title: Helix unfolding in unsolvated peptides.
abstract: The conformations of unsolvated Ac-K(AGG)(5)+H(+) and Ac-(AGG)(5)K+H(+) peptides (Ac = acetyl, A = alanine, G = glycine, and K = lysine) have been examined by ion mobility measurements over a wide temperature range (150-410 K). The Ac-K(AGG)(5)+H(+) peptide remains a globule (a compact, roughly spherical structure) over the entire temperature range, while both an alpha-helix and a globule are found for Ac-(AGG)(5)K+H(+) at low temperature. As the temperature is raised the alpha-helix unfolds. Rate constants for loss of the helix (on a millisecond time scale) have been determined as a function of temperature and yield an Arrhenius activation energy and preexponential factor of 38.2 +/- 1.0 kJ mol(-1) and 6.5 +/- 3.7 x 10(9) s(-1), respectively. The alpha-helix apparently does not unfold directly into the globule, but first converts into a long-lived intermediate which survives to a significantly higher temperature before converting. According to molecular dynamics simulations, there is a partially untwisted helical conformation that has both a low energy and a well-defined geometry. This special structure lies between the helix and globule and may be the long-lived intermediate.
authors.affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
authors.forename: B S
authors.initials: BS
authors.lastname: Kinnear
authors.forename: M R
authors.initials: MR
authors.lastname: Hartings
authors.forename: M F
authors.initials: MF
authors.lastname: Jarrold
mesh_terms: D000409:Alanine / Q000737:chemistry; D005998:Glycine / Q000737:chemistry; D007700:Kinetics; D010455:Peptides / Q000737:chemistry*; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D021241:Spectrometry, Mass, Electrospray Ionization; D013696:Temperature
chemical_list: D010455:Peptides; D000409:Alanine; D005998:Glycine