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Published February 21, 2019 | Accepted Version + Supplemental Material
Journal Article Open

Hole Hopping Across a Protein–Protein Interface

Abstract

We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122) are inserted between the Cu^I center and a Re photosensitizer coordinated to a H126 imidazole (Re = Re^I(H126)(CO)_3(dmp)^+, dmp = 4,7-dimethyl-1,10-phenanthroline). Optical excitation of this mutant in aqueous media (≤40 μM) triggers 70 ns electron transport over 23 Å, yielding a long-lived (120 μs) Re^I(H126)(CO)_3(dmp&•–)WWCu^(II) product. The Re126FWCu^I mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2–2 mM, {Re126WWCu^I}_2 and {Re126FWCu^I}_2 are formed with the dmp ligand of the Re photooxidant of one molecule in close contact (3.8 Å) with the W122′ indole on the neighboring chain. In addition, {Re126WWCu^I}_2 contains an interfacial tryptophan quadruplex of four indoles (3.3–3.7 Å apart). In both mutants, dimerization opens an intermolecular W122′ → //*Re ET channel (// denotes the protein interface, *Re is the optically excited sensitizer). Excited-state relaxation and ET occur together in two steps (time constants of ∼600 ps and ∼8 ns) that lead to a charge-separated state containing a Re(H126)(CO)3(dmp•–)//(W122^•+)′ unit; then (Cu^I)′ is oxidized intramolecularly (60–90 ns) by (W122^•+)′, forming Re^I(H126)(CO)_3(dmp^•–)WWCu^I//(Cu^(II))′. The photocycle is closed by ∼1.6 μs Re^I(H126)(CO)_3(dmp^•–) → //(Cu^(II))′ back ET that occurs over 12 Å, in contrast to the 23 Å, 120 μs step in Re126WWCu^I. Importantly, dimerization makes Re126FWCu^I photoreactive and, as in the case of {Re126WWCu^I}_2, channels the photoproduced "hole" to the molecule that was not initially photoexcited, thereby shortening the lifetime of Re^I(H126)(CO)_3(dmp^•–)//Cu^(II). Although two adjacent W124 and W122 indoles dramatically enhance Cu^I → *Re intramolecular multistep ET, the tryptophan quadruplex in {Re126WWCu^I}_2 does not accelerate intermolecular electron transport; instead, it acts as a hole storage and crossover unit between inter- and intramolecular ET pathways. Irradiation of {Re126WWCu^(II)}_2 or {Re126FWCu^(II)}_2 also triggers intermolecular W122′ → //*Re ET, and the Re(H126)(CO)_3(dmp^•–)//(W122^•+)′ charge-separated state decays to the ground state by ∼50 ns Re^I(H126)(CO)_3(dmp^•–)^+ → //(W122^•+)′ intermolecular charge recombination. Our findings shed light on the factors that control interfacial hole/electron hopping in protein complexes and on the role of aromatic amino acids in accelerating long-range electron transport.

Additional Information

© 2019 American Chemical Society. Received: December 13, 2018; Revised: January 15, 2019; Published: January 23, 2019. This work was supported by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health under Award No. R01DK019038. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Further support was provided the Czech Science Foundation (GAČR) Grant No. 17-011375, the Czech Ministry of Education (MŠMT) Grant No. LTAUSA18026, "IT4Innovations National Supercomputing Center – LM2015070", EPSRC Grant No. (UK) EP/R029687/1, the STFC Rutherford Appleton Laboratory (UK), and the Arnold and Mabel Beckman Foundation. We thank Dr. Hana Kvapilová (JH Institute) for the help with measuring TRIR spectra. Yuling Shen (Caltech) is acknowledged for the help with protein preparation. The authors declare no competing financial interest.

Attached Files

Accepted Version - nihms-1010470.pdf

Supplemental Material - jp8b11982_si_001.pdf

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Additional details

Created:
August 19, 2023
Modified:
October 20, 2023