Water Molecule Adsorption on Short Alanine Peptides:  How Short Is the Shortest Gas-Phase Alanine-Based Helix?

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-A8K+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.