Chlorophyll light absorption

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

As mentioned above, in all photophobic action spectra measured so far additional peaks and/or shoulders occur at 565 and/or 615 nm, besides the chlorophyll a peaks, indicating the participation of phycobiliproteins and, hence, PS II in the active light absorption (Fig. 4). Moreover, activity of both PS I and PS II was confirmed by inhibitor experiments (see reviews by Nultsch 10°),Nultsch and Hader102), and Hader47)) and also with the aid of a dual wavelength technique39 40 101), described in detail below. 

The action spectmm of positive and negative phototaxis of Anabaena variabilis was measured recently106). This species contains no C-phycoerythrin. Accordingly, maximum activity is found at around 615 nm (Fig. 7). In addition, in this form a second maximum occurs at around 675 nm, and a third small, but distinct, one at 440 nm, both indicating that chlorophyll a is also involved in the active light absorption (see above). The utilization via photosynthesis, however, could be excluded in this case, since the trichomes oriented themselves perfectly well to the light direction in the presence of photosynthetic inhibitors, such as DCMU and DBMIB, at concentrations in which the photosynthetic oxygen evolution was almost completely inhibited. 

The photooxidation of chlorophyll indicated in Eq. 23-31 is accompanied by bleaching in the principal light absorption band. However, since there is so much light-gathering chlorophyll for each reaction center, the effect is small. The study of the process has been aided greatly by preparation of isolated bacterial photochemical reaction centers. 

In contrast to the chlorophylls, which absorb light in two regions of the visible spectrum, the carotenoids exhibit intense absorption in just one, 350-500 nm. Figure E8.4 compares the absorption spectra of four common carotenoids. As with the chlorophylls, the absorption maxima of the carotenoids vary with polarity of the solvent. /3-Carotene in diethyl ether has a kmax of449.8 nm, but in the more polar acetone, the A.max is 454 nm. 

Vostokov SV, Lisitsyn BE, Konovalov BV, Soloviev DM, Gagarin VI (2002) Mesoscale variability of chlorophyll a concentration, particulate organic matter content and spectral index of light absorption by phytoplankton in the upper layer of northeastern part of the Black Sea. In Zatsepin AG, Flint MV (eds) Multi-disciplinary investigations of the northeastern part of the Black Sea. Nauka, Moscow, p 235 (in Russian)... 

In 1944, Emerson, Stauffer, and Umbreit proposed that the function of light energy in photosynthesis is the formation of energy-rich phosphate bonds (p. 107). In their model (hereafter, the Emerson model ), the free energy necessary to drive both C02 fixation and reduction came from these energy-rich phosphate bonds (i.e., ATP), produced as a result of light absorption by a chlorophyll-containing protein. 

In vivo light absorption spectra (400-700 nm) were recorded with a Varian-Cary spectrophotometer equipped with an integrating sphere. Samples were taken to an approximate optical density of 0.1. Data were corrected for scattering by subtracting a baseline value as measured at 725 nm. The spectrally averaged, chlorophyll a specific absorption cross section (aph, m2 mg Chi a-1) was calculated as follows ... 

In both treatments, the in vivo light absorption characteristics were invariable over the day (data not shown). Maximum values in the chlorophyll-specific absorption coefficient were observed near 675 nm (Chi a), 470 nm (carotenoids) and 430 nm (Chi a) (Fig. 4). The absorption cross section was 0.057 m2 mg Chi a 1 under iron limitation and... 

The primary processes of photochemistry involve the light absorption event, which we have already discussed, together with the subsequent deexcitation reactions. We can portray such transitions on an energy level diagram, as in Figure 4-9 for chlorophyll. In this section we discuss the various deexcitation processes, including a consideration of their rate constants and lifetimes. 

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