Characterizing Intermediates Along the Transition from Polyproline I to Polyproline II Using Ion Mobility Spectrometry-Mass Spectrometry

Abstract

Polyproline exists predominately as the all-cis polyproline I (PPI) helix in aliphatic alcohols, whereas the all-trans polyproline II (PPII) helix is favored in aqueous solutions. Previous ion mobility spectrometry-mass spectrometry (IMS-MS) work demonstrates that the gas-phase conformations of polyproline ions can be related to the corresponding PPI and PPII helices in solution [J. Phys. Chem. B 2004, 108, 4885]. Here, we use IMS-MS to examine the detailed intermediate steps associated with the process of Polyproline-13 (Pro13) conversion from the PPI helix to the PPII helix upon solvent exchange. Collision cross section distributions of Pro13 [M + 2H](2+) ions obtained at different transition times indicate the presence of two major conformers, identified as the PPI and PPII helices, and six conformers that appear as subpopulations of polyproline. Further analysis shows a transition mechanism with sequential cis-trans isomerizations followed by a parallel process to establish PPII and two smaller subpopulations at equilibrium. Temperature-dependent studies are used to obtain Arrhenius activation parameters for each step of the mechanism, and molecular dynamics simulations provide insight about the structures of the intermediates. It appears that prolines sequentially flip from cis to trans starting from the N-terminus. However, after the first few transitions, possible steps take place at the center of the peptide chain; subsequently, several pathways appear to be accessible at the same time. Our results reflect the existence of stable subpopulations in polyprolines and provide new insight into the structural changes during the transition process of polyproline peptides converting from PPI to PPII in aqueous solution.

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

“…It is clear that the majority of the population cannot originate in the gas phase and, thus, there remain only two alternatives: it either reflects the population of this conformer in solution, which survives the electrospray process, or the conditions in the electrospray droplets. A possible way to address this question would be to change the solution conditions, for example by changing the solvent or concentration of the solute and measuring the cis-Pro to trans-Pro ratio in the ion mobility spectra [19,66].…”

Measuring the Elasticity of Poly‐l‐Proline Helices with Terahertz Spectroscopy

“…[13] Proline is unique amongst naturally occurring amino acids as the only residue able to readily form both cis and trans configurations about its peptide bond linkages, [14] thereby permitting two different helical structures to exist for poly-l-proline. [15] Theall-cis right-handed helix (Form I, PP-I) is tightly wound, [16] while the all-trans left-handed helix (Form II, PP-II) adopts al ess dense geometry (Figure 1). [17] Theavailability of these similar, yet fundamentally different, poly-l-proline helices makes them excellent choices for exploring the connection between molecular structure,l owfrequency vibrational motions,and bulk elastic constants.…”

“…These results are consistent with previous studies of the poly-l-proline transformation, which found that large activation energy barriers exist along the conversion coordinate. [15,29] Themeasurement of biopolymer elasticity through acombined approach of THz-TDS experiments and ss-DFT simulations enables quantification of molecular rigidities to be achieved in arelatively straightforward way.This methodology has yielded the previously unmeasured Youngs moduli of the widespread poly-l-proline polypeptide in both its helical forms,a nd revealed them to be considerably more elastic than expected. This prompts contemplation of their role as an analytical standard for rigidity.…”

Measuring the Elasticity of Poly‐l‐Proline Helices with Terahertz Spectroscopy

“…Proline is unique amongst naturally occurring amino acids as the only residue able to readily form both cis and trans configurations about its peptide bond linkages,14 thereby permitting two different helical structures to exist for poly‐ l ‐proline 15. The all‐ cis right‐handed helix (Form I, PP‐I) is tightly wound,16 while the all‐ trans left‐handed helix (Form II, PP‐II) adopts a less dense geometry (Figure 1).…”