Induction-driven stabilization of the anion-π interaction in electron-rich aromatics as the key to fluoride inclusion in imidazolium-cage receptors

Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion-π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion-π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH(3)CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-π interactions and (C-H)(+)···F(-)-type ionic hydrogen bonds. (1)H NMR, (19)F NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F(-), a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F(-) as well as Gibbs free energy. The 2-F(-) complex is more stable than the 1-F(-) complex by 1.87 kcal mol(-1), which is attributable to the stronger anion-π interaction between F(-) and triethylbenzene.     

Stability and structure of mixed-ligand metal ion complexes that contain Ni2+, Cu2+, or Zn2+, and Histamine, as well as adenosine 5'-triphosphate (ATP4-) or uridine 5'-triphosphate (UTP(4-): an intricate network of equilibria

With a view on protein–nucleic acid interactions in the presence of metal ions we studied the “simple” mixed‐ligand model systems containing histamine (Ha), the metal ions Ni²⁺, Cu²⁺, or Zn²⁺ (M²⁺), and the nucleotides adenosine 5′‐triphosphate (ATP^4?) or uridine 5′‐triphosphate (UTP^4?), which will both be referred to as nucleoside 5′‐triphosphate (NTP^4?) . The stability constants of the ternary M(NTP)(Ha)^2? complexes were determined in aqueous solution by potentiometric pH titrations. We show for both ternary‐complex types, M(ATP)(Ha)^2? and M(UTP)(Ha)^2?, that intramolecular stacking between the nucleobase and the imidazole residue occurs and that the stacking intensity is approximately the same for a given M²⁺ in both types of complexes: The formation degree of the intramolecular stacks is estimated to be 20 to 50 %. Consequently, in protein–nucleic acid interactions imidazole–nucleobase stacks may well be of relevance. Furthermore, the well‐known formation of macrochelates in binary M²⁺ complexes of purine nucleotides, that is, the phosphate‐coordinated M²⁺ interacts with N7, is confirmed for the M(ATP)^2? complexes. It is concluded that upon formation of the mixed‐ligand complexes the M²⁺? N7 bond is broken and the energy needed for this process corresponds to the stability differences determined for the M(UTP)(Ha)^2? and M(ATP)(Ha)^2? complexes. It is, therefore, possible to calculate from these stability differences of the ternary complexes the formation degrees of the binary macrochelates: The closed forms amount to (65±10) %, (75±8) %, and (31±14) % for Ni(ATP)^2?, Cu(ATP)^2?, and Zn(ATP)^2?, respectively, and these percentages agree excellently with previous results obtained by different methods, confirming thus the internal validity of the data and the arguments used in the evaluation processes. Based on the overall results it is suggested that M(ATP)^2? species, when bound to an enzyme, may exist in a closed macrochelated form only, if no enzyme groups coordinate directly to the metal ion.     

Controlled self-assembly of re-engineered insulin by Fe(II)

Self-assembly of proteins mediated by metal ions is crucial in biological systems and a better understanding and novel strategies for its control are important. An abiotic metal ion ligand in a protein offers the prospect of control of the oligomeric state, if a selectivity over binding to the native side chains can be achieved. Insulin binds Zn(II) to form a hexamer, which is important for its storage in vivo and in drug formulations. We have re-engineered an insulin variant to control its self-assembly by covalent attachment of 2,2'-bipyridine. The use of Fe(II) provided chemoselective binding over the native site, forming a homotrimer in a reversible manner, which was easily followed by the characteristic color of the Fe(II) complex. This provided the first well-defined insulin trimer and the first insulin variant for which self-assembly can be followed visually.     

肌球蛋白中金属离子与ATP相互作用及其催化ATP水解的研究进展

生物体内细胞在氧化物质的过程中释放出的大量自由能,这些能量先形成高能磷酸化合物三磷酸腺苷（adenosine 5′-triphosphate,ATP）,当ATP水解为ADP（二磷酸腺苷,adenosine 5′-diphosphate）和无机磷酸时.     

Noncovalent complexation of glucosylthioureidocalix[4]arenes with carboxylates and their gas-phase characteristics: an ESI-FTICR mass spectrometric study

The noncovalent complexation of three glucosylcalix[4]arenes (1-3) towards 23 mono- and dicarboxylic acid anions were studied by ESI-FTICR mass spectrometry. Competitive complexation, collision-induced dissociation and gas-phase H/D-exchange experiments were performed to obtain information on selectivity of calixarenes towards carboxylates and characteristics of their complexes. The flexibility and number of glucose units of the host and the spatial disposition of the hydrogen bonding groups on the carboxylate guests were found to affect the selectivity of complexation strongly. The glucosylcalixarenes exhibited particular selectivity for dicarboxylic acid anions incorporating π-systems, and clear isomeric selectivity was observed for isophthalic among phthalic acid anions and for fumaric acid over maleic acid anion.     

Retention and selectivity effects caused by bonding of a polar urea-type ligand to silica: a study on mixed-mode retention mechanisms and the pivotal role of solute-silanol interactions in the hydrophilic interaction chromatography elution mode

The separation properties of five silica packings bonded with 1-[3-(trimethoxysilyl)propyl]urea in the range of 0–3.67 μmol m⁻² were investigated in the hydrophilic interaction chromatography (HILIC) elution mode. An increase of the ligand surface density promoted retention of non-charged polar compounds and even more so for acids. An opposite trend was observed for bases, while the amphoteric compound tyrosine exhibited a U-shaped response profile. An overall partitioning retention mechanism was incompatible with these observations; rather, the substantial involvement of adsorptive interactions was implicated. Support for the latter was provided by column-specific changes in analyte retention and concomitant selectivity effects due to variations of salt concentration, type of salt, pH value, organic modifier content, and column temperature. Silica was more selective for separating compounds differing in charge state (e.g. tyramine vs. 4-hydroxybenzoic acid), while in cases where structural differences of solutes resided in non-charged polar groups (e.g. tyramine vs. 5-hydroxydopamine, nucleoside vs. nucleobase) more selective separations were obtained on bonded phases. Hierarchical cluster analysis of the home-made urea-type and three commercial amide-type bonded packings evinced considerable differences in separation properties. The present data emphasise that the role of the packing material under HILIC elution conditions is hardly just the polar support for a dynamic coating with a water-enriched layer. Three major retention mechanisms are claimed to be relevant on bare silica and the urea-type bonded packings: (i) HILIC-type partitioning, (ii) HILIC-type weak adsorption such as hydrogen bonding between solutes and ligands or solutes and silanols (potentially influenced by individual degrees of solvation, salt bridging, etc.), (iii) strong electrostatic (ionic) solute–silanol interactions (attractive/repulsive). Even when non-charged polar bonded phases are used, solute–silanol interactions should not be discounted, which makes them a prime parameter to be characterised by HILIC column tests. Multi/mixed-mode type separations seem to be common under HILIC elution conditions, associated with a great deal of selectivity increments. They are accessible and controllable by a careful choice of the type of packing, the mobile phase composition, and the temperature.