Rebecca J. Rapf

Photochemical Kinetics of Pyruvic Acid in Aqueous Solution

Pyruvic acid in the atmosphere is found in both the gas and aqueous phases, and its behavior gives insight into that of other α-keto acids. Photolysis is a significant degradation pathway for this molecule in the environment, and in aqueous solution the major photoproducts are higher-molecular-weight compounds that may contribute to secondary organic aerosol mass. The kinetics of the aqueous-phase photolysis of pyruvic acid under aerobic and anaerobic conditions was investigated in order to calculate the first-order rate constant, Jaq, in solution. Analysis of the exponential decay of pyruvic acid was performed by monitoring both pyruvic acid and its photolytic products over the course of the reaction by (1)H NMR spectroscopy. Detection of major and minor products in the 0.1, 0.05, and 0.02 M pyruvic acid photolyses clearly demonstrates that the primary reaction pathways are highly dependent on the initial pyruvic acid concentration and the presence of dissolved oxygen. The Jaq values were calculated with approximations based on the dominant pathways for limiting cases of the mechanism. Finally, a model study using the calculated rate constants demonstrates the importance of aqueous-phase photolysis as a sink for pyruvic acid in the atmosphere, compared with gas-phase photolysis and OH oxidation.

Mechanistic Description of Photochemical Oligomer Formation from Aqueous Pyruvic Acid

The aqueous phase photochemistry of pyruvic acid, an important oxidation product of isoprene, is known to generate larger oligomeric species that may contribute to the formation of secondary organic aerosol in the atmosphere. Using high resolution negative mode electrospray ionization mass spectrometry, the aqueous photochemistry of dilute solutions of pyruvic acid (10, 1, and 0.5 mM) under anaerobic conditions was investigated. Even at the lowest concentration, covalently bonded dimers and trimers of pyruvic acid were observed as photochemical products. We calculate that it is energetically possible to photochemically generate parapyruvic acid, a dimer of pyruvic acid that is known to form via dark oligomerization processes. Subsequent photochemical reactions of parapyruvic acid with pyruvic acid form larger oligomeric products, such as 2,4-dihydroxy-2-methyl-5-oxohexanoic acid. A robust and relatively simple photochemical mechanism is discussed that explains both the conditional dependence and wide array of products that are observed.

Photochemical Synthesis of Oligomeric Amphiphiles from Alkyl Oxoacids in Aqueous Environments

The aqueous phase photochemistry of a series of amphiphilic α-keto acids with differing linear alkyl chain lengths was investigated, demonstrating the ability of sunlight-initiated reactions to build molecular complexity under environmentally relevant conditions. We show that the photochemical reaction mechanisms for α-keto acids in aqueous solution are robust and generalizable across alkyl chain lengths. The organic radicals generated during photolysis are indiscriminate, leading to a large mixture of photoproducts that are observed using high-resolution electrospray ionization mass spectrometry, but these products are identifiable following literature photochemical mechanisms. The alkyl oxoacids under study here can undergo a Norrish Type II reaction to generate pyruvic acid, increasing the diversity of observed photoproducts. The major products of this photochemistry are covalently bonded dimers and trimers of the starting oxoacids, many of which are multi-tailed lipids. The properties of these oligomers are discussed, including their spontaneous self-assembly into aggregates.

pH Dependence of the Aqueous Photochemistry of α-Keto Acids

α-Keto acids are important, atmospherically relevant species, and their photochemistry has been considered in the formation and processing of aerosols. Despite their atmospheric relevance, the photochemistry of these species has primarily been studied under extremely low pH conditions. Using a variety of analytical techniques, we characterize the extent of hydration and deprotonation for solutions of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, as a function of pH. We find that changes in the initial solution composition govern the accessibility of different photochemical pathways, resulting in slowed photolysis under high pH conditions and a shift in photoproducts that can be predicted mechanistically.

Environmental processing of lipids driven by aqueous photochemistry of α-Keto acids

Sunlight can initiate photochemical reactions of organic molecules though direct photolysis, photosensitization, and indirect processes, often leading to complex radical chemistry that can increase molecular complexity in the environment. α-Keto acids act as photoinitiators for organic species that are not themselves photoactive. Here, we demonstrate this capability through the reaction of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, with a series of fatty acids and fatty alcohols. We show for five different cases that a cross-product between the photoinitiated α-keto acid and non-photoactive species is formed during photolysis in aqueous solution. Fatty acids and alcohols are relatively unreactive species, which suggests that α-keto acids are able to act as radical initiators for many atmospherically relevant molecules found in the sea surface microlayer and on atmospheric aerosol particles.

Comment on “Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid”

mechanism to explain the aqueous photolysis of pyruvic acid and its photoproducts. 1−12 The first steps of the mechanism are generally agreed upon: pyruvic acid absorbs a photon, undergoes intersystem crossing and internal con-version, and then initiates further chemistry from the T1 surface. From here, the mechanisms in the literature diverge. In 2006, Guzman ́et al. 6 suggested a mechanism that proceeds by proton-coupled electron transfer to explain two products: dimethyltartaric acid and an oxo-C7 species, for which two possible structures were provided. Griffith et al. 2 later presented a diff erent mechanism, in which hydrogen abstraction by triplet-state pyruvic acid initiates radical chemistry that yields dimethyltartaric acid and other observed products, 2,8,9 including acetoin and lactic acid. Griffith et al.’s identification of acetoin and lactic acid was based on three independent NMR techniques: gHMBCad ( 13 C− 1 H correlation), 1 H gCOSY ( 1 H− 1 H correlation), and DOSY (viscosity corrected diffusion constant data). 2 The high level of agreement in the results from these techniques between standards and post-photolysis solutions off ers irrefutable evidence that both acetoin and lactic acid are generated from the photolysis of aqueous pyruvic acid under certain conditions. Their identification was initially disputed by Eugene et al.; 3 however, Griffith et al. 4 addressed these concerns. In their recent publication, Eugene and Guzman ́ 1 claim to have

Reactivity of Electronically Excited SO2 with Alkanes

We studied the reaction of electronically excited sulfur dioxide in the triplet state (3SO2) with a variety of alkane species, including propane, n-butane, isobutane, n-pentane, n-hexane, cyclohexane, n-octane, and n-nonane. Reaction rate constants for the photoinitiated reaction of SO2 with all of these species were determined and found to be in the range from 3.7 × 10-13 to 5.1 × 10-12 cm3molecule-1s-1. We found that reaction proceeds via a hydrogen abstraction to form HOSO• and organic radical (R•) species and that reactivity is correlated with the energy required to break a C-H bond and the length of the alkane chain. Abstraction rates were found to be fastest for reaction with hydrogen on a tertiary carbon. Similarly, abstraction from secondary carbons is found to be faster than from primary carbons. The reactivity of 3SO2 with alkanes increases with chain length as additional secondary carbons are added.