The Crystal Structure and the Absolute Configuration of (+)-2-Benzylglutamic Acid Hydrobromide Dihydrate

The crystal structure of (+)-2-benzylglutamic acid hydrobromide dihydrate has been determined using three-dimensional X-ray intensity data. The crystal is orthorhombic, with four formula units in a unit cell with the dimensions: a=8.38, b=10.54, and c=17.68 Å, the space group being P212121. The structure was solved by the heavy atom method, and its parameters were refined by the least-squares method using anisotropic thermal parameters, the final R being 0.073. The absolute configuration of the glutamic acid has also been determined to be d-(+)-2-benzylglutamic acid, [α]d20+1.65° (c 10, 4 n HCl). The conformation of the carbon skeleton of the glutamic acid is different from any of the glutamic acids so far analyzed, due to the internal rotations of the C–C bonds. A detailed comparison of the conformations of several glutamic acids and glutamines will be given. Nine independent hydrogen bonds make a rather unique and complicated three-dimensional network. Among these, two between the carboxyl groups (donor) and the water molecules (acceptor) are found to be fairly strong, their bond lengths being 2.55 and 2.51 Å.