1,3-Dihydroxyacetone (DHA) is the active ingredient in commercial personal care products for the production of a "sunless" tan on human skin. The DHA molecule is the simplest ketose sugar, and it reacts with amino acids on the surface of the skin in a biochemical pathway known as the Maillard reaction. However, DHA is known to be photochemically active and will produce free radicals with high quantum yield under ultraviolet (UV) excitation. The Forbes Group report a detailed study in Applied Magnetic Resonance of the free radicals produced by DHA photolysis as a function of wavelength and solvent.
Contrary to previous optical and steady-state electron paramagnetic resonance (EPR) studies, X-band time-resolved (TR)-EPR spectra reveal a complex reactivity pattern: Norrish I α-cleavage is observed in almost all cases, but when good H-atom donors are present, H-atom abstraction by the solvent is observed. Changing the wavelength of excitation from 308 to 248 nm can reverse this observation. Comparison of TREPR spectra obtained from commercially available sunless tanning lotions shows that many free radicals, including those from DHA, can be detected using direct detection TREPR upon photolysis of the lotions themselves. The results suggest that caution should be observed in the use of these products in conjunction with UV exposure.
Carrying out organic reactions in the solid state is an attractive avenue for green chemistry research, as this removes the requirement for volatile solvents. However, very little is known about the mechanisms of such reactions, and in particular the structures and lifetimes of the reactive intermediates involved. The Forbes Group, in collaboration with the group of Miguel Garcia–Garibay at UCLA and Dr. Valery Tarasov from the Semenov Institute of Chemical Physics in Moscow, Russia, recently investigated the photochemical cleavage reaction of a ketone in a solid nanocrystalline suspension that lead to a single product in 100% yield in just a few hours. Using a highly specialized time–resolved method, postdoc Natalia Lebedeva and former graduate student Ryan White detected the electron paramagnetic resonance spectrum of the triplet state of a radical pair "trapped" in the interior of a nanocrystal of dimethoxydicumylketone that was suspended in water.
Two highly unusual spectroscopic features appeared in the spectra: 1) non–Boltzmann spin state populations or "spin polarization," which means that some of the transitions in the spectrum appeared in emission rather than absorption and 2) the spectrum was detected at room temperature. Normally, triplet states relax their spin states so rapidly that very low temperatures are required to observe them. The electron-electron dipolar coupling from Dr. Tarasov's spectral simulation give an estimate of 5 Ä for the inter–radical separation in the nanocrystal. It is interesting that the spectrum was obtained using solution techniques -pumping the aqueous suspension through the spectrometer probe during the experiment- yet the acquired spectrum was more typical of a solid–state powder pattern triplet state. The Forbes Group is currently exploring the generality of the phenomenon by investigating other nanocrystals with different reactivities and structures.