H–d-Phe–d-Pro–Gly methyl ester hydrochloride monohydrate

The conformation of the title tripeptide methyl ester hydrochloride monohydrate, 1-[2-(methoxycarbonylmethylaminocarbonyl)pyrrolidin-1-ylcarbonyl]-2-phenylethanaminium chloride monohydrate, C17H24N3O4 +·Cl−·H2O, is extended, but the structure cannot be classified as any typical secondary structure. Interactions through water molecules and chloride ions were formed, in addition to peptide–peptide hydrogen bonds, stabilizing the molecular packing. In comparison with the previous β-turn structure of the Phe–d-Pro–Gly analogue [Doi, Ichimiya & Asano (2007 ▶). Acta Cryst. E63, o4691], it was suggested that the difference between the chiralities of Phe and Pro residues of the title compound is important to induce the β-turn structure.

The molecular structure of (I) is shown in Fig. 1. The peptide is a chloride salt and its N-terminal (N10 atom) is protonated. The peptide molecule is somewhat extended, but the structure is not classified to any typical secondary structures from torsion angles. The Pro residue shows a ring puckering with amplitude of Q2 = 0.361 (2) Å and phase of φ2 = 293.1 (2) ° (Cremer & Pople, 1975), which is slightly different from those of the β-turn structure of Boc-Phe-D-Pro-Gly-OMe (Doi et al., 2007).
A peptide-peptide hydrogen bond is formed between N10 and O18 atoms. This interaction makes the molecular arrangement propagated along the b axis, but no sheet structure is created (Fig. 2). Molecular packing is stabilized by the interactions with chloride ion (Cl) and water molecule (O1).
CD spectra of (I) showed no clear proof of special structures existed in acetonitril solution (data not shown), and the structure of (I) was somewhat extended. In contrast to the β-turn structure of the diastereomeric tripeptide (Boc-Phe-D-Pro-Gly-OMe), these results indicate that the chirality of Phe different from that of Pro is important for folding of this tripeptide motif.

S2. Experimental
The title compound was synthesized by a conventional liquid-phase method and the protected peptide, Boc-D-Phe-D-Pro-Gly-OMe (Boc = t-Butyloxycarboxy; OMe = methylester), was obtained. Boc group was removed by using HCl/dioxane, and the hydrocloride salt was obtained. Crystals were grown from aqueous acetonitrile solutions by vapor diffusion method.

S3. Refinement
The non-H atoms were refined anisotropically. H atoms were treated as riding atoms with distances C-H = 0.95-1.00 Å, N-H (-NH 3 + ) = 0.91 Å and N-H (CONH) = 0.88 Å; U iso (H) = 1.2U iso (C), U iso (H) = 1.5U eq (C methyl ), U iso (H) = 1.2U eq (N CONH ) and U iso (H) = 1.5U eq (N NH3 ). H atoms of the water molecule were found in a difference Fourier map considering hydrogen-bond networks and fixed during refinements with U iso (H) = 1.2U eq (O). The absolute structure was based on the starting materials and was established by Flack parameter.  A view of (I) with displacement ellipsoids drawn at the 50% probability level with the aid of PLATON (Spek, 2003).
Dotted lines represent hydrogen bonds.

Figure 2
Packing diagram of (I). Side chains of amino acids are omitted for clarity. Dotted lines represent hydrogen bonds. Circles and filled-circles represent chloride ion (Cl) and water (O1) molecules.  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.