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Time Resolved FT EPR Identification of (E) and (Z) Conformational Isomers of Glycyl Radicals Formed upon Photoinduced Oxidation of Glycine Esters in Aqueous Solutions

Using time-resolved Fourier transform electron paramagnetic resonance (FT EPR) spectroscopy, the photooxidation of glycine methyl and ethyl esters by spin-polarized triplets of anthraquinone-2, 6-disulfonate, was studied in aqueous solution in the pH range 9-10. Quinone radical anions showing strong emissive spin-polarization (CIDEP) were formed, indicating fast electron transfer from the glycine esters to the spin-polarized quinone triplet as the primary reaction. The lifetime of the simultaneously formed glycine ester aminium radical cations was too short for detection by FT EPR in the nanosecond time-scale. Fast deprotonation of aminium radicals from nitrogen and  -carbon sites led to the generation of two successor radicals, the aminyl radical • NH-CH2-CO2R and the glycyl radical NH2-• CH-CO2R (R = Me or Et) in the ratio of 1:9, respectively. Glycyl radicals were found to be present in two isomeric forms (Z and E), as unambiguously identified from their FT EPR spectra. The assignment to the two isomers was supported by quantum chemical DFT calculations. The calculated hfc constants for aminyl radicals and for (E)/(Z) glycyl radicals are in accordance with experimental values. The concentration ratio of (Z) and (E) isomers was determined to be 7:3, as predicted by the DFT calculations. No interconversion was observed within 1  s at room temperature. Results are discussed in the light of their relevance to the oxidation mechanism and, in particular, to the enantioselective synthesis of aliphatic amino acids and their derivatives.

photooxidation; glycyl radicals; rate constant; oxidation mechanism

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