The crucial step within photosystems I and II is the use of the absorbed light energy to transfer an electron from a donor molecule to an acceptor molecule. The particular donor and acceptor molecules are different in the two photosystems. The site in a photosystem at which this event occurs is known as the reaction centre. The central role of using the excitation energy to extract an electron from one molecule and transfer it to another is, in each photosystem, carried out by a special form of chlorophyll a complexed with a specific protein. When the reaction centre chlorophyll receives excitation energy, it is raised to an excited electronic state. In this excited state it can reduce (transfer an electron to) the acceptor molecule. The oxidized chlorophyll then withdraws an electron from the donor molecule, and so returns to its original state. The loss of an electron is accompanied by a fall in the absorption spectrum of the reaction centre chlorophyll, in the region of its red peak. The spectral change is maximal at about 700 nm in photosystem I and at about 680 nm in photosystem II: accordingly, the specialized forms of chlorophyll are referred to as P700 and P680, respectively. Since the reaction centre chlorophylls constitute only a very small proportion of the total chlorophyll a - one P700 and one P680 per 500 total chlorophyll molecules in green plants - their spectral changes are insignificant with respect to the total absorption spectrum of the system.
Excitation of reaction centre chlorophyll can be brought about by direct absorption of a photon. Since P700 and P680 constitute a tiny proportion of the total pigment, however, this does not take place very often. In fact, virtually all the excitation energy received by the reaction centre is energy initially captured by the vastly more numerous light-harvesting or antenna pigment molecules of that photosystem and transferred to the reaction centre. The mechanism by which this takes place is known as inductive resonance transfer, first postulated by the theoretical chemist T. Forster in 1947.
Absorption of a photon by one molecule is followed by vibrational energy dissipation, bringing the excited electron to its lowest excited state. This can be in a state of resonance with one of the upper vibrational levels of the excited state of another molecule (not, initially, in an excited state). The energy is transferred from the first molecule to the second, i.e. the first molecule reverts to the ground state and the second molecule is raised to an excited electronic state. For efficient transfer the fluorescence emission peak of the donating molecule must overlap the absorption spectrum of the receiving molecule. Since the fluorescence emission spectrum of any molecule is a mirror image on the long-wavelength side of the absorption spectrum, with the peak shifted to longer wavelength, it follows that for efficient energy transfer the absorption peak of the donating molecule should be at a shorter wavelength than that of the receiving molecule. In addition the molecules must not be too far apart: efficient transfer can take place at distances up to about 5 nm.
The light-harvesting, as opposed to reaction centre, pigment molecules are made up of the great majority of the chlorophyll a molecules, chlorophylls b, c1 (or c3) and c2, the various carotenoids and the biliproteins. All these (except carotenoids) fluoresce actively in vitro and so the assumption that energy transfer among them takes place by inductive resonance presents no problem. Carotenoids, on the other hand, show virtually no fluorescence in vitro, and the view is sometimes expressed that energy transfer from carotenoids to other pigments must involve some other mechanism. It has nevertheless been shown that carotenoid fluorescence does in fact occur, but at very low yield: 6 x 10~5 in b-carotene453a and 5 x 10~5 in fucoxanthin.680a Absorption of a photon by a carotenoid molecule raises its energy level to the first excited singlet state, indicated by 1Bu. This is followed by a fast non-radiative relaxation to an optically forbidden singlet state, 2Ag, at a lower energy level ('optically forbidden' meaning that the molecule cannot arrive at this state directly by absorption of a photon). In the case of b-carotene, Gillbro and Cogdell (1989) believe that the fluorescence emission is from the 1Bu excited singlet state, and that its very short lifetime excludes the Forster mechanism as a candidate for efficient energy transfer from carotenoids in the light-harvesting antenna. As a possible alternative they suggest a short-range electron exchange interaction. In the case of fucox-anthin, Katoh et al. (1991) present evidence that the fluorescence emission originates from the 2Ag state, and suggest that energy transfer takes place by the Forster mechanism to the Qy singlet state (corresponding to the ^670 nm absorption band) of chlorophyll a.
In higher plants and green algae, energy absorbed by chlorophyll b is transferred to chlorophyll a with about l00% efficiency: energy absorbed by carotenoids is transferred (with lower efficiency), probably first to chlorophyll b and then to chlorophyll a. In all those algae containing chlorophyll c, the energy absorbed by this pigment is transferred efficiently to chlorophyll a. In those algae that contain major light-harvesting carotenoids - fucoxanthin, peridinin or siphonaxanthin - with substantial absorption in the 500 to 560 nm region, there is efficient energy transfer directly from the carotenoid to chlorophyll a. In red and blue-green algae, the sequence of transfer, 80 to 90% efficient overall, is phycoerythrin (or phycoerythrocyanin) ! phycocyanin ! allophycocyanin ! chlorophyll a. In cryptophytes, which usually have only one biliprotein, this sequence is not possible: direct energy transfer from the biliprotein to chlorophyll a may occur, with quite high efficiency from phycocyanin, but lower efficiency from phycoerythrin.
In every case (with the exception of chlorophyll d-containing organisms -see below), no matter which pigment first captures the light, the absorbed energy always ends up in chlorophyll a. This is to be expected since chlorophyll a has its absorption peak at a longer wavelength than any of the other pigments and, as noted earlier, energy migration by this mechanism is in the direction of the molecules absorbing at the greatest wavelength. Among the bulk chlorophyll a-protein complexes, the excitation energy moves at random until it reaches a reaction centre, where it is immediately trapped and used for electron transfer.
In the unusual cyanophyte, Acaryochloris marina, the major light-absorbing pigment is chlorophyll d, with an in vivo Qy absorption band at about 715 nm,920,908 compared to ^676 nm for chlorophyll a. The photosystem I reaction centre chlorophyll in this organism appears to be a form of chlorophyll d: the spectral change when it is excited and loses an electron is maximal at about 740 nm, and so it is referred to as P740.595 Acaryochloris marina does contain a small amount of chlorophyll a - about 3% of the chlorophyll d content - and there is evidence that the photosystem II reaction centre chlorophyll in this organism is in fact chlorophyll a, and that 'uphill' transfer of energy from chlorophyll d to chlorophyll a takes place.907,908 The biliproteins in A. marina provide energy only to photosystem II.594
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