NMR investigation of imidazolium-based ionic liquids and their aqueous mixtures.

(1)H and (13)C NMR spectroscopy is employed to investigate the interaction of water with two imidazolium-based ionic liquids (ILs), 1-hexyl-3-methylimidazolium bromide ([C(6)mim]Br) and 1-octyl-3-methylimidazolium bromide ([C(8)mim]Br), at IL concentrations well above the critical aggregation concentration (CAC). The results are compared with those of the neat samples. To this aim, a detailed analysis of the changes in the (1)H chemical shifts, (13)C relaxation parameters, and 2D ROESY data due to the presence of water is performed. The results for both neat ILs are consistent with a packed structure where head-to-head, head-to-tail, and tail-to-tail contacts occur and where the site of maximal mobility restriction is at the polar head. At the lowest investigated water content, the presence of water influences mainly the environment around the IL polar head, slowing down the motional dynamics of the aromatic ring with respect to the alkyl chain. At higher water contents this difference diminishes, the motional freedom of the whole molecule increasing. The presence of ROESY cross-peaks between protons in the polar and apolar IL regions, as well as between protons in non-neighboring alkyl groups, at all investigated water contents suggests that the alkyl tails are not fully segregated in hydrophobic domains, as expected for micelle-like structures.

Competitive binding exchange between alkali metal ions (K+, Rb+, and Cs+) and Na+ ions bound to the dimeric quadruplex [d(G4T4G4)]2: a 23Na and 1H NMR study.

A comparative study of the competitive cation exchange between the alkali metal ions K+, Rb+, and Cs+ and the Na+ ions bound to the dimeric quadruplex [d(G4T4G4)]2 was performed in aqueous solution by a combined use of the 23Na and 1H NMR spectroscopy. The titration data confirm the different binding affinities of these ions for the G-quadruplex and, in particular, major differences in the behavior of Cs+ as compared to the other ions were found. Accordingly, Cs+ competes with Na+ only for the binding sites at the quadruplex surface (primarily phosphate groups), while K+ and Rb+ are also able to replace sodium ions located inside the quadruplex. Furthermore, the 1H NMR results relative to the CsCl titration evidence a close approach of Cs+ ions to the phosphate groups in the narrow groove of [d(G4T4G4)]2. Based on a three-site exchange model, the 23Na NMR relaxation data lead to an estimate of the relative binding affinity of Cs+ versus Na+ for the quadruplex surface of 0.5 at 298 K. Comparing this value to those reported in the literature for the surface of the G-quadruplex formed by 5'-guanosinemonophosphate and for the surface of double-helical DNA suggests that topology factors may have an important influence on the cation affinity for the phosphate groups on DNA.

Binding of Mg2+, Cd2+, and Ni2+ to liquid crystalline NaDNA: polarized light microscopy and NMR investigations.

The interaction of the divalent metal ions Mg(2+), Cd(2+), and Ni(2+) with liquid crystalline NaDNA solutions (molar ratios Me(2+)/DNA-phosphate

Molecular dynamics simulations of A. T-rich oligomers: sequence-specific binding of Na+ in the minor groove of B-DNA.

Molecular dynamics simulations have been employed to probe the sequence-specific binding of sodium ions to the minor groove of B-DNA of three A. T-rich oligomers having identical compositions but different orders of the base pairs: C(AT)(4)G, CA(4)T(4)G, and CT(4)A(4)G. Recent experimental investigations, either in crystals or in solution, have shown that monovalent cations bind to DNA in a sequence-specific mode, preferentially in the narrow minor groove regions of uninterrupted sequences of four or more adenines (A-tracts), replacing a water molecule of the ordered hydration structure, the hydration spine. Following this evidence, it has been hypothesized that in A-tracts these events may be responsible for structural peculiarities such as a narrow minor groove and a curvature of the helix axis. The present simulations confirm a sequence specificity of the binding of sodium ions: Na(+) intrusions in the first layer of hydration of the minor groove, with long residence times, up to approximately 3 ns, are observed only in the minor groove of A-tracts but not in the alternating sequence. The effects of these intrusions on the structure of DNA depend on the ion coordination: when the ion replaces a water molecule of the spine, the minor groove becomes narrower. Ion intrusions may also disrupt the hydration spine modifying the oligomer structure to a large extent. However, in no case intrusions were observed to locally bend the axis toward the minor groove. The simulations also show that ions may reside for long time periods in the second layer of hydration, particularly in the wider regions of the groove, often leading to an opening of the groove.

