Search for: All records

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. 

Abstract Proline residues within proteins lack a traditional hydrogen bond donor. However, the hydrogens of the proline ring are all sterically accessible, with polarized C−H bonds at Hα and Hδ that exhibit greater partial positive character and can be utilized as alternative sites for molecular recognition. C−H/O interactions, between proline C−H bonds and oxygen lone pairs, have been previously identified as modes of recognition within protein structures and for higher‐order assembly of protein structures. In order to better understand intermolecular recognition of proline residues, a series of proline derivatives was synthesized, including 4R‐hydroxyproline nitrobenzoate methyl ester, acylated on the proline nitrogen with bromoacetyl and glycolyl groups, and Boc‐4S‐(4‐iodophenyl)hydroxyproline methyl amide. All three derivatives exhibited multiple close intermolecular C−H/O interactions in the crystallographic state, with H⋅⋅⋅O distances as close as 2.3 Å. These observed distances are well below the 2.72 Å sum of the van der Waals radii of H and O, and suggest that these interactions are particularly favorable. In order to generalize these results, we further analyzed the role of C−H/O interactions in all previously crystallized derivatives of these amino acids, and found that all 26 structures exhibited close intermolecular C−H/O interactions. Finally, we analyzed all proline residues in the Cambridge Structural Database of small‐molecule crystal structures. We found that the majority of these structures exhibited intermolecular C−H/O interactions at proline C−H bonds, suggesting that C−H/O interactions are an inherent and important mode for recognition of and higher‐order assembly at proline residues. Due to steric accessibility and multiple polarized C−H bonds, proline residues are uniquely positioned as sites for binding and recognition via C−H/O interactions.