engleski

Oxidation of Cyclic Dipeptides by Photoinduced H-Atom Abstraction. A Laser Flash FT EPR and Optical Spectroscopy Study

Laser flash photolysis with the Fourier transform electron paramagnetic resonance (FT EPR) and optical spectroscopy detection methods on the nanosecond time scale have been employed in order to investigate the oxidation mechanism of cyclic dipeptides glycine, alanine, and sarcosine anhydrides intitated by SO4• - or 9, 10-anthraquionone-2, 6-disulfonate (2, 6-AQDS) triplet in oxygen free aqueous solutions. A direct hydrogen abstraction from the the ring C-H position of an anhydride by both oxidants is proposed as the primary reaction, rather then an electron transfer from nitrogen followed by  C-H deprotonation. The overall second-order rate constants for the reaction with SO4• - were determined to be 7.2 x 107 M-1 s-1, 1.2 x 108 M-1 s-1, and 5.2 x 108 M-1 s-1 for gylicine anhydride, alanine anhydride, and sarcosine anhydride, respectively. The rate constants for 2, 6-AQDS triplet as oxidizing species are about two times lower. Tha radical intermediate products derived from cyclic dipeptides observed on the microsecond time scale were assigned to the general structure of piperazine-2, 5-dione-3-yl radical. These are spin polarized by the mechanisms of chemically induced dynamic electron polarization (CIDEP). For SO4• - as the oxidant the spectra are exhibiting and E/A* polarization pattern originating partially from F-pairs of two piperazine-2, 5-dione-3-yl-radicals.

laser flash photolysis; oxidation; cyclic dipeptides

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