Anion-induced zwitterion formation in gas-phase arginine

Gas-phase studies of biological molecules provide important information not available in solution. In particular, all naturally occurring amino acids are known to be zwitterionic in solution, whereas most adopt canonical structures in the gas-phase. However, complexation with solvent molecules and cations can induce zwitterion formation.

This thesis presents experimental and theoretical results that provide evidence for the stabilisation of the zwitterionic structure of gas-phase arginine by simple anions in the gas-phase. Mass spectrometry (MS) experiments conducted in a quadrupole ion trap (QIT) are presented. In these experiments, the loss of ammonia as opposed to water from the Br-•Arg cluster suggests that the arginine within the cluster is present in its zwitterionic form. Computational calculations also suggest that the lowest-energy gas-phase structure of Br-•Arg is one in which the arginine is present in its zwitterionic form. Thus, the experimental and theoretical data both support the idea that the zwitterionic structure of gas-phase arginine may be induced by complexation with anions. This work provides the first experimental evidence for anion-induced zwitterion formation in a gas-phase amino acid within a simple anion-molecule complex.

In addition, experimental results are presented for a series of deprotonated dicarboxylic acids, RCOO-, complexed to neutral arginine in the gas-phase. The results from collisionally induced dissociation MS experiments in a QIT suggest an enhancement in stability of the RCOO-•Arg cluster for acids that are a similar length to arginine.