ENDOR-assisted study of the stable EPR spectrum of X-irradiated alpha-L-sorbose single crystals: MLCFA and simulation...

After X irradiation of single crystals of alpha-L-sorbose at 295 K, previous electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) results have indicated the formation of at least 10 different free radicals, and also that conceivably each carbon in the pyranose ring is a possible radical center. The radicals appear to be formed mostly by net H-abstraction reactions followed by standard elimination (e.g. beta-OH elimination) reactions or proton shifts, in turn leading to ring opening and fragmentation. In the present work, EPR spectra were recorded at room temperature with the external magnetic field along each of the three crystallographic axes subsequent to careful annealing at different temperatures using a high-temperature cavity. Each of the three sets of spectra was subjected to a maximum likelihood common factor analysis (MLCFA) that contributed to a better understanding of the spectral decays. Furthermore, the most stable spectra were simulated by optimization of previous ENDOR and EI-EPR results. The optimized EPR parameters resulted in excellent simulations of the experimental stable sorbose spectra and hence provided an improved insight of their spectral compositions.