Bacteriochlorophyll absorption spectrum

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

The reduction of ring IV in chlorophylls a or b changes the optical absorption spectrum of the molecule dramatically. Whereas the long-wavelength absorption band of a cytochrome is relatively weak (see fig. 14.4), chlorophyll a has an intense absorption band at 676 nm (fig. 14.5). Chlorophyll b has a similar band at 642 nm. Bacteriochlorophylls a and b have strong absorption bands in the region of 770 nm (see fig. 15.5). The chlorophylls thus absorb red or near-infrared light very well. 

Since the bacteriochlorophyll present in the light-harvesting complex accounts for the majority of all the bacterial pigments, its absorption bands can readily be identified even in the spectrum of the unfractionated membrane. On the other hand, the pigments belonging to the reaction center amount to only "1% of the total BChl and its absorption is often masked by the bulk pigments. The BChl a present in the reaction center may be identified however in a purified reaction-center preparation isolated from the chromatophore membrane. This may be illustrated with Chromatium vinosum following fractionation and isolation of the reaction-center complex and the three antenna complexes from the chromatophore membrane. Fig. 2 (B) shows the absorption spectrum of the unfractionated Chromatium chro-... 

Fig. 4. Difference absorption spectrum of photoreduced bacleriopheophytin (B

There are totally 11 chromophores in the bacterial photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) virids. Since the excitation process of the reaction center is the primary event of the photo-induced electron transfer in the reaction center, the detailed analysis of the absorption spectrum is one of the key steps for the understanding of photochemistry of the system. The chromophores included in the PSRC are bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll in L- and M-branches (Bl and B ), bacteriopheophytin in L- and M-branches (Hl and Hm), menaquinone (MQ), ubiquinone (UQ) and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit. 

The biochemical architecture of photosynthetic bacteria is not as complex as that of green plants. For example, photosynthetic bacteria have only one photosystem, while green plants have two. The reaction center protein from several species of photosynthetic bacteria can be isolated from the photosynthetic membrane. Reaction centers from the species Rhodopseudomonas sphaeroides have been extensively studied. Although minor details will change from one species to another, the important features are nearly identical. The reaction center protein has a molecular weight of about 70,000 daltons. Within the reaction center protein extracted from the carotenoidless mutant strain R26 of the species R. sphaeroides, are found four molecules of bacteriochlorophyll a, two molecules of bacteriopheophytin a, one atom of nonheme iron, and, depending on the isolation procedure used, one or two molecules of ubiquinone. The absorption spectrum of the isolated reaction center has been well characterized. It is shown in Fig. 4. Based on in vitro absorption spectra, the bands at 870, 800, and 600 nm have been assigned to the bacteriochlorophyll a molecule. Bands at 760 and 530 nm have been attributed to the bacteriopheophytin a. 

Photochemical oxidation results in small changes in the absorption spectrum of the reaction center with the exception of a large amount of bleaching in the red-most band at 870 nm. Accompanying this bleaching is an ESR signal, which is narrowed by V2 over the signal observed in the cation of monomeric bacteriochlorophyll a. Just as for... 

Fig. 4. The absorption spectrum of purified reaction centers from the photosynthetic bacteria R. sphaeroides. The dashed line shows the red portion of the absorption spectrum taken in the presence of actinic light. The light is strong enough to photooxidize all of the bacteriochlorophyll a that is photochemically active.

A covalent Unkage between two bacteriochlorophyll a molecules results in a species which in the presence of the proper bifunctional ligand (e.g., ethanol) undergoes folding like the chlorophyll a dimer. It does not reproduce the in vivo absorption spectrum, absorbing at 803 nm instead of 870 nm. Yet, it does have a delocalized cation and a delocalized triplet state. 

FIGURE 118.3 Absorption spectrum of a suspension of Rhodopseudomonas viridis reaction centers (plus 100 pM Na ascorbate) dispersed by a detergent (LDAO). The absorption band at 970 nm is due to the primary donor P, and the large massif around 820 nm is mainly due to the two bacteriochlorophylls other than P, and to the two bacteriopheophytins. (Reaction centers kindly provided by Dr. J. Breton.)... 

These changes are illustrated in Fig. 2, where the visible absorption spectra of cells from exponentially-growing and carbon-starved cultures are shown for comparison. The shift of the three-peaked carotenoid band is clearly indicated by the marked variation in the depth of the trough to the left of the Qx(0, 0) bacteriochlorophyll band at 588 nm. There is also a change of the band s fine structure, which is lower in the spectrum of exponential cultures. This additional difference is enhanced by the variation of the extent of overlapping between carotenoid peak III and the Qx(0, 1) band of bacteriochlorophyll near 555 nm. 

The spectrum of the light emitted by tungsten / halogen lamps is better than the one from fluorescent lamps, taking into account that phototrophic bacteria use infrared light, with absorption maxima at 800 and 850 nm corresponding to the absorption of bacteriochlorophylls. 

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