In plants chlorophyll is associated with specific proteins, forexample, chlorophyll- binding proteins are referred to as CP I,CP 47 and CP 43. With improving biochemical techniques for useon the membrane systems there has been an ever increasing successin the isolation and characterisation of these proteins.
This figure shows a schematic representation of the majorsubfractions that can be isolated from membranes. InPS I (photosystem I) an initial solubilisation produces largeparticles (called PS I-110). These particles contain twochlorophyll-protein complexes: the reaction centre chlorophyll-protein (CP I) and a chlorophyll + complex (LHC I,light-harvesting complex) (). PS I-110 also contains 6 to 8 of lower molecular weight (8 to 25kDa, where 1dalton=1 a.m.u.) that do not bind to chlorophyll, calledSubunits II-VII. CP I, the reaction centre P700 chlorophyll-protein, can be isolated from any of these mixtures by treatmentwith SDS (sodium dodecyl sulfate) or LiDS (lithium dodecylsulfate) followed by electrophoresis.
As is detailed by the Zenith Bud Cultivator graph, the strong vegetating signature effectively covers Chlorophyll a, b, Carotenoids and Flavoproteins.
Theaccessory pigment role is a singlet-singlet energy transfer from thecarotenoid to the bacteriochlorophyll, while the protective role is atriplet-triplet energy transfer from the bacteriochlorophyll to thecarotenoid.
Drawing upon recent work with photosynthetic bacteria, evidenceis presented as to how the carotenoids are organized within bothportions of the photosynthetic unit (the light harvesting antenna andthe reaction centre) and how they discharge both their functions.
They serve as accessory light harvesting pigments,extending the range of wavelengths over which light can drivephotosynthesis, and they act to protect the chlorophyllous pigments fromthe harmful photodestructive reaction which occurs in the presence ofoxygen.
Chlorophyll is essentially two parts: a substituted ring and (the long carbon chain). The porphyrin ring isan excellent ligand, with the four nitrogen atomsbinding strongly to a co-ordinated metal atom in a square planararrangement. There are many examples of this including heme and vitaminB12.
The PS II reaction centre is significantly more complex than thereaction centre of PS I, where P700 is clearly localised on thegreen complex CP I. P680, the reaction centre chlorophyll of PSII, is difficult to determine because the P680+ Pheo- charge separation decays within a nanosecond.P680 is currently considered to be a chlorophyll- dimer, at least in the ground state.
Improved extraction procedures gave oxygen evolving PS II(photosystem II) particles (BBY's). These particles are largepieces of granal membranes, probably lipid depleted (,, ,). Other treatments have been employed to isolatethe core particles from PS II. These core particles contain tworeaction chlorophyll- proteins, CP 47 and CP 43 and several nonchlorophyll binding (D1, D2), but are free fromchlorophyll + complexes. Core particles which retain manganesehave been successfully isolated with the two chlorophyll-proteins and a limited number of other polypeptides. CP 47 andCP 43 can be purified from the other components of PS II by theuse of SDS-PAGE or HPLC (high performance liquid chromatography)but they have no photochemical activity in isolation.
The nature of the reaction centre of chlorophyll, P700, is stillunknown, as there is conflicting evidence. It has been suggestedthat this could be explained if there is a pair ofelectronically interacting chlorophyll- molecules in the ground(reduced) state (P700), and that the unpaired electron of theP700+ (oxidised) state is localised on only one of thechlorophyll's (). The other 40 to 50 chlorophyll- molecules ofCP I act as antennas, and are thought to be responsible for the721nm fluorescence emission maximum (,).
Initial experiments done by Ogawa ()and Thornber () isolatedtwo complexes by SDS-PAGE from SDS-solubilised membranes. One ofthese complexes, CP I, had a high apparent molecularweight and contained only chlorophyll-. CP I is the most stableof the complexes and retained the photochemical activity ofP700, the reaction centre of chlorophyll in PS I. It has achlorophyll to P700 ratio of ~45 (, , ,, ) and abeta-carotene to P700 ratio of ~8.
Treatment of cholorophyll- with acid removes the magnesium ionreplacing it with two hydrogen atoms giving an olive-brownsolid, -. Hydrolysis of this (reverse ofesterification) splits off and gives-.Similar compounds are obtained if chlorophyll- is used.
Chlorophyll's most important use, however, is in nature, in .It is capable of channelling the energy of sunlight into chemical energythrough the process of photosynthesis. In this process the energyabsorbed by chlorophyll transforms carbon dioxide and water intocarbohydrates and oxygen: