In order to understand the single crystal X-ray structures of a series of synthetic peptides mimicking the Pro138-Gly144 segment of human AQP4  , we have crystallized and determined the X-ray structures of some of these peptides. The crystal structure of the peptides Boc-Pro-Pro-OH (1), Boc-Aib-Aib-OMe in two polymorphs (2 and 3), Boc-Val-Val-OMe (4), and the tripeptide Boc-allylSer-Aib-Val-OMe (5), were elucidated.Peptide 1, C15H24N2O5, was designed and synthesized to the corresponding N-terminal Pro138-Pro139 segment of the 310-helix loop C (Pro138-Pro139-Ser140-Val141-Val142-Gly143-Gly144) of the AQP4. The compound crystallized in the orthorhombic space group P212121 with one molecule in the asymmetric unit. tBoc-Pro bond and Pro-Pro along the peptide bond show cis- and trans-conformations (cis-trans), approximately with ω = -8.2° and ω = -174.35°, respectively. Proline residues in 1 adopt incipient Poly-L-proline type I (PPI) and type II (PPII) backbone conformations. Orthorhombic and monoclinic form of the same compound has been determined so far by different authors. Finally, the backbone conformation of 1 is compared with these two forms (orthorhombic and monoclinic form) as well as with the Pro138-Pro139 segment of AQP4 at 1.8 Å and 3.2 Å resolutions.The peptide C14H26N2O5 was obtained in two polymorphic forms (2 and 3), both exist in the monoclinic space group P21/c and P21/n, respectively with one molecule in the asymmetric unit. The Aib residues in Boc-Aib-Aib-OMe adopt (φ, ψ)-values which are characteristics of helical conformations. The dihedral angles of the peptide backbone show that the Aib residues lie in left- or right-handed 310- /α-helical regions in the Ramachandran plot. The torsion angles indicate values for folded conformations.13Peptide 4, C16H30N2O5, was designed and synthesized to the corresponding Val141-Val142 segment of the AQP4 loop C. It was crystallized in the orthorhombic space group P212121 with three molecules in the asymmetric unit. Each molecule in the asymmetric unit adopts a β- strand/polyproline II backbone conformation. The main chain functional groups are hydrogen bonded into tapes carrying the characteristics of parallel β-sheets. Each tape has a left-handed twist and thus forms a helix, with six peptide molecules needed to complete a full 360° rotation. A comparison of hydrogen bond lengths and twisting mode is made with other related structures of protected dipeptides. Additionally, a comparison of the backbone conformation is made with that of the Val141-Val142 segment of the water channel aquaporin-4.Peptide 5, C21H37N3O7, crystallize in the monoclinic space group C2 with a single molecule in the asymmetric unit. A single crystal X-ray diffraction studies of 5 reveals that it adopts a bend (or turn-like structure) without any intramolecular hydrogen bonding. Most of the dihedral angles of the tripeptide 5 fall within the helical region of the Ramachandran plot.