Aqueous-Phase Reactions of Glyoxal and Amino Acids in Evaporating Cloud Droplets

Approximately 15% of organic carbon in atmospheric water droplets has been reported to be in proteins and free amino acids such as glycine and serine. New NMR, ATR, and electrospray-MS data show that reactions between glyoxal and free amino acids, both present in cloudwater at micromolar concentrations, are triggered by droplet evaporation, leaving behind non-volatile, higher-mass products as a cloud-processed aerosol. The drying process partially dehydrates glyoxal and forms a reactive carbonyl, which undergoes a nucleophilic attack by the amine group, ultimately forming an imine. Mass spectral data suggest that under conditions where glycine concentrations are limiting, a cyclic imine is formed. ATR data indicate that reactions between glyoxal and amino acids are operational when pH > pKa of the carboxylic acid group, a range which includes most atmospheric water droplets.