Hole Hopping Across a Protein-Protein Interface
We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCu , where two adjacent tryptophan residues (W124 and W122) are inserted between the Cu center and a Re photosensitizer coordinated to a H126 imidazole (Re = Re (H126)(CO) (dmp) , dmp = 4,7-dimethyl-1,10-...
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| Published in: | The journal of physical chemistry. B Vol. 123; no. 7; p. 1578 |
|---|---|
| Main Authors: | , , , , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
United States
21.02.2019
|
| ISSN: | 1520-5207, 1520-5207 |
| Online Access: | Get more information |
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| Summary: | We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCu
, where two adjacent tryptophan residues (W124 and W122) are inserted between the Cu
center and a Re photosensitizer coordinated to a H126 imidazole (Re = Re
(H126)(CO)
(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
(H126)(CO)
(dmp
)WWCu
product. The Re126FWCu
mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2-2 mM, {Re126WWCu
}
and {Re126FWCu
}
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
}
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)
(dmp
)//(W122
)' unit; then (Cu
)' is oxidized intramolecularly (60-90 ns) by (W122
)', forming Re
(H126)(CO)
(dmp
)WWCu
//(Cu
)'. The photocycle is closed by ∼1.6 μs Re
(H126)(CO)
(dmp
) → //(Cu
)' back ET that occurs over 12 Å, in contrast to the 23 Å, 120 μs step in Re126WWCu
. Importantly, dimerization makes Re126FWCu
photoreactive and, as in the case of {Re126WWCu
}
, channels the photoproduced "hole" to the molecule that was not initially photoexcited, thereby shortening the lifetime of Re
(H126)(CO)
(dmp
)//Cu
. Although two adjacent W124 and W122 indoles dramatically enhance Cu
→ *Re intramolecular multistep ET, the tryptophan quadruplex in {Re126WWCu
}
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
}
or {Re126FWCu
}
also triggers intermolecular W122' → //*Re ET, and the Re(H126)(CO)
(dmp
)//(W122
)' charge-separated state decays to the ground state by ∼50 ns Re
(H126)(CO)
(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. |
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| Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
| ISSN: | 1520-5207 1520-5207 |
| DOI: | 10.1021/acs.jpcb.8b11982 |