skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Solvation stabilizes intercarbonyl n→π* interactions and polyproline II helix
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 ring 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.  more » « less
Award ID(s):
2004110 1616490
PAR ID:
10336810
Author(s) / Creator(s):
Date Published:
Journal Name:
Physical Chemistry Chemical Physics
Volume:
24
Issue:
22
ISSN:
1463-9076
Page Range / eLocation ID:
13571 to 13586
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; (, 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. 
    more » « less
  2. ; ; (, 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–Hαand 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 Oγand Pro C–Hαwere 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Oi•••HNi+3or COi•••HNi+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 Oγcapable 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. 
    more » « less
  3. ; ; ; ; ; ; ; ; (, Biochemistry)
    Despite the importance of proline conformational equilibria (trans versus cis amide, exo versus endo ring pucker) on protein structure and function, there is a lack of convenient ways to probe proline conformation. 4,4-Difluoroproline (Dfp) was identified to be a sensitive 19F NMR-based probe of proline conformational biases and of cis-trans isomerism. Within model compounds and disordered peptides, the diastereotopic fluorines of Dfp exhibit similar chemical shifts (FF = 0–3 ppm) when a trans X–Dfp amide bond is present. In contrast, the diastereotopic fluorines exhibit a large (FF = 5–12 ppm) difference in chemical shift in a cis X–Dfp prolyl amide bond. DFT calculations, X-ray crystallography, and solid-state NMR spectroscopy indicated that the FF directly reports on the relative preference of one proline ring pucker over the other: a fluorine which is pseudo-axial (i.e. the pro-4R-F in an exo ring pucker, or the pro-4S-F in an endo ring pucker) is downfield, while a fluorine which is pseudo-equatorial (i.e. pro-4S-F when exo, or pro-4R-F when endo) is upfield. Thus, when a proline is disordered (a mixture of exo and endo ring puckers, as at trans-Pro in peptides in water), it exhibits a small . In contrast, when the Pro is ordered (i.e. when one ring pucker is strongly preferred, as in cis-Pro amide bonds, where the endo ring pucker is strongly favored), a large  is observed. Dfp can be used to identify inherent induced order in peptides and to quantify proline cis-trans isomerism. Using Dfp, we discovered that the stable polyproline II helix (PPII) formed in the denatured state (8 M urea) exhibits essentially equal populations of the exo and endo proline ring puckers. In addition, the data with Dfp suggested the specific stabilization of PPII by water over other polar solvents. These data strongly support the importance of carbonyl solvation and n* interactions for the stabilization of PPII. Dfp was also employed to quantify proline cis-trans isomerism as a function of phosphorylation and the R406W mutation in peptides derived from the intrinsically disordered protein tau. Dfp is minimally sterically disruptive and can be incorporated in expressed proteins, suggesting its broad application in understanding proline cis-trans isomerization, protein folding, and local order in intrinsically disordered proteins. 
    more » « less
  4. ; ; (, Biochemistry)
    Acyl capping groups stabilize -helices relative to free N-termini by providing one additional C=Oi•••Hi+4–N hydrogen bond. The electronic properties of acyl capping groups might also directly modulate -helix stability: electron-rich N-terminal acyl groups could stabilize the -helix by strengthening both i/i+4 hydrogen bonds and i/i+1 n* interactions. This hypothesis was tested in peptides X–AKAAAKAAAKAAAAKAAGY-NH2, X=different acyl groups. Surprisingly, the most electron-rich acyl groups (pivaloyl, iso-butyryl) strongly destabilized the -helix. Moreover, the formyl group induced nearly identical -helicity as the acetyl group, despite being a weaker electron donor for hydrogen bonds and for n* interactions. Other acyl groups exhibited intermediate -helicity. These results indicate that the electronic properties of the acyl carbonyl do not directly determine -helicity in peptides in water. In order to understand these effects, DFT calculations were conducted on -helical peptides. Using implicit solvation, -helix stability correlated with acyl group electronics, with the pivaloyl group exhibiting closer hydrogen bonds and n* interactions, in contrast to the experimental results. However, DFT and MD calculations with explicit water solvation revealed that hydrogen bonding to water was impacted by the sterics of the acyl capping group. Formyl capping groups exhibited the closest water-amide hydrogen bonds, while pivaloyl groups exhibited the longest. In -helices in the PDB, the highest frequency of close amide-water hydrogen bonds is observed when the N-cap residue is Gly. The combination of experimental and computational results indicates that solvation (hydrogen bonding of water) to the N-terminal amide groups is a central determinant of -helix stability. 
    more » « less
  5. ; ; (, chemRxiv)
    In proteins, proline-aromatic sequences exhibit increased frequencies of cis-proline amide bonds, via proposed C–H/π interactions between the aromatic ring and either the proline ring or the backbone C–Hα of the residue prior to proline. These interactions would be expected to result in tryptophan, as the most electron-rich aromatic residue, exhibiting the highest frequency of cis-proline. However, prior results from bioinformatics studies on proteins and experiments on proline-aromatic sequences in peptides have not revealed a clear correlation between the properties of the aromatic ring and the population of cis-proline. An investigation of the effects of aromatic residue (aromatic ring properties) on the conformation of proline-aromatic sequences was conducted using three distinct approaches: (1) NMR spectroscopy in model peptides of the sequence Ac-TGPAr-NH2 (Ar = encoded and unnatural aromatic amino acids); (2) bioinformatics analysis of structures in proline-aromatic sequences in the PDB; and (3) computational investigation using DFT and MP2 methods on models of proline-aromatic sequences and interactions. C–H/π and hydrophobic interactions were observed to stabilize local structures in both the trans-proline and cis-proline conformations, with both proline amide conformations exhibiting C–H/π interactions between the aromatic ring and Hα of the residue prior to proline (Hα-trans-Pro-aromatic and Hα-cis-Pro-aromatic interactions) and/or with the proline ring (trans-ProH-aromatic and cis-ProH-aromatic interactions). These C–H/π interactions were strongest with tryptophan (Trp) and weakest with cationic histidine (HisH+). Aromatic interactions with histidine were modulated in strength by His ionization state. Proline-aromatic sequences were associated with specific conformational poses, including type I and type VI β-turns. C–H/π interactions at the pre-proline Hα, which were stronger than interactions at Pro, stabilize normally less favorable conformations, including the ζ or αL conformations at the pre-proline residue, cis-proline, and/or the g+ χ1 rotamer or αL conformation at the aromatic residue. These results indicate that proline-aromatic sequences, especially Pro-Trp sequences, are loci to nucleate turns, helices, loops, and other local structures in proteins. These results also suggest that mutations that introduce proline-aromatic sequences, such as the R406W mutation that is associated with protein misfolding and aggregation in the microtubule-binding protein tau, might result in substantial induced structure, particularly in intrinsically disordered regions of proteins. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email