations. electron and energy transfer processes [22][23][24][25] and radiative and non-radiative decay, the latter leading to the general concept of the energy gap law (EGL). [20,21] They have been utilized as model systems to understand basic photophysical processes, e.g.…”