Solid-state 13C NMR study of poly(vinyl alcohol) gels.

Poly(vinyl alcohol) (PVA) with 55% and 61% syndiotacticity, and their related dry and hydrated gels obtained by two different freeze-thawing cycles have been investigated using the solid-state 13C CP-MAS NMR technique. From a comparative analysis of the spectra, evidence was obtained that the gelation process largely disrupts the intramolecular hydrogen-bonded network of the PVA. The addition of water to the dry gels favours their swelling, destroying intra-chain hydrogen bonds between hydroxyl groups as a function of the degree of tacticity and the gelation procedure, and promotes the formation of new networks of interchain hydrogen bonds. Information on the dynamics of the polymeric domains in the kilohertz range has been obtained from the analysis of the spin relaxation times T1rho(1H) and T1rho(13C) indicating that homogeneous arrangements of the amorphous or swollen polymeric chains exist, independent of the preparation method or the tacticity of the PVA chains.

Multinuclear NMR investigation of the NaDNA/ethidium bromide anisotropic system.

A combined use of (31)P, (23)Na, (2)H and (17)O NMR spectroscopies and polarized light microscopy has been employed to investigate the effect of the ethidium bromide (EB) binding on the liquid crystalline phase of concentrated double stranded DNA solutions. The optical textures and the (31)P and (23)Na NMR spectra of the DNA anisotropic solutions show that the intercalation of EB induces significant modifications either in the arrangements of the DNA rods and the surrounding ionic atmosphere. On the contrary, no indication of significant changes of the orientational order of the water molecules around DNA emerges from the water (2)H and (17)O NMR spectra.

Structure and Bonding of Diiodine Adducts of the Sulfur-Rich Donors 1,3-Dithiacyclohexane-2-thione (ptc) and 4,5-Ethylenedithio-1,3-dithiole-2-thione (ttb).

The reactions of I(2) with ptc and ttb (title ligands) have been investigated in CHCl(3) solution at different temperatures by spectrophotometry. A least-squares method procedure provided evidence for the formation of the 1:1 adducts. Crystals of the latter have been analyzed by X-ray diffraction methods (both monoclinic, P2(1)/c; ptc.I(2), a = 8.691(6) Å, b = 9.010(6) Å, c = 13.237(5) Å, beta = 103.43(2) degrees, Z = 4, R = 0.0305; ttb.I(2), a = 12.090(6) Å, b = 6.433(5) Å, c = 15.731(6) Å, beta = 99.30(2) degrees, Z = 4, R = 0.0419). Both structures show that the thionic sulfur (in any case a CS(3) group inserted in a ring) is bound almost collinearly with the diiodine molecule. The d(S-I) separations are 2.755(2) and 2.805(3) Å in the ptc.I(2) and ttb.I(2) adducts, respectively, while d(I-I) is practically the same (2.812(2) Å). An evident stereochemical difference is that the S-I-I moiety is nearly coplanar with the CS(3) group in ptc.I(2) while it is upright in ttb.I(2). However, the feature is not expected to cause a major electronic difference. In order to reproduce the structural features, different ab initio approaches have been attempted, with the best results being obtained with the density functional method (DFT). Despite the S-I distances which are slightly longer than the experimental ones (by ca. 0.25 Å), the distribution of filled and empty frontier molecular orbitals (MOs) allows a good interpretation of the visible spectra. Also a rationalization of the sigma electronic density distributed over the three centers S-I-I has been attempted by qualitative MO theory (EHMO method). Provided the good agreement with the higher level calculations, the perturbation theory arguments highlight the variable sp hybridization at the central iodine atom as the electronic factor of importance. The strength of the donor (D) affects significantly the redistribution of six electrons over four atomic orbitals, and the classic model is revised as a four-orbital/six-electron one. Thus, it is pointed out that a major four-electron repulsion is exerted over the D-I or the I-I linkages with major consequences for their respective lengths.