Crystal structure of l-leucyl-l-isoleucine 2,2,2-trifluoroethanol monosolvate

Unlike several other dipeptides with two hydrophobic residues, l-Leu-l-Ile has not previously been obtained as an alcohol solvate, forming instead two different hydrates. Formation of a co-crystal has here been achieved by using a 2,2,2-trifluoroethanol solution. As expected, the resulting structure is divided into hydrophilic and hydrophobic layers.


Chemical context
Dipeptides with at least one hydrophobic residue (i.e. lacking a functional group) such as Val, Leu, Ile and Phe have a high propensity to form crystal structures that are divided into hydrophobic and hydrophilic layers (Gö rbitz, 2010). The latter include two C(8) head-to-tail chains with two of the three Nterminal amino H atoms acting as donors and the C-terminal carboxylate group as acceptor, and also a C(4) or C(5) chain using the peptide >N-H group as donor and, respectively, the peptide carbonyl group or the carboxylate group as acceptor. The third amino H atom finds an acceptor in a polar side chain or, when both residues are hydrophobic, in a co-crystallized solvent molecule. l-Leu-l-Val has thus been obtained as a series of alcohol solvates (Gö rbitz & Torgersen, 1999), but also as a non-layered hydrate (Gö rbitz & Gundersen, 1996). The same is true for l-Leu-l-Leu (Gö rbitz, 1998(Gö rbitz, , 2001. l-Leul-Ile (LI) has, on the other hand, been obtained as two distinct hydrates; a 0.75 hydrate (Gö rbitz, 2004; CSD refcode ETIWIN) that is isostructural to the Leu-Val analogue (Gö rbitz & Gundersen, 1996), and a 2.5 hydrate with extensive water channels (Gö rbitz & Rise, 2008; CSD refcode HIZCOJ). Crystallization using methanol, ethanol or 2-propanol as precipitating agents did not result in formations of alcohol solvates. ISSN 2056-9890 Recently we have become interested in the use of fluorinated alcohols like 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol during crystallization, not only due to their superior abilities to dissolve a large range of organic molecules (abandoning the use of water if that is desirable), but also as crystal engineering tools to manipulate hydrogen-bonding patterns in solid-state structures by being incorporated into the crystal lattice by virtue of their strong hydrogen-bond-donating capacity. The crystal structure of the LI TFE solvate (I) presented here provides an example of how this can take place.

Structural commentary
The four molecules (two dipeptides and two solvent species) in the asymmetric unit are shown in Fig. 1. The structure is well behaved with normal bond lengths and bond angles. Disorder for TFE molecule D was easily resolved (see Refinement details). The molecular conformations of the two peptide molecules are quite different in terms of the side-chain conformations, Table 1. The overall molecular conformation of molecule B is very close to that of molecule B in the 2.5 hydrate (Gö rbitz & Rise, 2008). A substantial 24.5 deviation from the idealized trans orientation at 180 for 2 2 of molecule B is needed to relieve a short contact between H91B and F2C, Fig. 2.

Supramolecular features
The unit cell and crystal-packing arrangement is illustrated in Fig. 3a), hydrogen-bond parameters are listed in Table 2 The asymmetric unit of (I), solvent molecules being shown in different positions relative to the peptide molecules than they have in the unit cell to avoid extensive overlap. The minor disorder orientation for TFE molecule D is shown in wireframe representation. The amino group of molecule A has an unusual eclipsed conformation (blue shade) resulting from formation of an intramolecular hydrogen bond to O1A, while a normal staggered conformation (red shade) is observed for molecule B. Thermal displacement ellipsoids are shown at the 50% probability level.

Figure 2
In the experimental crystal structure of (I) (left) the ethyl group of the Ile residue of molecule B is rotated to relieve a short distance between H91B and F2C. If the C7B-C8B-C9B-C10B torsion angle had been exactly 180 , this distance would have been too short (right). The terminal methyl group, with C10B as a sphere, is not involved in any short contacts. Table 1 Selected torsion angles ( ).  Table 2 Hydrogen-bond geometry (Å , ). While the two molecules in the asymmetric unit of structures like l-Met-l-Ala 2-propanol solvate (Gö rbitz, 2000; CSD refcode CAQTOD) and l-Leu-l-Phe 2-propanol solvate (Gö rbitz, 1999; CSD refcode COCGOQ) are quite similar and related by pseudotranslational symmetry along a 10 Å long axis, the differences between the conformations (as discussed above) and relative positions of LI molecules A and B are readily observed in Fig. 3b). The C(5) hydrogen-bonded chain is part of an S5 hydrogen-bonded sheet, one out of four distinct types of sheets observed in layered dipeptide crystal structures (Gö rbitz, 2010). This sheet is compared in Fig. 4 to the corresponding sheet of l-Leu-l-Val 2-propanol solvate (Gö rbitz & Torgersen, 1999), where the third amino hydrogen atom is accepted by the co-crystallized alcohol molecule (shaded blue in Fig. 4b).
At the same time, the hydroxyl group serves as a hydrogenbond donor to the peptide carbonyl group, which is not involved in any other strong hydrogen bonds (in distinction to the related S4 pattern). Precisely the same function is taken by TFE molecule D in Fig. 4a), but solvent molecule C is different; it seeks out and forms a hydrogen bond to the carboxylate group of peptide molecule B, uniquely abandoning its role as a hydrogen-bond acceptor (red shade in Fig. 4b). The third amino H atom of molecule A is then left to participate in only a bent intramolecular interaction that leads to the inherently less favorable eclipsed amino conformation shown in Fig. 1.
In summary, TFE has been shown to be co-crystallized with L-Leu-L-Ile, thus radically changing the hydrogen bonding pattern. Is is the first dipeptide alcohol solvate where an Hydrogen bonds in (a) the crystal structure of (I) and (b) the crystal structure of l-Leu-l-Val 2-propanol solvate (Gö rbitz & Torgersen, 1999; CSD refcode JUCSEF01). Peptide C atoms and solvent C atoms carrying hydroxyl groups are shown as small spheres, other side-chain and solvent atoms have been omitted for clarity. The archetype S5 pattern in (b) is characterized by the presence of one syn and one anti head-to-tail C(8) chain with alternating molecules being related by Screw symmetry (light grey shades), as well as a C(5) chain involving an amide >N-H donor and a carboxylate acceptor. An S4 pattern has the same symmetry, but a C(4) chain to O C< carbonyl acceptor, while consecutive molecules in T5 and T4 sheets are related by Translation rather than by a screw operation (Gö rbitz, 2010). See text for details on the red and blue shades. alcohol molecule does not act as a hydrogen bond acceptor, but rather forms a strong hydrogen bond donor to a peptide carboxylate acceptor.

Synthesis and crystallization
l-Leu-l-Val was purchased from Sigma-Aldrich and used as received. Colorless plates of the title compound were grown by vapor diffusion of acetonitrile into 30 ml of a saturated trifluoroethanol solution of the dipeptide.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. Solvent molecule D is disordered over a major and a minor position with occupancies 0.825 (5) and 0.175 (5), respectively. The O1 and C1 atoms of the minor component were constrained to have the same set of anisotropic displacement parameters as the corresponding atoms of the major component, while C2 and the three F atoms were refined isotropically.   Crystal structure of L-leucyl-L-isoleucine 2,2,2-trifluoroethanol monosolvate Carl Henrik Görbitz

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. One of the solvent molecules is disordered over two positions.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (