Chlorophyll molecules

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

This symmetry is important in bringing the two chlorophyll molecules of the "special pair" into close contact, giving them their unique function in initiating electron transfer. They are bound in a hydrophobic pocket close to the symmetry axis between the D and E transmembrane a helices of both... 

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). 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

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... 

Chlorophyll molecules form circular rings in the light-harvesting complex LH2... 

The structure of the LH2 complex of R. acidophila is both simple and elegant (Figure 12.17). It is a ring of nine identical units, each containing an a and a P polypeptide of 53 and 41 residues, respectively, which both span the membrane once as a helices (Figure 12.18). The two polypeptides bind a total of three chlorophyll molecules and two carotenoids. The nine heterodimeric units form a hollow cylinder with the a chains forming the inner wall and the P chains the outer wall. The hole in the middle of the cylinder is empty, except for lipid molecules from the membrane. 

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. 

Figure 12.22 Schematic diagram showing the flow of excitation energy in the bacterial photosynthetic apparatus. The energy of a photon absorbed by LH2 spreads rapidly through the periplasmic ring of bacterio-chlorophyll molecules (green). Where two complexes touch in the membrane, the energy can be transmitted to an adjacent LH2 ring. From there it passes by the same mechanism to LHl and is finally transmitted to the special chlorophyll pair in the reaction center. (Adapted from W. Kiihlbrandf, Structure 3 521-525, 1995.)...

Modeling of the reaction center inside the hole of LHl shows that the primary photon acceptor—the special pair of chlorophyll molecules—is located at the same level in the membrane, about 10 A from the periplasmic side, as the 850-nm chlorophyll molecules in LH2, and by analogy the 875-nm chlorophyll molecules of LHl. Furthermore, the orientation of these chlorophyll molecules is such that very rapid energy transfer can take place within a plane parallel to the membrane surface. The position and orientation of the chlorophyll molecules in these rings are thus optimal for efficient energy transfer to the reaction center. 

Photosynthetic Units Consist of Many Chlorophyll Molecules hut Only a Single Reaction Center... 

Chlorophyll, the substance responsible for the green color of leaves, has one magnesium atom per chlorophyll molecule and contains 2.72% magnesium by mass. What is the molar mass of chlorophyll ... 

Magnesium and calcium are by far the most important members of the group. Magnesium is, in effect, the doorway to life it is present in every chlorophyll molecule and hence enables photosynthesis to take place. Calcium is the element of rigidity and construction it is the cation in the bones of our skeletons, the shells of shellfish, and the concrete, mortar, and limestone of buildings. 

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

Illuminated chlorophyll molecules can form excitation states which are able to transfer energy on to the oxygen molecule, raising it from its ground state to the more reactive excited state known as singlet oxygen. 

Tributsch H (1972) Reaction of excited chlorophyll molecules at electrodes and in photosynthesis. Photochem Photobiol 16 261-269... 

Because a chlorophyll molecule contains a closed circuit of ten conjugated double bounds to absorb light, spectrophotometric (UV-Vis) and fluorometric measurements are satisfactory to identify and estimate amounts of chlorophyll a and chlorophyll b, usually the only ones present in fresh plant extracts. The basis of numerous spectrophotometric determinations reported in literature is that chlorophylls strongly absorb at 500 to 700 nm in the visible region and show a large typical band around 400 nm. 

Chlorophyll absorbs photons having a wavelength of about 680 nm and an energy of about 1.8 eV. The primary act of the photosynthetic reaction is excitation of the chlorophyll molecules during photon absorption ... 

Such a process can naturally be expected to play a certain part in the mechanism of directed energy transport in biological systems, in particular, in the transfer of absorbed energy from the antenna chlorophyll molecules to the reactive center in the photosynthetic system of plants. In Ref. [30], energy exchange between molecules of the photosynthetic pigments chlorophyll a and pheophytin a was studied experimentally with pigments introduced into the polar matrix. 

Note that the chlorophyll molecules are all not identical in the photosynthesis apparatus. Many molecules in a collecting unit absorb light so that a large volume... 

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