Generation and characterization of the selenocysteinyl radical: direct evidence from time-resolved UV/Vis, electron paramagnetic resonance, and Fourier transform infrared spectroscopy.
The selenocysteinyl radical 1 has been generated for the first time by laser flash photolysis (lambda(exc) = 266 nm) of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2 and of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3 in acetonitrile and characterized by time-resolved (TR) UV/Vis, Fourier transform infrared (FTIR), and electron paramagnetic spectroscopy in combination with theoretical methods. A detailed product study was conducted using gas chromatography and one- and two-dimensional NMR spectroscopy. In the case of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3, the (9-fluorenylideneamino)oxycarbonyl moiety serves as a photolabile protection group providing a "caged selenocysteinyl radical" suitable for biophysical applications. Cleavage of the diselenide bridge or the selenium-carbonyl bond by irradiation is possible in high quantum yields. Because of the lack of a good IR chromophore in the mid-IR region, the selenocysteinyl radical 1 cannot be monitored directly by TR FTIR spectroscopy. TR UV/Vis spectroscopy revealed the formation of the selenocysteinyl radical 1 from both precursors. The selenocysteinyl radical 1 has a lifetime tau approximately 63 mus and exhibits a strong band located at lambda(max) = 335 nm. Calculated UV absorptions of the selenocysteinyl radical (UB3LYP/6-311G(d,p)) are in good agreement with the experimental results. The use of TR UV/Vis spectroscopy permitted the determination of the decay rates of the selenocysteinyl radical in the presence of two quenchers. The product studies demonstrated the reversible photoreaction of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2. Products of the photolysis of the "caged selenocysteinyl radical" precursor 3 are dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2, carbon dioxide, and some further smaller fragments. In addition, the photodecomposition of the (9-fluorenylideneamino)oxycarbonyl moiety produced 9-fluorenone-oxime 4, 9-fluoren-imine 5, and 6 and 7 as products of the dimerization of two 9-fluorenoneiminoxy radicals 8.
Kolano
Christoph
C
Lehrstuhl für Organische Chemie II der Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany. c.kolano@pci.unizh.ch
Bucher
Götz
G
Schade
Olaf
O
Grote
Dirk
D
Sander
Wolfram
W
D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals; D009682:Magnetic Resonance Spectroscopy; D017279:Selenocysteine / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*
D005609:Free Radicals; D017279:Selenocysteine