DL-Isoleucine

The crystal structures of N-acetyl-DL-isoleucine, N-acetyl-DL-alloisoleucine and their ammonium salts show that these four compounds exist as racemic compounds around room temperature. The two ammonium salts are arranged around a 21 screw axis, forming a helical column which consists of ammonium ions and single enantiomeric anions similar to the crystals of the ammonium salts of optically active N-acetyl-L-isoleucine and N-acetyl-D-alloisoleucine. The ammonium ion and the carboxylate ion in the helix are connected by three hydrogen bonds, the fourth hydrogen bond being formed between the ammonium ion and an external acetyl amino group of the neighboring helical column. The fourth hydrogen bond is formed between the ammonium ion and an external acetyl amino group of the neighboring 21 column. Ammonium N-acetyl-DL-alloisoleucinate was revealed to exist as an unstable racemic compound due to conformational similarity between the racemic and optically active compounds in the solid state and was optically resolved by fractional crystallization at 293 K.

DL‑isoleucine single crystals were grown by the slow evaporation method at ambient temperature. Their vibrational properties were studied at ambient temperature as a function of pressure by Raman scattering. At ambient conditions the mode assignment was done in terms of the Potential Energy Distribution (PED) through density functional theory calculations. Both nitrogen and neon were used as pressure transmitting media. The pressure-dependent investigation shows modifications in the Raman spectra recorded between 30 and 3200 cm -1 that were interpreted as phase transitions undergone by the crystal between 1.3 and 1.9 GPa and between 3.6 and 5.1 GPa. Finally, stress was simulated on the unit cell of the crystal from ambient up to 5.0 GPa.

I studied the elastic properties of crystalline L- and DL-forms of leucine and isoleucine within the framework of density functional theory with van der Waals interactions. The energy gaps of the considered crystals are 7.48-7.60 eV. Chiral molecules have the same chemical composition. Therefore, the study of crystalline amino acids provides a better understanding of how the structure of molecules affects mechanical properties of molecular crystals. Complete set of elastic constants for L-leucine, L-isoleucine, DL-leucine and DL-isoleucine were calculated. Linear compressibility of crystals has high anisotropy. The crystalline L- and DL-forms of leucine and isoleucine have different mechanical properties. Linear compressibility has a negative value for DL-isoleucine. My calculations predict that L-leucine and L-isoleucine are ductile compounds, while DL-leucine and DL-isoleucine are brittle compounds.