Helix unfolding in unsolvated peptides

The conformations of unsolvated Ac-K(AGG)(5)+H+ and Ac-(AGG)(5)K+H+ peptide s (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 st ructure) 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 temperat ure is raised the alpha -helix unfolds. Rate constants for loss of the heli x (on a millisecond time scale) have been determined as a function of tempe rature and yield an Arrhenius activation energy and preexponential factor o f 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 firs t converts into a long-lived intermediate which survives to a significantly higher temperature before converting. According to molecular dynamics simu lations, there is a partially untwisted helical conformation that has both a low energy and a well-defined geometry. This special structure lies betwe en the helix and globule and may be the long-lived intermediate.