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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. 

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.