Mechanistic aspects of photooxidation of polyhydroxylated molecules on metal oxides

Abstract

Polyhydroxylated molecules, including natural carbohydrates, are known to undergo photooxidation on wide-gap transition metal oxides irradiated by ultraviolet light. In this study, we examine mechanistic aspects of this photoreaction on aqueous TiO(2), α-FeOOH, and α-Fe(2)O(3) particles using electron paramagnetic resonance (EPR) spectroscopy and site-selective deuteration. We demonstrate that the carbohydrates are oxidized at sites involved in the formation of oxo-bridges between the chemisorbed carbohydrate molecule and metal ions at the oxide surface. This bridging inhibits the loss of water (which is the typical reaction of the analogous free radicals in bulk solvent) promoting instead a rearrangement that leads to elimination of the formyl radical. For natural carbohydrates, the latter reaction mainly involves carbon-1, whereas the main radical products of the oxidation are radical arising from H atom loss centered on carbon-1, -2, and -3 sites. Photoexcited TiO(2) oxidizes all of the carbohydrates and polyols, whereas α-FeOOH oxidizes some of the carbohydrates, and α-Fe(2)O(3) is unreactive. These results serve as a stepping stone for understanding the photochemistry on mineral surfaces of more complex biomolecules such as nucleic acids.

Figure 1

First-derivative X-band EPR spectra from ethylene glycol (solid line) and glycerol (dashed line) on aqueous anatase nanoparticles (355 nm laser irradiation at 77 K). These spectra were obtained at 50 K with a modulation field of 5 G at a modulation frequency of 100 kHz; the microwave power is 2 mW. The low-field line from lattice trapped electrons is not shown; the broad line in the high field is from surface trapped electrons. The arrow indicates a feature arising from the formyl radical.

Figure 2

EPR spectra of photoirradiated aqueous solutions of TiO₂ nanoparticles containing various carbohydrates. The first derivative EPR spectra were obtained at 50 K, using a field modulation of 2 G (100 kHz). The structures of the carbohydrates (see the plot legend) are given in Scheme 1; the numbering corresponds to that used in the text and Scheme 1. The arrow indicates the line of the formyl radical. The part of the spectrum where lines from organic radicals overlap with lines from lattice and trapped electrons is excluded from the plot.

Figure 3

EPR spectra of photoirradiated D-ribose isotopomers on TiO₂ in D₂O. The dotted line corresponds to protiated D-ribose in TiO/H₂O; the site of H/D exchange is indicated in the plot labels. The numbering scheme is given in the structure inset at the top left and Scheme 1. The vertical arrow indicates the line of the formyl radical. The dash-dot vertical lines indicate features from ^•C(3) radicals (open circle) and ^•C(2) radicals (open square), as explained in the text. The short solid vertical lines in trace (i) indicate the fine structure arising from the d₁-^•C(2) radical. See Figure 9S for simulations of EPR spectra from the individual radicals.

Figure 4

A comparison of EPR spectra for 5,5'-protiated and deuterated forms of 2-deoxy-D-ribose on photoirradiated TiO₂.

Scheme 1

Structural formulas for polyols and carbohydrates.

Scheme 2

Possible reaction mechanisms accounting for the observed preference of reaction (2) on the metal oxide surface over reaction (1) that typically occurs in the bulk.

Scheme 3

Structural formulas for ribose-derived -H radicals (^•C(1–5)) and some of the secondary radicals (^•C_d(1–3)) formed via acid-catalyzed dehydration of the primary radicals.