High-temperature ion mobility measurements have been performed for alpha-helical Ac-A15K+H+ and globular Ac-KA15+H+ peptides. The alpha-helical and globular conformations do not melt into random coils as the temperature is raised. Instead, both conformations survive to the point where the peptide signals vanishes due to fragmentation. This occurs at 600 K for the globular Ac-KA15+H+ peptide and at 725 K for the alpha-helical Ac-A15K+H+. For the helical Ac-A15K+H+ peptide it appears that fragmentation is triggered by disruption of the helical conformation.

Recent studies have shown that protonated polyalanine peptides (Ala n +H) + are not helical in the gas phase, 1,2 despite alanine's high helix propensity in solution. 3 Here we show that substituting an alkali metal ion (Li + , Na + , K + , Rb + , or Cs + ) for the proton leads to a transformation from a random globule to a rigid helix. While metal ions have been found to enhance the helicity of small peptides in aqueous solution, [4][5][6][7][8] this often involves residues with high metal affinities locking in the helical conformation by formation of metal-mediated cross-links. The peptides discussed here do not have metal-complexing side chains. The enhanced helicity apparently results because the oxyphilic metal ions coordinate to CO groups at the C-terminus. This "caps" the helix 9,10 and allows favorable interactions between the metal ion and the helix dipole. [11][12][13] The interactions between metal ions and proteins play many important roles in energy metabolism and signaling. Metal ions often cause conformational changes when they bind to proteins, which can profoundly affect their properties. [14][15][16][17] Metal ion interactions also provide a valuable tool in the design of proteins. [18][19][20][21] There have been a number of experimental and theoretical studies of the interactions between small peptides and metal ions in the gas phase. These studies have focused on the energetics and location of metal binding, [22][23][24][25] on the conformations

High-resolution ion mobility measurements and molecular dynamics (MD) simulations have been used to study the conformations of unsolvated valine-based peptides with up to 20 residues. In aqueous solution, valine is known to have a high propensity to form β-sheets and a low propensity to form α-helices. A variety of protonated valine-based peptides were examined in vacuo:  Val n +H+, Ac-Val n -Lys+H+, Ac-Lys-Val n +H+, Val n -Gly-Gly-Val m +H+, Val n -LPro-Gly-Val m +H+, Val n -DPro-Gly-Val m +H+, Ac-Val n -Gly-Lys-Val m +H+, Ac-Val n +H+, and Arg-Val n +H+. Peptides designed to be β-hairpins were found to be random globules or helices. The β-hairpin is apparently not favored for valine-based peptides in vacuo, which is in agreement with the predictions of MD simulations. Peptides designed to be α-helices appear to be partial α/partial π-helices. Insertion of Gly-Gly, LPro-Gly, or DPro-Gly into the center of a polyvaline peptide disrupts helix formation. Some of the peptides that were expected to be random globules (because their most basic protonation site is near the N-terminus where protonation destabilizes the helix) were found to be helical with the proton located near the C-terminus. Helix formation appears to be more favorable in unsolvated valine-based peptides than in their alanine analogues. This is the reverse of what is observed in aqueous solution, but appears to parallel the helix propensities determined in polar solvents.

Water adsorption measurements have been performed under equilibrium conditions for unsolvated Ac-A(n)K+H(+) and Ac-KA(n)+H(+) peptides with n = 4 - 10. Previous work on larger alanine peptides has shown that two dominant conformations (helices and globules) are present for these peptides and that water adsorbs much more strongly to the globules than to the helices. All the Ac-KA(n)+H(+) peptides studied here (which are expected to be globular) adsorb water strongly, and so do the Ac-A(n)K+H(+) peptides with n < 8. However, for Ac-A(n)K+H(+) with n = 8-10 there is a substantial drop in the propensity to adsorb water. This result suggests that Ac-A(8)K+H(+) is the smallest Ac-A(n)K+H(+) peptide to have a significant helical content in the gas phase. Water adsorption measurements for Ac-V(n)K+H(+) and Ac-L(n)K+H(+) with n = 5-10 suggest that the helix emerges at n = 8 for these peptides as well.

Equilibrium constants for the adsorption of the first water molecule onto a variety of unsolvated alanine-based peptides have been measured and Delta H degrees and DeltaS degrees have been determined. The studies were designed to examine the effects of conformation, charge, and composition on the propensity for peptides to bind water. In general, water adsorption occurs significantly more readily on the globular peptides than on helical ones: several of the singly charged helical peptides were not observed to adsorb a water molecule even at -50 degrees C. These results place a limit on the free energy change for interaction between a water molecule and the helical peptide group. Molecular dynamics simulations reproduce most of the main features of the results. The ability to establish a network of hydrogen bonds to several different hydrogen-bonding partners emerges as a critical factor for strong binding of the water molecule. Whether the charge site is involved in water adsorption depends on how well it is shielded. Peptides containing a protonated histidine bind water much more strongly that those containing a protonated lysine because the delocalized charge on histidine is difficult to shield. The entropy change for adsorption of the first water molecule is correlated with the enthalpy change.