Stability and IR Spectroscopy of Zwitterionic Form of β-Alanine in Water Clusters

Amino acids are the building blocks of proteins, and their detection in outer space thus has implications on the origin of life. They form a zwitterionic structure in aqueous environments while adopting a neutral configuration in the gas phase. We perform an experimental and computational study on the number of water molecules needed for zwitterion formation of β-alanine. Our density functional theory investigation reveals that a minimum of five water molecules are required to form and stabilize the zwitterion. A characteristic connecting water molecule located between the COO^- and NH₃⁺ groups is found to enhance the stability. This water molecule is also involved in a characteristic infrared active vibration at ≈1560 cm^-1, which is slightly shifted with the number of surrounding water molecules and is located in a spectral region outside of water vibrations. A corresponding infrared signal is found in high-resolution experimental spectra of β-alanine and water in a solid para-hydrogen matrix.