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1. Self-Assembly and Rearrangement of a Polyproline II Helix Peptide on Gold

   https://doi.org/10.1021/acs.langmuir.0c03583  

   Hostert, Jacob D. ; Loney, Charles N. ; Pramounmat, Nuttanit ; Yan, Katherine ; Su, Zihang ; Renner, Julie N. ( May 2021 , Langmuir)
2. Solvation stabilizes intercarbonyl n→π* interactions and polyproline II helix

   https://doi.org/10.1039/D2CP00857B  

   Zondlo, Neal J. ( June 2022 , Physical Chemistry Chemical Physics)

   n→π* interactions between consecutive carbonyls stabilize the α-helix and polyproline II helix (PPII) conformations in proteins. n→π* interactions have been suggested to provide significant conformational biases to the disordered states of proteins. To understand the roles of solvation on the strength of n→π* interactions, computational investigations were conducted on a model n→π* interaction, the twisted-parallel-offset formaldehyde dimer, as a function of explicit solvation of the donor and acceptor carbonyls, using water and HF. In addition, the effects of urea, thiourea, guanidinium, and monovalent cations on n→π* interaction strength were examined. Solvation of the acceptor carbonyl significantly strengthens the n→π* interaction, while solvation of the donor carbonyl only modestly weakens the n→π* interaction. The n→π* interaction strength was maximized with two solvent molecules on the acceptor carbonyl. Urea stabilized the n→π* interaction via simultaneous engagement of both oxygen lone pairs on the acceptor carbonyl. Solvent effects were further investigated in the model peptides Ac-Pro-NMe 2 , Ac-Ala-NMe 2 , and Ac-Pro 2 -NMe 2 . Solvent effects in peptides were similar to those in the formaldehyde dimer, with solvation of the acceptor carbonyl increasing n→π* interaction strength and resulting in more compact conformations, in both the proline endo and exo ringmore »puckers, as well as a reduction in the energy difference between these ring puckers. Carbonyl solvation leads to an energetic preference for PPII over both the α-helix and β/extended conformations, consistent with experimental data that protic solvents and protein denaturants both promote PPII. Solvation of the acceptor carbonyl weakens the intraresidue C5 hydrogen bond that stabilizes the β conformation.« less
3. Electronic and Steric Control of n→π* Interactions: Stabilization of the α‐Helix Conformation without a Hydrogen Bond

   https://doi.org/10.1002/cbic.201800785  

   Wenzell, Nicole A. ; Ganguly, Himal K. ; Pandey, Anil K. ; Bhatt, Megh R. ; Yap, Glenn P. A. ; Zondlo, Neal J. ( January 2019 , ChemBioChem)
4. A Library Approach to Cationic Amphiphilic Polyproline Helices that Target Intracellular Pathogenic Bacteria

   https://doi.org/10.1021/acsinfecdis.8b00124  

   Nepal, Manish ; Mohamed, Mohamed F. ; Blade, Reena ; Eldesouky, Hassan E. ; N. Anderson, Tiffany ; Seleem, Mohamed N. ; Chmielewski, Jean ( July 2018 , ACS Infectious Diseases)
5. Glycine in Water Favors the Polyproline II State

   https://doi.org/10.3390/biom10081121  

   Andrews, Brian ; Zhang, Shuting ; Schweitzer-Stenner, Reinhard ; Urbanc, Brigita ( August 2020 , Biomolecules)

   Conformational preferences of amino acid residues in water are determined by the backbone and side-chain properties. Alanine is known for its high polyproline II (pPII) propensity. The question of relative contributions of the backbone and side chain to the conformational preferences of alanine and other amino acid residues in water is not fully resolved. Because glycine lacks a heavy-atom side chain, glycine-based peptides can be used to examine to which extent the backbone properties affect the conformational space. Here, we use published spectroscopic data for the central glycine residue of cationic triglycine in water to demonstrate that its conformational space is dominated by the pPII state. We assess three commonly used molecular dynamics (MD) force fields with respect to their ability to capture the conformational preferences of the central glycine residue in triglycine. We show that pPII is the mesostate that enables the functional backbone groups of the central residue to form the most hydrogen bonds with water. Our results indicate that the pPII propensity of the central glycine in GGG is comparable to that of alanine in GAG, implying that the water-backbone hydrogen bonding is responsible for the high pPII content of these residues.