Six conformers of neutral aspartic acid identified in the gas phase.

The multiconformational landscape of the proteogenic amino acid aspartic acid, the simplest amino acid with two carboxylic groups, has been explored for the first time in the gas phase. Solid aspartic acid (m.p. > 300 degrees C) was vaporized by laser ablation and expanded in a supersonic jet where rotational spectra of various conformers were obtained by Fourier transform microwave spectroscopy. Six different conformers have been conclusively identified by their distinct rotational and (14)N nuclear quadrupole coupling constants. The relative stability of the conformers rests on a delicate balance of the different intramolecular hydrogen bonds established between the two carboxylic groups and the amino group.