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.