Light Induced Fragmentation of Polyfunctional Carboxylated Compounds on Hydrated Metal Oxide Particles: From Simple Organic Acids to Peptides

Abstract

Light induced reactions of carboxylic, hydroxycarboxylic, and aminocarboxylic acids, carboxylated aromatics, and α-amino acids and peptides adsorbed on hydrated anatase (TiO2), goethite (α-FeOOH), and hematite (α-Fe2O3) have been studied at low temperature (77−200 K), by means of electron paramagnetic resonance (EPR) spectroscopy, and at 295 K, by means of transient absorption spectroscopy. In the room-temperature solution, the photocatalytic decomposition of carboxylated molecules, in the absence of synergistic adsorption by hydroxyl groups (e.g., in serine), is inefficient due to weak surface binding. The yield of radical formation increases significantly at low temperature, as the carboxylated molecules adsorb on the surface. The main photodegradation path is decarboxylation initiated by charge transfer from the metal oxide to the adsorbate. Below 120 K, for carboxylic acids and nonaromatic amino acids and peptides, the decarboxylation is the only reaction pathway, yielding the corresponding C-centered radicals. At higher temperature these radicals become mobile and abstract H from the parent molecules. For aromatic amino acids (histidine, tyrosine, and tryptophan) charge transfer followed by deprotonation is the predominant photoreaction. For polycarboxylated aromatics, the outcome of the charge transfer is determined by the stability of the trapped-hole species. In addition to trapping holes, some adsorbates, such as mellitic acid, trap surface electrons. The photocatalytic activity of the anatase, goethite, and hematite are similar for aromatic, carboxylic, and hydroxycarboxylic acids; for hematite, this activity reduces markedly for α-amino acids and peptides. The rutile is inactive toward these carboxylated molecules. These findings are related to martian soil chemistry, biomedical chemistry, and photocatalysis.

Photocatalytic Decomposition of Carboxylated Molecules on Light-Exposed Martian Regolith and Its Relation to Methane Production on Mars

“…N 2 -purged aqueous solutions containing the adsorbate were flash frozen at 77 K in Suprasil tubes and subsequently irradiated with 355 nm laser light (15 mJ/pulse, 100-1000 pulses). We stress that the organic molecules that we studied do not absorb at this wavelength, so the UVA light was absorbed by the oxide particles (Shkrob and Chemerisov, 2009). The irradiated samples were transferred into a cryostat of a continuous-wave EPR spectrometer operating at 9.45 GHz, and the radicals and spin centers were detected in situ at 50-200 K (representative spectra obtained at different temperatures are shown in Figs.…”

“…The irradiated samples were transferred into a cryostat of a continuous-wave EPR spectrometer operating at 9.45 GHz, and the radicals and spin centers were detected in situ at 50-200 K (representative spectra obtained at different temperatures are shown in Figs. 2b and 3b of Shkrob and Chemerisov, 2009). First-derivative EPR spectra were taken at several temperatures, microwave power levels (typically, 0.2 mW), and field modulation amplitudes (typically, 2-5 G); the field modulation frequency was 100 kHz.…”

“…We outline only general aspects of the experimental approach, as the full details of the method have been given in a previous publication (Shkrob and Chemerisov, 2009).…”

“…Our examination is based on our recent study (Shkrob and Chemerisov, 2009) in which the photoinduced catalytic oxidation of aliphatic and aromatic carboxylated and polycarboxylated compounds, including L-a-amino acids and peptides, were systematically examined with electron paramagnetic resonance (EPR) spectroscopy, a technique that allows for detection and identification of reaction intermediates that have unpaired electrons. As the metal oxides are semiconductors, the photoexcitation of these materials results in charge separation, which yields trapped electrons (e À ) and electron deficiencies called holes (h þ ).…”

“…This single-light wavelength is fully representative of the photochemistry across the entire region of phototransparency of the martian atmosphere where it overlaps with the absorption spectra of the oxide materials (for the latter, see Fig. 1S in Shkrob and Chemerisov, 2009).…”