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.

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 exhibited cis-proline and a type VIa2 β-turn (BcisD). The cis-proline conformation was stabilized by a C-H/O interaction between Pro C-H_α and the Ser side-chain oxygen. NMR data were consistent with stabilization of cis-proline by a C-H/O interaction in solution. The other crystallographically observed molecule had trans-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 and trans-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_γ and Pro C-H_α were 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+3 or CO_i•••HN_i+4 hydrogen bonds, with Ser as the i residue and Pro as the i + 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_γ capable of engaging in a hydrogen bond with the amide N-H of the i + 2 (type I β-turn or 3₁₀-helix; Ser χ₁ t) or i + 3 (α-helix; Ser χ₁ g⁺) residue. Non-proline cis amide bonds can also be stabilized by C-H/O interactions.