Photochemistry of Pyruvic Acid in Water and Ice

• Main Author: Guzmán, Marcelo Ioel
• Format: Thesis
• Language: English
• Published: 2007
• Subjects: ice core
• Online Access: https://thesis.library.caltech.edu/4688/; https://thesis.library.caltech.edu/4688/1/Chapter_0_Title_Abstract_Table_of_Contents.pdf; https://thesis.library.caltech.edu/4688/2/Chapter_1_Introduction.pdf; https://thesis.library.caltech.edu/4688/3/Chapter_2_Background.pdf; https://thesis.library.caltech.edu/4688/4/Chapter_3_Hydration_in_Ice.pdf; https://thesis.library.caltech.edu/4688/5/Chapter_4_Photolysis_in_Water.pdf; https://thesis.library.caltech.edu/4688/6/Chapter_5_EMR_in_Ice.pdf; https://thesis.library.caltech.edu/4688/7/Chapter_6_Photolysis_Ice.pdf; https://thesis.library.caltech.edu/4688/8/Chapter_7_CO_and_CO2_Production_in_Ice_Core_Records.pdf; https://resolver.caltech.edu/CaltechETD:etd-11292006-181629

The 321 nm band photodecarboxylation of aqueous pyruvic acid, PA, solutions was studied in the range 5 ≤ [PA] ≤ 100 mM. Prompt and delayed CO₂ production stages are detected using TEMPO as a scavenger. These two processes are resolved in the photolysis of frozen solutions: after simultaneous evolution during illumination, additional CO₂ is quantified in the dark (Eₐ ~ 22 kJ mol⁻¹, T < 268 K). Photodecarboxylation rates of deareated aqueous PA solutions drop a two-fold in the frozen state at 253 K relative to the fluid at 293 K. Benzoylformic acid frozen solutions decarboxylate with no post-illumination CO₂ emissions. ¹H-NMR studies of the hydration of aqueous PA between the carbonyl and its hydrated gem-diol species in the frozen state indicate that the fraction of the carbonyl-form of PA, approaches ~ 20% at temperatures below 263 K, regardless of the initial PA concentration in the range 0.1 M ≤ [PA] ≤ 4.6 M. PA is cooperatively hydrated while dissolved in increasingly viscous aqueous microfluids often called quasi liquid layers (QLL) down to vitrification. Electron magnetic resonance signals found in frozen aqueous PA solutions UV-irradiated at 77 K correspond to distant triplet radical pairs separated by ~ 0.9 nm. They originate by photoinduced electron transfer between a triplet excited state PA and ground state PA to produce ³[PA⁺⋅ PA⁻⋅]. The subsequent deprotonation of PA⁺⋅ into an acylcarbonyloxyl radical that undergoes ultrafast decarboxylation accounts for the unquenchable source of CO₂, even at cryogenic temperatures. Liquid chromatography with UV and ESI-MS detection in combination with ¹³C-isotope labeling experiments were used for products identification. The fate of the radical anion PA⁻⋅ after protonation into a ketyl-radical is recombination into 2,3-dimethyltartaric acid or addition to a second PA molecule. The later process leads to an unstable 2-oxodicarboxylic acid that eliminates CO₂ to produce 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic. In conclusion, the possibility of photolysis in ice core records due to penetrating Cerenkov radiation derived from cosmic rays is explored. Potential impacts of the photolysis of organic matter trapped in ice with the subsequent release of CO and CO₂ over the last two millennia is analyzed.