title: Direct probing of zwitterion formation in unsolvated peptides.
abstract: Molecular beam electric deflection measurements have been used to determine electric susceptibilities for small unsolvated alanine-based peptides. The electric susceptibility provides information about the charge distribution within the peptide and can be used to distinguish between zwitterionic and canonical forms. Measured electric susceptibilities for WAn peptides (n = 1-5) are similar to those for capped Ac-WAn-NH2 peptides (which cannot form zwitterions). Susceptibilities calculated using a simulated tempering-based approach are substantially larger for the zwitterionic form than for the canonical form. The measured susceptibilities are in good agreement with those calculated for the canonical form. For the larger peptides, the lowest potential energy structure found in the simulations is hairpin-like, while the lowest free energy structure found at room temperature is extended. The zwitterionic form is constrained by intramolecular interactions which make it entropically unfavorable.
authors.affiliation: Laboratoire de Spectrométrie Ionique et Moléculaire, UMR No. 5579, CNRS et Université Lyon 1, 43 bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France.
authors.forename: Rodolphe
authors.initials: R
authors.lastname: Antoine
authors.forename: Michel
authors.initials: M
authors.lastname: Broyer
authors.forename: Philippe
authors.initials: P
authors.lastname: Dugourd
authors.forename: Gary
authors.initials: G
authors.lastname: Breaux
authors.forename: Frederick C
authors.initials: FC
authors.lastname: Hagemeister
authors.forename: David
authors.initials: D
authors.lastname: Pippen
authors.forename: Robert R
authors.initials: RR
authors.lastname: Hudgins
authors.forename: Martin F
authors.initials: MF
authors.lastname: Jarrold
mesh_terms: D000409:Alanine / Q000737:chemistry*; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D010455:Peptides; D014364:Tryptophan; D000409:Alanine