Photosystem cation radicals

Davis, M.S., Forman, A., and Fajer, J., Ligated chlorophyll cation radicals their function in photosystem II of plant photosynthesis, PNAS, 76, 4170, 1979. 

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

The herbicidal activity of 1,1-dimethyl-4,4 -dimethylbipyridylium dication ( Paraquat ) seems to depend on the formation of the cation radical through reaction with a component of photosystem 1 in the chloroplast, followed by reaction with oxygen to form superoxide ion (Eq, 36) (for a summary of references, see >). The rate constant for the latter reaction has been reported... 

E4.1 (a) Organic radicals typically have -factors close to the free-electron value g = 2.00232, with only slight anisotropy. In an ESR study of carotenoid and chlorophyll cation radicals in photosystem n atZJ-band (130 GHz), Lakshmi et al measured = 2.00335, gy = 2.00251, gz = 2.00227 [11]. The line-width was 4-5 G. Are the gy, and gz features completely resolved under those conditions (The spectra are reproduced in Fig. 4.2) Would it be possible to resolve the features corresponding to these -factors at 385 GHz ... 

CHARACTERISATION OF TRIPLET AND QUINONE INDUCED CATION RADICAL STATES IN THE ISOLATED PHOTOSYSTEM TWO REACTION CENTRE. 

C02-fixation to formate is catalyzed by formate dehydrogenase, ForDH. Photogenerated MV+ mediates the reduction of C02 to formate [200]. Other bipyridinium radicals, such as JV,j V -dimethyl-2,2 -bipyridinium or JV,Ar -trime-thylene-2,2 -bipyridinium radical cation act also as charge carriers for ForDH. The photosystem that Was utilized for generation of MV+ and 002-fixation includes Ru(bpy)f+ as photosensitizer, cysteine as sacrificial electron donor and MV2+ as electron acceptor. The net photosynthetic process accomplished in this photosystem (Fig. 40) corresponds to the reduction of 0O2 to formate by cysteine, see Eq. (70). This is an endoergic transformation by ca. 12.5 kcal mol-1. 

The chemistry of radical anions, generated by PET processes, is less developed than that of radical cations, possibly due to the nonavailability of suitable photosystems to initiate photosensitized one-electron redox reactions. 

Kass, H., Fromme, P. Witt, H.T. and Lubitz, W. (Orientation and electronic structure ofthe primary donor radical cation in Photosystem I a single crystal EPR and ENDOR study, J. Phys. Chem. B 105, 1225-1239. 

The electron transfer pathway shown in Fig. 1 is driven by light-induced charge separation in the two photosystems. The first of these systems is PS II, shown on the left of Fig. 1. In PS II, the radical cation generated by the charge separation extracts... 

The j3-carotene radical cation can be induced in RCs of plant Photosystem II (PS II) by illumination of ferricyanide-treated BB Y-type RC particles (Noguchi et al., 1994). The radical was observed by FTIR... 

Noguchi T, Mitsuka T and Inoue Y (1994) Fourier transform infrared spectrum of the radical cation of 3-carotene photoinduced in Photosystem II. FEES Lett 356 I79-I82... 

Unlike radical cations, the quantum of chemistry originating from PET-gener-ated radical anions is still limited possibly due to the impending development of a suitable photosystem to initiate photosensitized one-elecron redox reactions in wide array of functionalities. Nevertheless, the radical anion chemistry follows, more or less, the analogous pattern of bond dissociation and addition (electrophilic/radical) reactions as observed for the radical cations. As there are not many examples to describe the separate categories, this section is subdivided... 

A further study on the same system at 130 GHz but this time on fully deuterated Photosystem II samples from Synechococcus lividus produced a better resolved spectrum with a much higher signal to noise ratio. This gave further proof that both sets of radicals were produced at all temperatures but with the chlorophyll produced predominantly at the higher temperatures. Measured principal g values of the P-carotenoid were found to be gxx = 2.00335, gyy = 2.00251, gzz = 2.00227 and those for the chlorophyll z were gxx = 2.00312, gyy = 2.00263, gzz = 2.00202, similar to those of the chlorophyll a radical-cation. 

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