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Time-Resolved FT EPR and Optical Spectroscopy Study on Photooxidation of Aliphatic -Amino Acids in Aqueous Solutions ; Electron Transfer from Amino vs Carboxylate Functional Group

Using time-resolved Fourier transform electron paramagnetic resonance, FT EPR, and optical spectroscopy, the photooxidation of glycine, -alanine, -aminoisobutyric acid, and model compounds &#61538; -alanine, methylamine and sodium acetate, by excited triplets of anthraquinone-2, 6-disulfonate dianion was studied in aqueous solutions in the pH range 5-13. Anthraquinone radical trianions showing strong emissive spin-polarization (CIDEP) were formed, indicating fast electron transfer from the quenchers to the spin-polarized quinone triplet as the primary reaction. None of the primary radicals formed upon one-electron oxidation of quenchers could be detected at the nanosecond time scale of FT EPR measurements because of their very fast transformation into secondary products. The latter were identified to be decarboxylated -aminoalkyl radicals for -amino acids anions and zwitterions, &#61538; -aminoalkyl radicals for &#61538; -alanine zwitterions, and methyl radicals for acetate anions ; corresponding aminyl radicals were the first EPR detectable products from &#61538; -alanine anions and methylamine. Thus, anthraquinone-2, 6-disulfonate triplet can take an electron from both NH2- and -CO2- functional groups forming aminium (+ NH2-) and acyloxyl (-CO2 ) radicals, respectively. Aminium radicals derived from &#61538; -alanine anions and CH3-NH2 stabilize by deprotonation into aminyl radicals, whereas these derived from -amino acids anions are known to suffer ultrafast decarboxylation ( ~ 10 ps). Analysis of the polarization patterns revealed that decarboxylation from acyloxyl radicals are considerably slower (ns < < 0.1 &#61549; s). Therefore, in the case of -amino acids, the isoelectronic structures NH2-CR2-CO2 and + NH2-CR2-CO2- probably do not constitute resonance mesomeric forms of one and the same species and the decarboxylation of aminium radicals is not preceded by the intramolecular carboxylate to amino group electron transfer. Absolute triplet quenching rate constants at zero ionic strength were in the range of 2 × 108 to 2 × 109 M-1 s-1 for R-NH2 and 2 × 107 to 108 M-1 s-1 for R-CO2- type of electron donors, reflecting in principle their standard reduction potentials. The strengths of acids: +NH3- CH2, +NH3- C(CH3)H, and +NH3- C(CH3)2, pKa <4, >6, and >7, respectively, were found to be remarkably strongly dependent on -C substitution. The conjugate bases of these -aminoalkyl radicals reduce anthraquinone-2, 6-disulfonate dianion ground state with ksec = 3 × 109 M-1 s-1.

amino acids; electron transfer; photooxidation; radicals

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