Electron Flow through Nitrotyrosinate in Pseudomonas aeruginosa Azurin

We have designed ruthenium-modified Pseudomonas aeruginosa azurins that incorporate 3-nitrotyrosine (NO₂YOH) between Ru­(2,2′-bipyridine)₂(imidazole)­(histidine) and Cu redox centers in electron transfer (ET) pathways. We investigated the structures and reactivities of three different systems: RuH107NO₂YOH109, RuH124NO₂YOH122, and RuH126NO₂YOH122. RuH107NO₂YOH109, unlabeled H124NO₂YOH122, and unlabeled H126NO₂YOH122 were structurally characterized. The p<i>K</i>_a’s of NO₂YOH at positions 122 and 109 are 7.2 and 6.0, respectively. Reduction potentials of 3-nitrotyrosinate (NO₂YO^–)-modified azurins were estimated from cyclic and differential pulse voltammetry data: oxidation of NO₂YO^–122 occurs near 1.1 versus NHE; oxidation of NO₂YO^–109 is near 1.2 V. Our analysis of transient optical spectroscopic experiments indicates that hopping via NO₂YO^– enhances Cu^I oxidation rates over single-step ET by factors of 32 (RuH107NO₂YO^–109), 46 (RuH126NO₂YO^–122), and 13 (RuH124NO₂YO^–122)