Bacteriochlorophylls special pair

The hole in the bacteriochlorophyll special pair is filled by electron transfer from a cytochrome. In Rps. viridis, the cytochrome donor is tightly bound to the PRC at the membrane surface. This subunit contains four hemes in a nearly linear array oriented perpendicular to the membrane. The reduction potentials of the hemes alternate from high (>250 mV Vi. NHE) to low (<50mV) as the distance from (Bchl)2 increases. The heme closest to the special pair, cytochrome C559, has the highest potential and fills the (Bchl)2 + hole in about 200 ns. The next well-characterized process is ET from cytochrome C556 to cytochrome C559 in 2 ts over a distance of 27.9 A. ... 

Figure 11 Polarized room temperature near-IR absorption spectra of a Blastochloris viridis bacterial reaction center single crystal of 200 xm thickness. Spectra were taken with the crystal encapsulated and preserved in a thin-walled capillary while illuminated with a 633nm HeNe laser. The spectral band seen near 1300nm is of the photogenerated bacteriochlorophyll special pair cation P+. The amplitude of this peak allows the extent of photoconversion of the sample and its kinetics to be directly monitored (Drawn from data in Baxter et al. )...

Note the data points marked by x in the near infrared region between 1000 and 1300 nm, as reported by Dutton, Kaufmann, Chance and Rentzepis". This combination of extinction coefficients is almost certainly due to the oxidation of the primary electron donor, P870, i.e., the bacteriochlorophyll special-pair [BChl]2, since it is not found for species such as BChF, BChl", or B (generated electrochemi-cally). Picosecond measurements have revealed the maximum absorbance increase at 1250 nm in Rb. sphaeraides and C. vinosum and at 1300 nm mRp. viridis. Picosecond measurements in this wavelength region can thus provide direct information on the reaction course of P870 photooxidation. 

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

The two BPh are situated along two symmetric branches of pigments emanating from the bacteriochlorophyll special pair (5-7). The two BPh are distinquishable by the position of their absorption peaks (8) the BPh... 

Fig. 13. Arrhenius plot of k(T) for electron transfer from cytochrome c to the special pair of bacteriochlorophylls in the reaction center of c-vinosum.

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). 

Figure 12.21 Schematic diagram of the relative positions of bacteriochlorophylls (green) in the photosynthetic membrane complexes LHl, LH2, and the reaction center. The special pair of bacteriochlorophyll molecules in the reaction center is located at the same level within the membrane as the periplasmic bacteriochlorophyll molecules Chi 875 in LHl and the Chi 850 in LH2. (Adapted from W. Kiihlbrandt, Structure 3 521-525, 1995.)...

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

The vibrational frequency of the special pair P and the bacteriochlorophyll monomer B have also been extracted from the analysis of the Raman profiles [39,40,42,44,51]. Small s group has extensively performed hole-burning (HB) measurements on mutant and chemically altered RCs of Rb. Sphaeroides [44,45,48-50]. Their results have revealed low-frequency modes that make important contribution to optical features such as the bandwidth of absorption line-shape, as well as to the rate constant of the ET of the RCs. 

Figure 8. Model of excitonic interactions for the special pair (P) and accessory bacteriochlorophylls (B).

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Rapid multistep Coulombic energy transfer takes place as the excitation energy is transferred between the antenna chromophores and the special pair of bacteriochlorophyll molecules (P) in the reaction centre. 

Analogies of Rare Earth Porphyrin Doubledeckers with the Special Pair of Bacteriochlorophylls in Bacterial Photosynthesis... 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

Spectral measurements suggesting exciton splitting were among early observations that led to the conclusion that the bacteriochlorophyll involved in the initial photochemical process exists as a dimer or special pair (Fig. 23-31),319/324 a conclusion verified by the structure determination. The special pair of BChl b lies in the center of the helical bundle that is embedded in the membrane. Nearly perpendicular to the rings of the special pair are two more molecules of BChl b. The central magnesium atoms of all four bacteriochlorophylls are held by imidazole groups of histidine side chains.319 325 Below the chlorophylls are... 

A rise in absorbance at 1250 nm is interpreted as formation of the bacteriochlorophyll cation radical BChl+ in the special pair. Other spectral changes support the formation of BPh as the first reduction product (A- in Eq. 23-31). However, this is thought to occur in two steps323 with the monomeric BChl (BA in Fig. 23-31B) receiving the electron in 2.8 ps and passing it to the pheophytin (Phe ( >A in Fig. 23-31B) in 0.7 ps (Eq. 23-32 corresponding steps for PSII of green plants are also shown). 

The quinone QA (the secondary acceptor) is next reduced by the BPh radical in 200 ps with development of a characteristic EPR signal321 330 at g = 1.82. Over a much longer period of time ( 320 ns) an electron passes from the tetraheme cytochrome subunit to the Chl+ radical in the special pair.323/323a y ie relatively slow rate of this reaction may be related to the fact that the bacteriochlorophyll of the special pair is 2.1 nm (center-to-center) from the nearest heme in the... 

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