Glucosylthioureidocalix[4]arenes: Synthesis, conformations and gas phase recognition of amino acids.

The gas-phase recognition of native amino acids and the conformational properties of three glucosylthioureidocalix[4]arenes () were studied theoretically and experimentally using ab initio calculations, ESI-FTICR,(1)H and (13)C NMR MS. The conformational and complexation properties of the glucocalixarenes were dependent on the number of glucose units at the upper rim and the length of the alkyl chains at the lower rim of the calixarene skeleton. ESI-MS experiments showed the compounds to form 1 : 1 complexes with the amino acids, with a marked preference for amino acids containing an aromatic nucleus and an additional H-bonding group in their side chain (Trp, Tyr, Phe >> Ser, Leu and Asp). The experimental data were rationalized by the results of ab initio calculations. ESI-MS competitions carried out with enantiomeric-labelled (EL) amino acids showed enantiomeric selectivities ranging from 0.61 (Phe(D)/Phe(L) with ligand ) to 2.58 (Tyr(D)/Tyr(L) with ligand ). In gas-phase hydrogen-deuterium (H/D) exchange reactions, diglucosylcalix[4]arene exhibited extremely slow exchange rates, which were attributed to the close proximity and strong hydrogen bonding between the facing glucosylthioureido groups. H/D exchange rates were much higher for the tetraglucosylcalix[4]arenes and and their amino acid complexes, and the more rigid tetrapropoxy derivative showed more selective H/D exchange reactions than the calixarene . Bi- or trimodal H/D exchange distribution was observed for the tetraglucosyl derivatives indicating that these ligands exist in multiple isomeric forms in gas phase.

Ab initio studies on nanoscale friction between fluorinated diamond surfaces: effect of model size and level of theory.

Interactions between two fluorinated diamond surfaces placed in contact with each other were investigated with quantum chemical Hartree-Fock and Møller-Plesset perturbation theory and basis sets def-SV(P), def-TZVP, and 6-31G. Two models, C(6)H(6)F(3)-C(24)H(24)F(12) and C(13)H(16)F(6)-C(22)H(24)F(10), were used to examine how model size and level of theory affect the atomic-scale friction, especially the coefficient of friction. Also of interest was a comparison of the interaction energies of the two models with different stacking configurations. The averages of the calculated friction coefficients fell within the range of values 0.28-0.43.

Ab initio studies on nanoscale friction between graphite layers: effect of model size and level of theory.

Ab initio methods were used to investigate the nanoscale friction between two graphite layers placed in contact. The interaction energies were calculated for four two-layer models in series, C(6(n+1))2H(6n+1))-C(6)(n)2H(6)(n) with n = 1, 2, 3, and 4, and additionally for C(54)H(18)-C(6)H(6) and C(150)H(30)-C(6)H(6). The study was done with the Hartree-Fock method using basis sets 3-21G and 6-31G and with the second-order Møller-Plesset theory using basis set 6-31G. A density functional method (B3PW91) was also tested for reference purposes. The main interest was how the model size and level of theory affect the nanoscale friction coefficient. Most of the calculated friction coefficients fell within the range of values of 0.07-0.14.

Hydrogen bonding in phosphonate cavitands: investigation of host-guest complexes with ammonium salts.

The H-bonding in alkylammonium complexes of phosphonate cavitands were studied by mass spectrometric methods and theoretical calculations. The alkylammonium ions included primary, secondary, and tertiary methyl- and ethylammonium ions. Their complexation with mono-, tetra-, and two di-phosphonate cavitands, which differ according to the number and position of H-bond acceptor P = O groups, was evaluated by using different competition experiments, energy-resolved CID, gas-phase H/D-exchange, and ligand-exchange reactions, together with ab initio theoretical optimization of the complexes. The phosphonate cavitands with two or more adjacent P = O groups were found to be selective towards secondary alkylammonium ions, due to simultaneous formation of two stable hydrogen bonds. In the ion-molecule reactions (both H/D- and ligand-exchange), the formation of two stable hydrogen bonds was observed either to slow down the reaction or to completely prevent it. This was, however, limited to situations where two hydrogen bonds are formed between the H-bond donor sites of the alkyl ammonium ion and the vicinal H-bond acceptor sites of the cavitand.

The complex formation of tetracyclohexylammonium C1-resorcinarene with various guests - an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry study.

The complex formation of a tetraammonium C1-resorcinarene (R+4HCl) was studied using electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Although R+4HCl easily loses its counter ions in the ESI process, a neutral self-assembled structure with an intramolecular circular hydrogen-bonded 16-membered -N(+)-H ... X(-) ... H-N(+)- array with ammonium ion as the charge-giving species was observed in the gas phase. In addition to chloride, several other counter ions were also studied. The size and basicity of the counter ion as well as the size of the charge-giving cation strongly affected the gas-phase stability of the self-assembled system. H/D exchange experiments showed that the ammonium substituents in the apical position of R affect the hydrogen-bonding system in the resorcinarene. The complexation of the saturated dicarboxylic acids was found to depend on the length of the carbon chain. The rigidity of the molecular skeleton of the acid improved the complexation considerably. The orientation and position of the carboxylic groups also had an effect on the complexation and consequently enabled stereochemical differentiation of the acids. Mass spectrometric observations were supported by theoretical calculations.

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.