Nucleic acids that contain multiple sequential guanines assemble into guanine quadruplexes (G-quadruplexes). Drugs that induce or stabilize G-quadruplexes are of interest because of their potential use as therapeutics. Previously, we reported on the interaction of the Cu(2+) derivative of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (CuTMpyP4), with the parallel-stranded G-quadruplexes formed by d(T(4)G( n )T(4)) (n = 4 or 8) (Keating and Szalai in Biochemistry 43:15891-15900, 2004). Here we present further characterization of this system using a series of guanine-rich oligonucleotides: d(T(4)G( n )T(4)) (n = 5-10). Absorption titrations of CuTMpyP4 with all d(T(4)G( n )G(4)) quadruplexes produce approximately the same bathochromicity (8.3 +/- 2 nm) and hypochromicity (46.2-48.6%) of the porphyrin Soret band. Induced emission spectra of CuTMpyP4 with d(T(4)G( n )T(4))(4) quadruplexes indicate that the porphyrin is protected from solvent. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry revealed a maximum porphyrin to quadruplex stoichiometry of 2:1 for the shortest (n = 4) and longest (n = 10) quadruplexes. Electron paramagnetic resonance spectroscopy shows that bound CuTMpyP4 occupies magnetically noninteracting sites on the quadruplexes. Consistent with our previous model for d(T(4)G(4)T(4)), we propose that two CuTMpyP4 molecules are externally stacked at each end of the run of guanines in all d(T(4)G( n )T(4)) (n = 4-10) quadruplexes.
Lead
(Pb)-containing solids find widespread commercial use in batteries,
piezoelectrics, and as starting materials for synthesis. Here, we
combine density functional theory (DFT) and thermodynamics in a DFT
+ solvent ion model to compare the surface reactivity of Pb oxides
and carbonates, specifically litharge, massicot, and cerussite, in
contact with water. The information provided by this model is used
to delineate structure–property relationships for surfaces
that are able to release Pb as Pb2+. We find that Pb2+ release is dependent on pH and chemical bonding environment
and go on to correlate changes in the surface bonding to key features
of the electronic structure through a projected density of states
analysis. Collectively, our analyses link the atomistic structure
to i) specific electronic states and ii) the thermodynamics of surface
transformations, and the results presented here can be used to guide
synthetic efforts of Pb2+-containing materials in aqueous
media or be used to better understand the initial steps in solid decomposition.
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