Chemistry and Photochemistry of Pyruvic Acid at the Air–Water Interface

Abstract

Interfacial regions are unique chemical reaction environments that can promote chemistry not found elsewhere. The air−water interface is ubiquitous in the natural environment in the form of ocean surfaces and aqueous atmospheric aerosols. Here we investigate the chemistry and photochemistry of pyruvic acid (PA), a common environmental species, at the air−water interface and compare it to its aqueous bulk chemistry using two different experimental setups: (1) a Langmuir−Blodgett trough, which models natural water surfaces and provides a direct comparison between the two reaction environments, and (2) an atmospheric simulation chamber (CESAM) to monitor the chemical processing of nebulized aqueous PA droplets. The results show that surface chemistry leads to substantial oligomer formation. The sequence begins with the condensation of lactic acid (LA), formed at the surface, with itself and with pyruvic acid, and LA + LA − H 2 O and LA + PA − H 2 O are prominent among the products in addition to a series of higher-molecular-weight oligomers of mixed units of PA and LA. In addition, we see zymonic acid at the surface. Actinic radiation enhances the production of the oligomers and produces additional surface-active molecules known from the established aqueous photochemical mechanisms. The presence and formation of complex organic molecules at the air−water interface from a simple precursor like PA in the natural environment is relevant to contemporary atmospheric science and is important in the context of prebiotic chemistry, where abiotic production of complex molecules is necessary for abiogenesis.

Water–Air Interfaces as Environments to Address the Water Paradox in Prebiotic Chemistry: A Physical Chemistry Perspective

“…We note that all types of oligomerization reactions, which could be promoted at the surface of water in natural environments, are potentially important for building the chemical complexity required for life. A recent study assessing both the photochemistry and the "dark" (i.e., no irradiation) chemistry of pyruvic acid by Kappes et al 14 at water−air interfaces observed complex oligomeric products at the surface of water under both conditions. The oligomers observed were likely formed by condensation reactions, and, intriguingly, in the case of the dark chemistry, formed spontaneously from a solution of simple monomeric precursors under ambient temperature.…”

“…We note that while leucine ethyl ester is a small organic molecule that is soluble (and therefore present) in bulk solution, even highly soluble molecules also exhibit some degree of surface activity. 14,31,32 These molecules, aided by the hydrophobic effect, 42,43 partition to the surface, leading to increased concentrations at the air− water interface. Following equilibration, the leucine ethyl ester system showed peptide bond formation, 1 which was detected by a new distinct vibrational feature in the reflection−absorption spectrum at ∼1625 cm −1 corresponding to the Amide I band of a dipeptide.…”

“…21 Aqueous prebiotic aerosols also would have had a significant organic content, some of which would partition to the surface of the droplets. 2,3,14,31,32 The pH of the prebiotic ocean has been estimated to range from ∼4 to ∼9, 33 but it generally is thought to have likely been slightly basic. In contrast, atmospheric aerosols are expected to have been acidic due to atmospheric CO 2 and SO 2 , which, upon uptake by an aqueous particle, form carbonic and sulfuric acid.…”

Aqueous Photochemistry of 2-Oxocarboxylic Acids: Evidence, Mechanisms, and Atmospheric Impact

“…In addition, experiments using liquid films of PA photolyzed in the presence of a 100 ppb ozone exhibited dominant surface chemistry over that registered in the bulk, pointing out the importance of this heterogeneous process [ 90 ]. Similarly, other attempts have aimed to investigate the m/z value of possible oligomers in heterogeneous chemistry [ 91 , 92 ], although regularly battling with the presence of impurities before starting the reactions.…”

Evaporation and molecular beam scattering from a flat liquid jet

“…Surface-sampling of Langmuir troughs enables chemical analysis of photochemical products at the liquid interface and in the bulk. 18 Mass spectrometry-based experiments on thin films as well as droplets in microemulsions and the gas phase have shown significantly enhanced surface chemical reaction rates compared to bulk liquids. [19][20][21] Molecular beam scattering, originally developed to probe interactions between pairs of gas-phase molecules 22,23 and gas-phase molecules with solid surfaces, 24,25 has proved to be a direct and powerful means for investigating the dynamics of elementary chemical reactions.…”