Conformations of, and NHO hydrogen bond in, piperidine-1-valeric acid and its dihydrate

Abstract

Effects of hydration on hydrogen bonding and conformation of the amino acid, piperidine-1-valeric acid (PIPVAL), have been studied by X-rays, FTIR and quantum chemical calculations. PIPVAL forms anhydrous and dihydrate crystals. In PIPVAL·2H₂O two zwitterionic molecules are linked by a pair of identical N⁺–H· · ·O hydrogen bonds of 2.684(4) Å into a cyclic centrosymmetric dimer. The dimers form a network of hydrogen bonds with water molecules. The conformation of anhydrous PIPVAL was studied by semiempirical and ab initio calculations. Two cyclic dimers were derived by the PM3 method (neutral and zwitterionic, with the former being 50 kcal mol^–1 more stable) and only the neutral one at the SAM1 and HF levels. The solid state FTIR spectra of the anhydrous and dihydrate crystals are considerably different. The spectrum of PIPVAL·2H₂O reflects the N⁺–H· · ·O hydrogen bond determined by X-rays. The broad and intense absorption in the 1600–400 cm^–1 region resulting from the much shorter hydrogen bond N⁺–H· · ·O of about 2.5 Å is observed in the anhydrous acid. This comparison clearly illustrates the specific features of amino acids associating with water, present in all living organisms.