Helix Unfolding in Unsolvated Peptides

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Abstract
The conformations of unsolvated Ac-K(AGG)5+H+ and Ac-(AGG)5K+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 α-helix and a globule are found for Ac-(AGG)5K+H+ at low temperature. As the temperature is raised the α-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 × 109 s-1, respectively. The α-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.