PubMed · 10233799
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1999. Three on SAD.. https://pubmed.ncbi.nlm.nih.gov/10233799/
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Infinite molecular helices [Ag(3,3'-Py2S)]X (3,3'-Py2S = 3,3'-thiobispyridine; X- = BF4-, ClO4-, and PF6-) have been rationally constructed or induced. Crystallographic characterization (X- = BF-, monoclinic P2(1)/n, a = 8.946(3) A, b = 14.130(2) A, c = 10.124(2) A, beta = 107.83(2) degrees, V = 1218.3(5) A3, Z = 4, R = 0.0351; X- = ClO4-, monoclinic P2(1)/n, a = 8.884(1) A, b = 14.305(3) A, c = 10.110(1) A, beta = 106.78(1) degrees, V = 1230.1(3) A3, Z = 4, R = 0.0417; X- = PF6-, monoclinic P2(1)/c, a = 10.959(2) A, b = 9.808(2) A, c = 14.065(3) A, beta = 112.03(2) degrees, V = 1401.4(5) A3, Z = 4, R = 0.0442) reveals that the skeletal structure is an oblong cylindrical cationic helix consisting of alternating Ag(I) and 3,3'-Py2S species and that its counteranions are pinched in two columns inside each helix. The formation of the helical coordination polymer appears to be primarily associated with a suitable combination of the skewed conformer of 3,3'-Py2S and the potential linear geometry of the N-Ag(I)-N bond. However, the framework of the nitrate analogue [Ag(3,3'-Py2S)NO3] (monoclinic P2(1)/c, a = 8.177(2) A, b = 10.291(1) A, c = 14.771(2) A, beta = 102.19(1) degrees, V = 1214.9(4) A3, Z = 4, R = 0.0300) is a two-dimensional network consisting of an 18-membered ring unit, where each 3,3'-Py2S acts as a N,N',S- tridentate ligand connecting three tetrahedral silver(I) ions with the monodentate nitrate weakly bonded to the silver (Ag.O = 2.65(1) A) rather than acting as a counteranion. The anion exchange of [Ag(3,3'-Py2S)NO3] with BF4-, ClO4-, or PF6- has been accomplished in aqueous media. The two-dimensional networks are easily converted into the helices via the anion exchange, but the reverse anion exchange proceeds slightly. Thermal analyses indicate a relationship between the thermal stabilities and the structural properties.
The cytoplasmic surface of bacteriorhodopsin is characterized by a group of carboxylates that function as a proton attractive domain [Checover, S., Nachliel, E., Dencher, N. A., and Gutman, M. (1997) Biochemistry 36, 13919-13928]. To identify these carboxylates, we selectively mutated them into cysteine residues and monitored the effects of the dynamics of proton transfer between the bulk and the surface of the protein. The measurements were carried out without attachment of a pH-sensor to the cysteine residue, thus avoiding any structural perturbation and change in the surface charge caused by the attachment of a reporter group, and the protein was in its BR state. The purple membranes were suspended in an unbuffered solution of pyranine (8-hydroxypyrene-1,3,6-trisulfonate) and exposed to a train of 1000 laser pulses (2.1 mJ/pulse, lambda = 355 nm, at 10 Hz). The excitation of the dye ejected the hydroxyl's proton, and a few nanoseconds later, a pair of free protons and ground-state pyranine anion was formed. The experimental observation was the dynamics of the relaxation of the system to the prepulse state. The observed signals were reconstructed by a numeric method that replicates the chemical reactions proceeding in the perturbed space. The detailed reconstruction of the measured signal assigned the various proton-binding sites with rate constants for proton binding and proton exchange and the pK values. Comparison of the results obtained by the various mutants indicates that the dominant proton-binding cluster of the wild-type protein consists of D104, E161, and E234. The replacement of D104 or E161 with cysteine lowered the proton binding capacity of the cluster to approximately 60% of that of the native protein. The replacement of E234 with cysteine disrupted the structure of the cluster, causing the two remaining carboxylates to function as isolated residues that do not interact with each other. The possibility of proton transfer between monomers is discussed.
The extent of ion pairing in chloride and perchlorate salts was studied by measurement of the Cl- and ClO4- resonances and the observation of the perchlorate stretching frequency by use of nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FT-IR), respectively, for a variety of ionophores in various solutions and in large unilaminar vesicles (LUVs). The NMR line widths of chloride and perchlorate were larger in solutions containing the neutral ionophores valinomycin (Val) and nonactin (Non) than in solutions containing the negatively charged ionophores nigericin (Nig), lasalocid (Las), and monensin (Mon). The viscosity-corrected perchlorate NMR line widths in solutions containing Val and Las were significantly negatively correlated (r2 > or = 0.99) with the dielectric constant of the solvent. Solvents with low dielectric constants favored ion pair formation. From methanolic solutions containing the Li+, Na+, K+, and Cs+ salts of Cl- and ClO4-, it was determined that the cation with the highest selectivity for the ionophore affords the most ion pairing. A decrease in pH from 7 to 3 had no significant effect on the NMR line widths of chloride and perchlorate in methanolic solutions containing Val, whereas a similar decrease in pH in a methanolic solution containing Mon caused a 2-fold increase in the line widths. The FT-IR difference spectrum of KClO4 in a methanolic solution containing Val showed splitting at the perchlorate stretching frequency. No band splitting was observed in the FT-IR difference spectrum of KClO(4) in methanolic solutions containing Las. The efflux of 35Cl in LUVs containing the neutral ionophore Val followed first-order kinetics with an efflux constant of 1.70 x 10(-3) x min(-1), as determined by 35Cl NMR spectroscopy. The induction of increased membrane permeability in LUVs by the ionophore was determined to be negligible for Val and Nig by fluorescence spectroscopy.