title: Theoretical study of the gas-phase structures of sodiated and cesiated leucine and isoleucine: zwitterionic structure disfavored in kinetic method experiments.
abstract: The most stable charge-solvated (CS) and zwitterionic (ZW) structures of sodiated and cesiated leucine and isoleucine were studied by density functional theory methods. According to the Boltzmann distribution in gas phase, both forms of LeuNa+ and IleNa+ exist, but in LeuCs+ and IleCs+, the ZW forms are dominant. Results for the sodiated compounds are consistent with the relationship found between decrease in relative stability of CS versus ZW form and aliphatic amino acid side chain length. The observed degeneracy in energy for IleNa+ conformers is at odds with kinetic method results. Additional calculations showed that kinetic method structural determinations for IleNa+ do not reflect relative order of populations in the lowest energy conformers. Since complexation of cationized amino acids into ion-bound dimers disfavors ZW structure by approximately 8 kJ mol(-1), it is suggested that for energy close conformers of sodium-cationized amino acids, the kinetic method may not be reliable for structural determinations.
authors.affiliation: Laboratory for Chemical Kinetics and Atmospheric Chemistry, Ruder Bosković Institute, Bijenicka 54, HR-10002, Zagreb, Croatia. marko@irb.hr
authors.forename: Marko
authors.initials: M
authors.lastname: Rozman
mesh_terms: D002586:Cesium / Q000737:chemistry; D005740:Gases; D007477:Ions / Q000737:chemistry; D007532:Isoleucine / Q000737:chemistry*; D007700:Kinetics; D007930:Leucine / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D012964:Sodium / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D005740:Gases; D007477:Ions; D007532:Isoleucine; D002586:Cesium; D012964:Sodium; D007930:Leucine