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.     

Calixarenes as hosts for ammonium cations: a quantum chemical study and mass-spectrometric investigations

Host-guest complexes of tetramethylcavitand with different ammonium cations were investigated by using a quantum chemical method at the density functional level (BP86, B3 LYP). The NH4+ cation is strongly bound to the host. Increasing methyl substitution at the cation decreases its inclination towards the complex formation. The calculated data are in line with results from electrospray ionization mass spectrometry (ESI-MS) experiments. They reveal stable aggregates only for the NH4+ cation and for the primary alkylammonium cations.     

Noncovalent complex formation of tetratosylated resorcarene with aromatic,aliphatic, and cyclic alkyl ammonium ions: a mass spectrometric study of complex properties

The ability of tetratosylated resorcarene to form complexes with aromatic ammonium ions was investigated by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The formation of noncovalent complexes, [1+guest]⁺ and [1 · 1+guest]⁺, as observed with singly charged aromatic anilinium and phenylene aminoammonium guests. Comparison of the complexation efficiencies of the aromatic and aliphatic ammonium ions showed the importance of proton affinity of conjugate amines in complex formation. In collision‐induced dissociation experiments, gas‐phase stability was found to be lower for complexes formed with aromatic ions and this behavior was not found to depend on the proton affinity of conjugate amines. Fast oxidation of the para and ortho aminoammonium ions and complexation of these ions with tetratosylated resorcarene was observed. Copyright © 2005 John Wiley & Sons, Ltd.     

Selective complexation of N-alkylpyridinium salts: binding of NAD+ in water

A new class of receptor molecules is presented that is highly selective for N-alkylpyridinium ions and electron-poor aromatics. Its key feature is the combination of a well-preorganized molecular clip with an electron-rich inner cavity and strategically placed, flanking bis-phosphonate monoester anions. This shape and arrangement of binding sites attracts predominantly flat electron-poor aromatics in water, binds them mainly by pi-cation, pi-pi, CH-pi, and hydrophobic interactions, and leads to their highly efficient desolvation. NAD(+) and NADP, the important cofactors of many redox enzymes, are recognized by the new receptor molecule, which embraces the catalytically active nicotinamide site and the adenine unit. Even nucleosides such as adenosine are likewise drawn into the clip's cavity. Complex formation and structures were examined by one- and two-dimensional NMR spectroscopy, Job plot analyses, and isothermal titration microcalorimetric (ITC) measurements, as well as quantum chemical calculations of (1)H NMR shifts. The new receptor molecule is a promising tool for controlling enzymatic oxidation processes and for DNA chemistry.     

Synthesis of poly(epsilon-lysine)-grafted dextrans and their pH- and thermosensitive hydrogelation with cyclodextrins.

To give pH sensitivity to a thermoreversible supramolecular-structured hydrogel system, poly(epsilon-lysine) (PL), as a cationic polymer, was grafted to dextran and used for inclusion complexation with alpha-cyclodextrins (alpha-CDs). The synthesized graft copolymer was characterized by 1H NMR spectroscopy, and the hydrogel formation was confirmed by X-ray diffraction and solid-state 13C NMR analysis. The hydrogelation was induced from a phase-separated structure of hydrated dextrans and hydrophobically aggregated inclusion complexes in buffer solution at pH 10.0. The prepared hydrogels showed thermoreversible gel-sol transitions as well as pH-sensitive phase transitions, which were recorded by the changes in UV/Vis transmittance. A rapid phase transition from gel to sol was observed upon decreasing the pH value to 4.0, which resulted from the dissociation process between the protonated guest polymer and alpha-CDs. The stimuli-responsive physical properties of the hydrogels were improved by modulating the degree of substitution of the grafted PL and the combination with alpha-CDs.     

