Polymorphism of l-nitroargininium bromide monohydrate

We have found that l-nitroargininium bromide monohydrate (l-NNA·HBr·H2O) has two polymorphic (α and β) forms. Vibrational spectra of l-NNA·HBr·H2O (α and β forms) and the previously known l-NNA·HCl·H2O are reported. On the basis of vibrational spectra it is concluded that α-form l-NNA·HBr·H2O is isostructural with l-NNA·HCl·H2O, in the structure of which carboxyl groups form hydrogen bonds with chloride ions. We have determined crystal and molecular structures of β-form l-NNA·HBr·H2O. Unlike l-NNA·HCl·H2O and therefore α-form l-NNA·HBr·H2O, in the structure of β-form carboxyl group forms strong hydrogen bond with water molecule. Similarity of the structure of β-form l-NNA·HBr·H2O with the structure of anhydrous l-nitroarginine methyl ester hydrochloride is revealed, which happens because OH3 grouping serves as methyl group. IR spectra of anhydrous specimens l-NNA·HCl and l-NNA·HBr (α and β forms) are reported as well.