The sterically hindered bis(phenol)-dipyrrin ligands LH and LH were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(L)] and [Cu(L)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(L)] and [Cu(L)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM cm) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave ( E = 0.05 and 0.04 V), as well as a reversible reduction wave ( E = -0.40 and -0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(L)] and [Cu(L)]) and anions ([Cu(L)] and [Cu(L)]) were generated. They all display an axial ( S = /) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(L)](SbF) was cocrystallized with [Cu(L)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 Å) and the C-C bonds connecting the peripheral rings to the dipyrrin. The cations show vis-NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)-dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.
A series of nine Ni(II) salophen complexes involving one, two, or three alkyl-imidazolium side-chains was prepared. The lengths of the side-chains were varied from one to three carbons. The crystal structure of one complex revealed a square planar geometry of the nickel ion. Fluorescence resonance energy transfer melting of G-quadruplex structures in the presence of salophen complex were performed. The G-quadruplex DNA structures were stabilized in the presence of the complexes, but a duplex DNA was not. The binding constants of the complexes for parallel and antiparallel G-quadruplex DNA, as well as hairpin DNA, were measured by surface plasmon resonance. The compounds were selective for G-quadruplex DNA, as reflected by equilibrium dissociation constant KD values in the region 0.1-1 μM for G-quadruplexes and greater than 2 μM for duplex DNA. Complexes with more and shorter side-chains had the highest binding constants. The structural basis for the interaction of the complexes with the human telomeric G-quadruplex DNA was investigated by computational studies: the aromatic core of the complex stacked over the last tetrad of the G-quadruplex with peripherical cationic side chains inserted into opposite grooves. Biochemical studies (telomeric repeat amplification protocol assays) indicated that the complexes significantly inhibited telomerase activity with IC50 values as low as 700 nM; the complexes did not significantly inhibit polymerase activity.
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