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1. Electronic Control of Polyproline II Helix Stability via the Identity of Acyl Capping Groups: the Pivaloyl Group Particularly Promotes PPII

   https://doi.org/10.1002/chem.202401454  

   Bhatt, Megh_R; Zondlo, Neal_J (June 2024, Chemistry – A European Journal)

   Abstract The type II polyproline helix (PPII) is a fundamental secondary structure of proteins, important in globular proteins, in intrinsically disordered proteins, and at protein‐protein interfaces. PPII is stabilized in part byn→π* interactions between consecutive carbonyls, via electron delocalization between an electron‐donor carbonyl lone pair (n) and an electron‐acceptor carbonyl (π*) on the subsequent residue. We previously demonstrated that changes to the electronic properties of the acyl donor can predictably modulate the strength ofn→π* interactions, with data from model compounds, in solution in chloroform, in the solid state, and computationally. Herein, we examined whether the electronic properties of acyl capping groups could modulate the stability of PPII in peptides in water. InX−PPGY‐NH₂peptides (X=10 acyl capping groups), the effect of acyl group identity on PPII was quantified by circular dichroism and NMR spectroscopy. Electron‐rich acyl groups promoted PPII relative to the standard acetyl (Ac−) group, with the pivaloyl andiso‐butyryl groups most significantly increasing PPII. In contrast, acyl derivatives with electron‐withdrawing substituents and the formyl group relatively disfavored PPII. Similar results, though lesser in magnitude, were also observed inX−APPGY‐NH₂peptides, indicating that the capping group can impact PPII conformation at both proline and non‐proline residues. The pivaloyl group was particularly favorable in promoting PPII. The effects of acyl capping groups were further analyzed inX–DfpPGY‐NH₂andX−ADfpPGY‐NH₂peptides, Dfp=4,4‐difluoroproline. Data on these peptides indicated that acyl groups induced order Piv‐ > Ac‐ > For‐. These results suggest that greater consideration should be given to the identity of acyl capping groups in inducing structure in peptides. 

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2. Synthesis and conformational preferences of peptides and proteins with cysteine sulfonic acid

   https://doi.org/10.1039/D3OB00179B  

   Bhatt, Megh R.; Zondlo, Neal J. (March 2023, Organic & Biomolecular Chemistry)

   Cysteine sulfonic acid (Cys-SO3H; cysteic acid) is an oxidative post-translational modification of cysteine, resulting from further oxidation from cysteine sulfinic acid (Cys- SO2H). Cysteine sulfonic acid is considered an irreversible post-translational modification, which serves as a biomarker of oxidative stress that has resulted in oxidative damage to proteins. Cysteine sulfonic acid is anionic, as a sulfonate (Cys-SO –; cysteate), in the ionization state that 3 is almost exclusively present at physiological pH (pKa ~ –2). In order to understand protein structural changes that can occur upon oxidation to cysteine sulfonic acid, we analyzed its conformational preferences, using experimental methods, bioinformatics, and DFT-based computational analysis. Cysteine sulfonic acid was incorporated into model peptides for α-helix and polyproline II helix (PPII). Within peptides, oxidation of cysteine to the sulfonic acid proceeds rapidly and efficiently at room temperature in solution with methyltrioxorhenium (MeReO3) and H2O2. Peptides containing cysteine sulfonic acid were also generated on solid phase using trityl-protected cysteine and oxidation with MeReO3 and H2O2. Using methoxybenzyl (Mob)-protected cysteine, solid-phase oxidation with MeReO3 and H2O2 generated the Mob sulfone precursor to Cys-SO – within fully synthesized peptides. These two solid-phase methods allow the synthesis of peptides containing either Cys-SO – or Cys-SO – in a 32 practical manner, with no solution-phase synthesis required. Cys-SO – had low PPII propensity 3 for PPII propagation, despite promoting a relatively compact conformation in φ. In contrast, in a PPII initiation model system, Cys-SO – promoted PPII relative to neutral Cys, with PPII initiation similar to Cys thiolate but less than Cys-SO – or Ala. In an α-helix model system, Cys- 2 SO – promoted α-helix near the N-terminus, due to favorable helix dipole interactions and 3 favorable α-helix capping via a sulfonate-amide side chain-main chain hydrogen bond. Across all peptides, the sulfonate side chain was significantly less ordered than that of the sulfinate. Analysis of Cys-SO – in the PDB revealed a very strong propensity for local (i/i or i/i+1) side 3 chain-main chain sulfonate-amide hydrogen bonds for Cys-SO –, with > 80% of Cys-SO – 33 residues exhibiting these interactions. DFT calculations conducted to explore these conformational preferences indicated that side chain-main chain hydrogen bonds of the sulfonate with the intraresidue amide and/or with the i+1 amide were favorable. However, hydrogen bonds to water or to amides, as well as interactions with oxophilic metals, were weaker for the sulfonate than the sulfinate, due to lower charge density on the oxygens in the sulfonate. 

