Molecular Mechanisms of Photosynthesis. Robert E. Blankenship
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transition the 0,0 band, with the higher energy satellites termed 0,1; 0,2; and so on. The first number is the vibrational quantum number of the ground electronic state before light absorption, and the second number the vibrational state of the excited electronic state after the transition. Of course, there are many vibrations in a chlorophyll molecule, and only one of these is the one responsible for the vibrational structure in the absorption spectrum.
The fluorescence spectrum of all chlorophylls peaks at slightly longer wavelengths than the absorption maximum. The fluorescence emission is polarized along the y molecular axis, as it is emitted from the Qy transition. The fluorescence spectrum usually has a characteristic “mirror image” relationship to the absorption. This is because the ground and excited states have similar shapes, so those molecular vibrations that are activated during electronic absorption are also likely to be activated upon fluorescence emission. However, in this case, the initial state is the ground vibrational state of the excited electronic state, and the final state is the excited vibrational state of the ground electronic state. This causes a shift of the emission to the longer‐wavelength side of the main transition, in what is known as the Stokes shift (see Appendix).
Table 4.2 brings together wavelengths of absorption maxima as well as molar extinction coefficients and fluorescence lifetimes and quantum yields of chlorophylls and bacteriochlorophylls in organic solvents. A comprehensive database of spectra and photophysical properties of chlorophylls and related pigments is available (Taniguchi and Lindsey, 2021). The spectral properties of these pigments are significantly altered when they are incorporated into protein complexes. In every case, the longest‐wavelength maximum shifts to longer wavelengths in pigment–proteins; sometimes the shift is more than 100 nm. We will examine the properties of these pigment–protein complexes in more detail in Chapters 5–7.
Table 4.2 Spectroscopic properties of chlorophylls and bacteriochlorophylls in vitroa
Source: Data from Scheer (1991) and Niedzwiedzki and Blankenship (2010).
| Pigment | λ max (nm) | ɛ max (mM−1 cm−1) b | τ f (ns) | φ f |
|---|---|---|---|---|
| Chlorophyll a | 662, 578,430 | 90.0 | 6.3 | 0.35 |
| Chlorophyll b | 644, 549, 455 | 56.2 | 3.2 | 0.15 |
| Chlorophyll c1 | 640, 593, 462 | 35.0 | 6.3 | |
| Chlorophyll d | 697, 456, 400 | 63.7 | 6.2 | |
| Chlorophyll f | 707, 440, 398 | 71.1 | ||
| Bacteriochlorophyll a | 773, 577, 358 | 90.0 | 2.9 | 0.2 |
| Bacteriochlorophyll b | 791, 592, 372 | 106 | 2.4 | |
| Bacteriochlorophyll c | 659, 429 | 75 | 6.7 | 0.29 |
| Bacteriochlorophyll d | 651, 423 | 79 | 6.3 | |
| Bacteriochlorophyll e | 649, 462 | 49 | 2.9 | |
| Bacteriochlorophyll f | 645, 467 | 3.4 | 0.13 | |
| Bacteriochlorophyll g | 762, 566, 365 | 76 | 2.7 |
a Most data taken from Scheer (1991) or Niedzwiedzki and Blankenship (2010). Solvents are not the same for all quantities.
b Values for ɛmax are for the longest wavelength absorbing Qy band.
4.5 Carotenoids
Carotenoids are found in all known native photosynthetic organisms, as well as in many nonphotosynthetic organisms (Britton et al., 1998; Frank et al., 2000; Polívka and Frank, 2010). There are many hundreds of chemically distinct carotenoids, so we will not give a comprehensive list. However, there are some consistent structural features that are common to most photosynthetic carotenoids. They are extended molecules with a delocalized π electron system. Carotenoids from oxygenic organisms usually contain ring structures at each end, and most carotenoids contain oxygen atoms, usually as part of hydroxyl or epoxide groups. Structures of several of the carotenoids found in photosynthetic systems are shown in Fig. 4.11.
Carotenoid biosynthesis consists of the building up of large molecules from a basic building block, the five‐carbon branched‐chain species isoprene (Britton et al., 1998). It is successively condensed into 10‐, 20‐, and 40‐carbon molecules, ending with the compound phytoene. Phytoene is a hydrocarbon consisting of eight isoprene units attached in a linear fashion. It is colorless, because most of the double bonds are isolated. The second stage in the biosynthesis consists of successive desaturation steps, producing a series of intermediates with an increasing number of conjugated double bonds. This has the effect of shifting the absorption into the visible region. The end product of this stage is the compound lycopene, which is responsible for the red color of tomatoes. Some of the intermediates, such as neurosporene, are the end point in carotenoid biosynthesis for some anoxygenic photosynthetic bacteria. In most organisms, there are two additional stages of the biosynthetic pathway: cyclization of the ends of the molecule followed by derivatization by hydroxylation or any of a wide variety of other processes.
Figure 4.11 Structures of several carotenoids and carotenoid precursors important in photosynthetic