Proline-Aromatic Sequences Stabilize Turns via C-H/π Interactions in Both cis-Proline and trans-Proline

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. However, previous bioinformatics studies on proteins and experiments on proline-aromatic sequences in peptides have not revealed a clear correlation between the electronic properties of the aromatic ring and the population of cis-proline. An investigation of the effects of aromatic residue on the conformation of proline-aromatic sequences was conducted using three approaches: NMR spectroscopy in model peptides of the sequence Ac-TGPAr-NH₂ (Ar = encoded and unnatural aromatic amino acids); bioinformatics analysis of structures in proline-aromatic sequences in the PDB; and quantum computational investigations. C-H/π and hydrophobic interactions were observed to stabilize local structures in both the trans-proline and cis-proline conformations, with each exhibiting C-H/π interactions between the aromatic ring and Hα of the residue prior to proline and/or with the proline ring. These C-H/π interactions were strongest with tryptophan and weakest with cationic histidine. 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 α_L conformations at the pre-proline residue, cis-proline, and/or the g⁺ χ₁ rotamer or α_L conformation at the aromatic residue. Proline-aromatic sequences, especially Pro-Trp sequences, are loci to nucleate turns, helices, loops, and other local structures in proteins.