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

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 crysta ls. In PIPVAL . 2H(2)O two zwitterionic molecules are linked by a pair of i dentical N+-H ... O hydrogen bonds of 2.684(4) Angstrom into a cyclic centr osymmetric dimer. The dimers form a network of hydrogen bonds with water mo lecules. The conformation of anhydrous PIPVAL was studied by semiempirical and ab initio calculations. Two cyclic dimers were derived by the PM3 metho d (neutral and zwitterionic, with the former being 50 kcal mol(-1) more sta ble) and only the neutral one at the SAM1 and HF levels. The solid state FT IR spectra of the anhydrous and dihydrate crystals are considerably differe nt. The spectrum of PIPVAL . 2H(2)O reflects the N+-H ... O hydrogen bond d etermined 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 Angstrom is observed in the anhydrous acid. This comparison clearly ill ustrates the specific features of amino acids associating with water, prese nt in all living organisms.