Stability and IR spectroscopy of zwitterionic form of v-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 v-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 v-alanine and water in a solid para-hydrogen matrix.