Structurally diagnostic ion-molecule reactions: acylium ions with alpha-, beta- and gamma-hydroxy ketones

Gas-phase reactions of four acylium ions and a thioacylium ion with three isomeric alpha-, beta- and gamma-hydroxy ketones are performed by pentaquadrupole mass spectrometric experiments. Novel structurally diagnostic reactions are observed, and found to correlate directly with interfunctional group separation. All five ions tested (CH(3)CO(+), CH(2)(double bond)CHCO(+), PhCO(+), (CH(3))(2)NCO(+) and (CH(3))(2)NCS(+)) react with the gamma-hydroxy ketone (5-hydroxy-2-pentanone) to form nearly exclusively a cyclic oxonium ion of m/z 85 that formally arises from hydroxy anion abstraction. With the beta-hydroxy ketone (4-hydroxy-2-pentanone), CH(2)(double bond)CHCO(+), PhCO(+) and (CH(3))(2)NCO(+) form adducts that undergo fast cyclization via intramolecular water displacement, yielding resonance-stabilized cyclic dioxinylium ions. With the alpha-hydroxy ketone (3-hydroxy-3-methyl-2-butanone), PhCO(+), (CH(3))(2)NCO(+) and (CH(3))(2)NCS(+) form stable adducts. Evidence that these adducts display cyclic structures is provided by the triple-stage mass spectra of the (CH(3))(2)NCS(+) adduct; it dissociates to (CH(3))(2)NCO(+) via a characteristic reaction-dissociation pathway that promotes sulfur-by-oxygen replacement. If cyclizations are assumed to occur with intramolecular anchimeric assistance, relationships between structure and reactivity are easily recognized.     

Host-enhanced π-π interaction for water-soluble supramolecular polymerization

Host-enhanced π-π interaction based on anthracene derivatives and cucurbit[8]uril can be used as the driving force for constructing water-soluble supramolecular polymers. For this purpose, two anthracene moieties were encapsulated into one cucurbit[8]uril cavity, forming a ternary complex. After encapsulation in the host, the distance between the two anthracene moieties was shortened, and the π-π interaction between them was enhanced significantly. To realize supramolecular polymerization, a bifunctional monomer consisting of two anthracene moieties and a short linker in between was carefully designed. Cyclization was avoided in this way. Thus, host-enhanced π-π interaction can function as a new driving force for supramolecular polymerization.     

Size- and structure-selective noncovalent recognition of saccharides by tetraethyl and tetraphenyl resorcinarenes in the gas phase

The noncovalent complexation of tetraethyl and tetraphenyl resorcinarenes with mono-, di-, and oligosaccharides was studied with negative-polarization electrospray ionization quadrupole ion trap and electrospray ionization Fourier-transform ion cyclotron resonance mass-spectrometric analysis. The saccharides formed 1:1 complexes with deprotonated resorcinarenes, which exhibited clear size and structure selectivity in their complexation. In the case of the monosaccharides, hexoses formed much more abundant and kinetically stable complexes than pentoses or deoxyhexoses. A comparison of the mono-, di-, and oligosaccharides revealed that both the relative abundance and stability of the complexes increase up to biose and triose, but start to decrease after that point, as the length of the oligosaccharide is increased. This behavior was rationalized by comparing the lowest-energy conformations of the complexes formed between the resorcinarene and oligosaccharides. This comparison was achieved by using theoretical calculations and X-ray crystal studies.     

Facial selectivity of the (R)-1,3-dimethyl-1-cyclohexyl cation in the gas phase

The model reaction between the (R)-1,3-dimethyl-1-cyclohexyl cation (I) and methanol has been investigated under gas-phase radiolytic conditions (750 Torr; 25-120 degrees C) with the aim of evaluating the intrinsic factors that govern the facial selectivity of biased carbocations. The peculiarity of the experimental approach allows the formation of different CH(3) (18)OH.I ionic adducts. Subsequent conversion of these adducts to give the corresponding E/Z covalent products follows different reaction coordinates, which are characterized by their own activation parameters. On the grounds of density functional theory (DFT) results, several [CH(3)OH.I] structures have been located on the relevant potential-energy surface (PES). The experimental results point to a gas-phase facial selectivity, which is mainly governed by entropic factors that arise as a result of the occurrence of different noncovalent ion-molecule "facial adducts" (FA). The formation of FAs may also play an important role in both the reaction dynamics and the positional selectivity. The present results cannot be interpreted by any of the models based on solution-phase experiments.