title: "Wet" Versus "Dry" Folding of Polyproline.
abstract: When the all-cis polyproline-I helix (PPI, favored in 1-propanol) of polyproline-13 is introduced into water, it folds into the all-trans polyproline-II (PPII) helix through at least six intermediates [Shi, L., Holliday, A.E., Shi, H., Zhu, F., Ewing, M.A., Russell, D.H., Clemmer, D.E.: Characterizing intermediates along the transition from PPI to PPII using ion mobility-mass spectrometry. J. Am. Chem. Soc. 136, 12702-12711 (2014)]. Here, we show that the solvent-free intermediates refold into the all-cis PPI helix with high (>90%) efficiency. Moreover, in the absence of solvent, each intermediate appears to utilize the same small set of pathways observed for the solution-phase PPII → PPI transition upon immersion of PPIIaq in 1-propanol. That folding in solution (under conditions where water is displaced by propanol) and folding in vacuo (where energy required for folding is provided by collisional activation) occur along the same pathway is remarkable. Implicit in this statement is that 1-propanol mimics a "dry" environment, similar to the gas phase. We note that intermediates with structures that are similar to PPIIaq can form PPII under the most gentle activation conditions-indicating that some transitions observed in water (i.e., "wet" folding, are accessible (albeit inefficient) in vacuo. Lastly, these "dry" folding experiments show that PPI (all cis) is favored under "dry" conditions, which underscores the role of water as the major factor promoting preference for trans proline. Graphical Abstract ᅟ.
authors.affiliation: Department of Chemistry, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Liuqing
authors.initials: L
authors.lastname: Shi
authors.affiliation: Department of Chemistry, Moravian College, Bethlehem, PA, 18018, USA.
authors.forename: Alison E
authors.initials: AE
authors.lastname: Holliday
authors.affiliation: Department of Chemistry, University of Southern Indiana, Evansville, IN, 47712, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Bohrer
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX, 77843, USA.
authors.forename: Doyong
authors.initials: D
authors.lastname: Kim
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX, 77843, USA.
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Servage
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX, 77843, USA. russell@chem.tamu.edu.
authors.forename: David H
authors.initials: DH
authors.lastname: Russell
authors.affiliation: Department of Chemistry, Indiana University, Bloomington, IN, 47405, USA. Clemmer@Indiana.EDU.
authors.forename: David E
authors.initials: DE
authors.lastname: Clemmer
mesh_terms: D010455:Peptides / Q000737:chemistry*; D011392:Proline; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012997:Solvents; D013816:Thermodynamics
chemical_list: D010455:Peptides; D012997:Solvents; C011083:polyproline; D011392:Proline
keywords: Collisional activation; Folding