Zwitterion Formation

The amino acids are the building blocks of a large variety of biological substances. They are known to exist in their zwitterionic forms in aqueous environments over a wide pH range. Therefore, efforts have turned to the small hydrate clusters, where water molecules are sequentially added to induce the H₂N–R–COOH ↔ ⁺H₃N–R–CO₂– transition.

The photoelectron spectrum of Gly-, displying features described in the text. Click for larger image.

The photoelectron spectrum of Gly⁻ (figure, above) displays the signature of a dipole-bound species (low EBE), with sufficient vibrational structure to characterize the core neutral as a higher energy, non-zwitterionic isomer of the amino acid. The fine structure has been assigned to excitation modes that are most active in the mid-infrared spectrum, as expected for detachment of a very diffuse electron where the vibrational excitation mechanism arises from vibronic interactions in the anion.

For more information see:
Diken et. al., J. Chem. Phys., 120, 9899-9902, 2004.

Acid Dissolution

Understanding the molecular-level mechanism of proton transport in aqueous solutions is an important challenge in physical chemistry. Acid dissolution is a ubiquitous chemical process which marks the earliest stages of proton transport within aqeuous media, and immediately introduces the role of zwitterion formation. The figure (below) depicts the dissociation of hydrogen bromide in a cluster of four water molecules. Based on previous work the water networks are believed to play an integral part of the charge separation process, leading to a solvent-separated ion-pair.

Hydrogen bromide dissociates in a cluster of four water molecules, leading to zwitterion formation.

For more information see:
Robertson and Johnson, Science, 298, 69, 2002.