Immobilized semiquinones

The primary acceptor in PS II is a plastoquinone, PQ, as ascertained from optical absorbance difference spectroscopy [46], Until recently, the EPR spectrum of the semiquinone escaped observation, and only the advent of preparation methods for PS II subchloroplast particles made its recording possible. As surmised earlier, the spectrum of the intact acceptor [47] very much resembled the very broad qui-none-iron acceptor complex in purple bacteria, whereas in iron-depleted PS II particles the narrow spectrum typical of an immobilized semiquinone was found [48], As in the bacterial photosystem, flash-induced reduction of Q, of the second quinone, Qb, or of both resulted in somewhat different EPR spectra, indicative of structural changes that influence the magnetic interaction between the semiquinone and the non, and/or between the two semiquinones [49],... 

All the spectral characteristics point to the presence of immobilized semiquinones as the simple radicals that originate the paramagnetism in melanins (S = 1/2). This view is also supported by many experiments... 

Immobilized semiquinones often are detected in membrane systems. In most instances analysis of their ESR spectra has been limited to measurement of a g value and a linewidth. More detailed analysis [154,155] could provide additional structural information to help identify the radical, its ionization state, and its local environment. The g value usually measured is that for the cross-over in the first derivative spectrum. While certainly useful in providing an approximate indication of the radical structure, it is generally frequency-dependent because of the anisotropy in the radical spectrum. It cannot give the more precise information on structure and environment that is available from the isotropic g value. The latter can readily be obtained from spectra of immobilized species by the method of Hyde and Pilbrow [156]. Related information is available from linewidth considerations [154] in general, spectra for neutral semiquinones are approximately twice as broad as those for semiquinone anions and are sensitive to deuteration of solvent. Information of this kind can be of great help in assigning a spectrum of an unknown to a given radical species. The point has been... 

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