Interaction of Glycine with Common Atmospheric Nucleation Precursors

The interaction between the simplest amino acid glycine in three different protonation states and common atmospheric nucleation precursors (H2O, NH3, and H2SO4) has been investigated using computational methods. Each nucleation step has been thoroughly sampled, and statistical Gibbs free energies of formation have been calculated using M06-2X/6-311++G(3df,3pd). From the stepwise Delta G values, the stabilities of the molecular clusters have been evaluated. Glycine in all three protonation states is found to have a favorable interaction with sulfuric acid with a higher cluster stabilizing effect than ammonia. The deprotonated glycine molecule is found to yield the highest stabilizing effect on the sulfuric acid clusters through the interaction of both the amino and carboxylic moieties, while the protonated glycine molecule is found to have a high stabilizing effect on the addition of water and ammonia. Furthermore, we find that a single sulfuric acid molecule is capable of stabilizing the glycine zwitterion. Sulfuric acid is found to be able to catalyze the spontaneous formation of the zwitterion and subsequently stabilize the formed ion. The formation of the glycine zwitterion occurs with a low Gibbs free energy barrier of 2.10 kcal/mol, indicating that this formation could occur rapidly in the atmosphere.