We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCu I , where two adjacent tryptophan residues (W124 and W122) are inserted between the CuI center and a Re photosensitizer coordinated to a H126 imidazole (Re = ReI(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) ReI(H126)­(CO)3(dmp•–)­WWCuII 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 (CuI)′ is oxidized intramolecularly (60–90 ns) by (W122•+)′, forming ReI(H126)­(CO)3(dmp•–)­WWCuI//(CuII)′. The photocycle is closed by ∼1.6 μs ReI(H126)­(CO)3(dmp•–) → //(CuII)′ 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 ReI(H126)­(CO)3(dmp•–)//CuII. Although two adjacent W124 and W122 indoles dramatically enhance CuI → *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 ReI(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.

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy) x (bpy) 3−x ] 2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x = 1−3, abbreviated as 1 2+ , 2 2+ , and 3 2+ ) undergo four (1 2+ ) or five (2 2+ and 3 2+ ) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc + /Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 2 2+ and 3 2+ being separated by 150−210 mV; reduction of the unsubstituted bpy ligand in 1 2+ and 2 2+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Mebpy ligand shifts the ν(CN) IR band by ca. −45 cm −1 , enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2 + , 3 + , and 3 0 show distinct ν(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy •− , confirming that each reduction is localized on a single ligand. NIR spectra of 1 0 , 1 − , and 2 − exhibit a prominent band attributable to the CN-Me-bpy •− moiety between 6000 and 7500 cm −1 , whereas bpy •− -based absorption occurs between 4500 and 6000 cm −1 ; complexes 2 + , 3 + , and 3 0 also exhibit a band at ca. 3300 cm −1 due to a CN-Me-bpy •− → CN-Me-bpy interligand charge-transfer transition. In the UV−vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 1 2+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(CN) IR band is shifted by −38 cm −1 and 5−7-fold-enhanced relative to the ground state.

Ion transfer voltammetry is used to estimate the acid dissociation constants Ka1 and Ka2 of the mono- and diprotonated forms of the biguanide drugs metformin (MF), phenformin (PF), and 1-phenylbiguanide (PB) in an aqueous solution. Measurements gave the pKa1 values for MFH(+), PFH(+), and PBH(+) characterizing the basicity of MF, PF, and PB, which are significantly higher than those reported in the literature. As a result, the monoprotonated forms of these biguanides should prevail in a considerably broader range of pH 1-15 (MFH(+), PFH(+)) and 2-13 (PBH(+)). DFT calculations with solvent correction were performed for possible tautomeric forms of neutral, monoprotonated, and diprotonated species. Extreme basicity of all drugs is confirmed by DFT calculations of pKa1 for the most stable tautomers of the neutral and protonated forms with explicit water molecules in the first solvation sphere included.