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3. Helical twists and β‐turns in structures at serine–proline sequences: Stabilization of cis ‐proline and type VI β‐turns via C–H/O interactions

   https://doi.org/10.1002/prot.26701  

   Oven, Harrison_C; Yap, Glenn_P_A; Zondlo, Neal_J (May 2024, Proteins: Structure, Function, and Bioinformatics)

   Abstract Structures at serine‐proline sites in proteins were analyzed using a combination of peptide synthesis with structural methods and bioinformatics analysis of the PDB. Dipeptides were synthesized with the proline derivative (2S,4S)‐(4‐iodophenyl)hydroxyproline [hyp(4‐I‐Ph)]. The crystal structure of Boc‐Ser‐hyp(4‐I‐Ph)‐OMe had two molecules in the unit cell. One molecule exhibitedcis‐proline and a type VIa2 β‐turn (BcisD). Thecis‐proline conformation was stabilized by a C–H/O interaction between Pro C–H_αand the Ser side‐chain oxygen. NMR data were consistent with stabilization ofcis‐proline by a C–H/O interaction in solution. The other crystallographically observed molecule hadtrans‐Pro and both residues in the PPII conformation. Two conformations were observed in the crystal structure of Ac‐Ser‐hyp(4‐I‐Ph)‐OMe, with Ser adopting PPII in one and the β conformation in the other, each with Pro in the δ conformation andtrans‐Pro. Structures at Ser‐Pro sequences were further examined via bioinformatics analysis of the PDB and via DFT calculations. Ser‐Pro versus Ala–Pro sequences were compared to identify bases for Ser stabilization of local structures. C–H/O interactions between the Ser side‐chain O_γand Pro C–H_αwere observed in 45% of structures with Ser‐cis‐Pro in the PDB, with nearly all Ser‐cis‐Pro structures adopting a type VI β‐turn. 53% of Ser‐trans‐Pro sequences exhibited main‐chain CO_i•••HN_i+3or CO_i•••HN_i+4hydrogen bonds, with Ser as theiresidue and Pro as thei + 1 residue. These structures were overwhelmingly either type I β‐turns or N‐terminal capping motifs on α‐helices or 3₁₀‐helices. These results indicate that Ser‐Pro sequences are particularly potent in favoring these structures. In each, Ser is in either the PPII or β conformation, with the Ser O_γcapable of engaging in a hydrogen bond with the amide N–H of thei + 2 (type I β‐turn or 3₁₀‐helix; Serχ₁t) ori + 3 (α‐helix; Serχ₁g⁺) residue. Non‐prolinecisamide bonds can also be stabilized by C–H/O interactions. 

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4. Bioinspired Design Rules for Flipping across the Lipid Bilayer from Systematic Simulations of Membrane Protein Segments

   https://doi.org/10.1039/D5ME00032G  

   Park, ByungUk; Van_Lehn, Reid C (May 2025, Molecular Systems Design & Engineering)

   The orientation of integral membrane proteins (IMPs) with respect to the membrane is established during protein synthesis and insertion into the membrane. After synthesis, IMP orientation is thought to be fixed due to the thermodynamic barrier for “flipping” protein loops or helices across the hydrophobic core of the membrane in a process analogous to lipid flip-flop. A notable exception is EmrE, a homodimeric IMP with an N-terminal transmembrane helix that can flip across the membrane until flipping is arrested upon dimerization. Understanding the features of the EmrE sequence that permit this unusual flipping behavior would be valuable for guiding the design of synthetic materials capable of translocating or flipping charged groups across lipid membranes. To elucidate the molecular mechanisms underlying flipping in EmrE and derive bioinspired design rules, we employ atomistic molecular dynamics simulations and enhanced sampling techniques to systematically investigate the flipping of truncated segments of EmrE. Our results demonstrate that a membrane-exposed charged glutamate residue at the center of the N-terminal helix lowers the energetic barrier for flipping (from ~12.1 kcal mol-1 to ~5.4 kcal mol-1) by stabilizing water defects and minimizing membrane perturbation. Comparative analysis reveals that the marginal hydrophobicity of this helix, rather than the marginal hydrophilicity of its loop, is the key determinant of flipping propensity. Our results further indicate that interhelical hydrogen bonding upon dimerization inhibits flipping. These findings establish several bioinspired design principles to govern flipping in related materials: (1) marginally hydrophobic helices with membrane-exposed charged groups promote flipping, (2) modulating protonation states of membrane-exposed groups tunes flipping efficiency, and (3) interhelical hydrogen bonding can be leveraged to arrest flipping. These insights provide a foundation for engineering synthetic peptides, engineered proteins, and biomimetic nanomaterials with controlled flipping or translocation behavior for applications in intracellular drug delivery and membrane protein design. 

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5. Cis-trans isomerization of peptoid residues in the collagen triple-helix

   https://doi.org/10.1038/s41467-023-43469-8  

   Qiu, Rongmao; Li, Xiaojing; Huang, Kui; Bai, Weizhe; Zhou, Daoning; Li, Gang; Qin, Zhao; Li, Yang (December 2023, Nature Communications)

   Abstract Cis-peptide bonds are rare in proteins, and building blocks less favorable to the trans-conformer have been considered destabilizing. Although proline tolerates the cis-conformer modestly among all amino acids, for collagen, the most prevalent proline-abundant protein, all peptide bonds must be trans to form its hallmark triple-helix structure. Here, using host-guest collagen mimetic peptides (CMPs), we discover that surprisingly, even the cis-enforcing peptoid residues (N-substituted glycines) form stable triple-helices. Our interrogations establish that these peptoid residues entropically stabilize the triple-helix by pre-organizing individual peptides into a polyproline-II helix. Moreover, noting that the cis-demanding peptoid residues drastically reduce the folding rate, we design a CMP whose triple-helix formation can be controlled by peptoid cis-trans isomerization, enabling direct targeting of fibrotic remodeling in myocardial infarction in vivo. These findings elucidate the principles of peptoid cis-trans isomerization in protein folding and showcase the exploitation of cis-amide-favoring residues in building programmable and functional peptidomimetics. 

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