engleski

Structural characterisation of p-semiquinone radical in a crystal: X-ray structure and EPR evidence

In membrane-located proteins, quinone system plays a significant role in electron-transfer reactions. The described work illustrates that semiquinone radical is stabilized in the solid state. Thin dark-red plate-like crystals of p-semiquinone radical were grown by evaporation of mildly alkaline, saturated water solution of hydroquinone. Although stable in air, larger single crystals decay after ca. 20 h exposure to X-rays or UV radiation producing amorphous yellow substance identified as a mixture of quinone and quinhydrone. Solid-state electronic paramagnetic resonance spectra of crystalline p-semiquinone detected its paramagnetic propetries revealing four symmetrically equivalent protons. Crystal strucure was determined at 100 and 150 K (monoclinic, space group P 21/c, a = 3.78 &Aring ; ; , b = 5.98 &Aring ; ; , c = 10.79 &Aring ; ; , β = 90.66°). In both of them, p-semiquinone molecule is centrosymmetric (four protons are, therefore, equivalent), with C-O bond length of 1.295 &Aring ; ; , corresponding to bond order of 1.5. Molecules are hydrogen bonded into infinite chains (O-H&middot ; ; &middot ; ; &middot ; ; O distance of 2.70 &Aring ; ; with the proton disordered between two oxygen atoms. This packing is very similar to that of quinhydrone ; the unit cell can be transformed into a half of the unit cell of quinhydrone.

radical; p-semiquinone; EPR

nije evidentirano