Accessory chlorophyll

This pair of chlorophyll molecules, which as we shall see accepts photons and thereby excites electrons, is close to the membrane surface on the periplasmic side. At the other side of the membrane the symmetry axis passes through the Fe atom. The remaining pigments are symmetrically arranged on each side of the symmetry axis (Figure 12.15). Two bacteriochlorophyll molecules, the accessory chlorophylls, make hydrophobic contacts with the special pair of chlorophylls on one side and with the pheophytin molecules on the other side. Both the accessory chlorophyll molecules and the pheophytin molecules are bound between transmembrane helices from both subunits in pockets lined by hydrophobic residues from the transmembrane helices (Figure 12.16). 

One apparent discrepancy between the spectroscopic data and the crystal structure is that no spectroscopic signal has been measured for participation of the accessory chlorophyll molecule Ba in the electron transfer process. However, as seen in Figure 12.15, this chlorophyll molecule is between the special pair and the pheophytin molecule and provides an obvious link for electron transfer in two steps from the special pair through Ba to the pheophytin. This discrepancy has prompted recent, very rapid measurements of the electron transfer steps, still without any signal from Ba- This means either... 

Illumination under the donor-limited condition, either in the absence or presence of oxygen, produces absorbance changes quite different from those under the acceptor-limited condition. There is, e.g., a prominent irreversible bleaching of (3-carotene and the accessory chlorophyll absorbing at 670 nm [see Fig. 4 (B)]. As a result of the oxidation of d> by the secondary acceptor SiMo, the lifetime of P680 is greatly increased and thus intensifying its destructive effect. Of course, this donor-limited photoinhibition can be readily prevented if an artificial secondary electron donor such as DPC or Mn is also added. 

In the core-antenna assembly, each CA-Chl a has at least one neighbor spaced at a center-to-center distance of <16 A. Relative to the Chl-a electron-transfer cofactors, namely, P700, the accessory chlorophyll (A), and the primary electron acceptor (Aq), however, the CA-Chl a molecules are at various distances, as can be seen in Table 11 which shows various distances relative to the three electron-transfer Chls a, the number of such core-antenna chlorophylls. 

Figure 19.28 Energy transfer from accessory pigments to reaction centers. Light energy absorbed by accessory chlorophyll molecules or other pigments can be transferred to reaction centers, where it drives photo induced charge separation. The green squares represent accessory chlorophyll molecules and the red squares represent carotenoid molecules the white squares designate protein.

The PSII structure of the unicellular green alga Chlamydomonas reinhardtii was modelled by combining results from the experimental data on components of the PSIl and standard homology approaches. This approach leads to a refined model of the PSII reaction center comprisi D1 and D2 proteins, accessory chlorophylls, a manganese cluster, two molecules of p-carotene and cytochrome b559. To our aims, the D1 protein structure was extrapolated from this model with a few... 

Important in the assignment of these transfer steps is obviously the identification of the absorption bands in the spectrum of the reaction center. In the near-infrared absorption region there are three bands visible for the R. sphaeroides reaction center. With the use of spectral differences induced by reduction or oxidation it was possible to assign the longest wavelength band in the Qy region (870 nm) to the special pair, the middle band (800 nm) to the accessory chlorophylls, and the third (770 nm) to the pheophytins. 

Charge separation probably occurs mainly as a result of one of the chlorophyll pair, with some involvement of the accessory chlorophyll. Rather than pheophytin acting as an acceptor, in this case the electron is transferred to the chlorophyll. Since the oxidation/reduction potential of the acceptor chlorophyll is negatively shifted, it plays a role in the subsequent photosynthetic reduction, hi the case of photosystem II (Fig. 3d), the interplanar and Mg Mg distances are further increased to 5.0 and 10.0 A [15], respectively, and the two chlorophylls in the pair are now regarded as almost independent. 

The 77 K absorption spectrum (a) and the 4th derivative (b) of the purified PSII reaction center complex prepared by isoelectric focusing in digitonin solution are shown in Fig. 2. The spectrum is well-resolved than in Triton preparation (1). Absorption in the red maximum region, as well as the peaks around 600 nm, is clearly separated into two components 670 and 680 nm in the red, which might be attributed to the accessory chlorophyll and P-680 plus pheophytin acceptor, respectively. The 77 K emission spectrum of Triton preparation (1) exhibits shoulders on both sides of the main peak at about 681 nm, originating from free chlorophyll and the aggregates. However, the contribution of these components was markedly reduced in the spectrum of the complex prepared by the present procedure a sharp emission peak at about 683 nm was observed in this case. 

---