Anhydrous Polyproline Helices and Globules

Ion mobility/time-of-flight methods and molecular modeling calculations have been used to examine the conformations of a range of polymer lengths and charge states of polyproline peptides, [Pro_n + zH]^z+ (n = 3-56; z = 1-6). Ions formed from 1-propanol solutions {[Pro_n + H]⁺ (n = 5-11) and [Pro_n + 2H]²⁺ (n = 10-22)} favor extended forms of the classical polyproline I helix. In these conformers, all proline residues are in the cis configuration, and protonation at the N-terminus allows hydrogen bonds to be formed with backbone carbonyl groups of the second and third proline residues in each polymer. Protonation of this all-cis form at the N-terminus also stabilizes the helix macrodipole. Singly charged ions formed from aqueous solutions favor globular and hairpin-like conformers that contain both cis- and trans-proline residues. Higher charge state ions (z = 3-6) formed from aqueous solutions favor relatively extended conformations, although these are not as extended as the polyproline II structural limit. As polymer size increases, higher charge state ions become more compact. Several conformer states of varying size that appear to be favored structural types are observed; however, we have not been able to identify the type of structures based on comparison of molecular modeling data and experimental measurements.