Javier Ramirez

Chiral recognition in gas-phase cyclodextrin: amino acid complexes--is the three point interaction still valid in the gas phase?

The validity of the “three-point interaction” model is examined in the guest exchange reaction involving complexes of cyclodextrins and amino acids. The amino acid guest is exchanged in the gas phase in the presence of a gaseous alkyl amine. The net reaction is proton transfer between the protonated amino acid and the alkyl amine. The amino acid is lost as a neutral species. This reaction is sensitive to the chirality of the amino acid. Several amino acids are examined as well as the respective methyl esters to determine the role of the three interacting groups (ammonium, carboxylic acid, and side chain) in enantioselectivity. We find that the three-point interaction model is indeed valid in the gas phase. Enantioselectivity is optimal when two points of attraction and one repulsion is present in the gas-phase complex. The results are supported by molecular modeling calculations. A mechanism for the exchange is proposed.

Differentially heated capillary for thermal dissociation of noncovalently bound complexes produced by electrospray ionization

Abstract A heated capillary consisting of two independently heated segments is used to dissociate gas-phase complexes of peptides and β-cyclodextrin. By fixing the temperature in the first segment and varying it in the second segment, the complex is first desolvated and then dissociated. The results indicate that the complex is dissociated in the gas phase rather than in the solution phase of an intact droplet. The thermal dissociation profile, the dissociation temperature and apparent Arrhenius parameters Ea and A are obtained. These values are compared to Ea obtained from blackbody infrared radiation dissociation.

Thermal dissociation of protonated cyclodextrin-amino acid complexes in the gas phase

Abstract Dissociation temperatures of protonated gas-phase amino acid-cyclodextrin complexes were determined by producing the complexes with electrospray ionization and passing them through a resistively heated capillary. The dissociation temperatures of the complexes increased with the number of hydrogen bonding interactions and the ability of the cyclodextrin host to sterically lock the amino acid guest in place. There was little correlation between the gas-phase basicity of the amino acid and the dissociation temperature. Molecular modeling was used to better understand the nature of the interactions.

Evidence for enzymatic activity in the absence of solvent in gas-phase complexes of lysozyme and oligosaccharides

Abstract Hen egg-white lysozyme complexed to substrate and nonsubstrate oligosaccharides are examined by electrospray ionization mass spectrometry. Heated capillary dissociation and collision induced dissociation (CID) are used to characterize the complexes. The relative order of stability obtained in gas phase agrees well with their solution-phase association constants. A proton transfer reaction occurs during the CID of substrate complexes that is not observed with non-substrate complexes. Oligosaccharide fragments are also observed with lysozyme–chitohexaose complex during CID. The possibility of lysozyme activity in the gas phase, or a gas-phase enzymatic activity, is explored.