107
spectrum (when the latter is plotted as fraction of the incident light absorbed). For
photosynthetic systems, the fluorescence excitation spectrum probes the absorptions of all
pigments whose excited state energy is transferred to chI.
IN VIVO ORGANIZATION OF PHOTOSYNTHETIC PIGMENTS
Photosynthetic pigments can be broadly separated into two categories of pigment types
depending on their function: antenna and reaction center pigments. There is good evidence
to suggest that all functional photosynthetic pigments are bound in specific pigment-protein
complexes (Thornber, 1986). The antenna versus reaction center functions of pigments are
determined largely by pigment-pigment and pigment-protein interactions in the local binding
environment. Differences in these binding environments may also induce significant (5-90 nm)
spectral shifts in the lowest energy absorptions of pigments compared to their absorption in
organic solvents. The binding environment also provides new avenues for decay of excited
state energy among the pigments. Of principal importance are transfer of excited state energy
between neighboring antenna pigments and photochemistry on the reaction center pigment.
These new processes alter the interpretation of fluorescence yield changes from that of
isolated pigments described in the previous section. In addition, there are other structural
constraints and physiological processes that alter fluorescence yield in vivo, complicating the
interpretation of fluorescence and photochemical measurements.
Reaction centers and photochemistry. The reaction center is the pigment in which the energy
of absorbed sunlight in the form of an excited electronic state of chI is converted into
chemical energy in the form of oxidants and reductants. The pigment and protein composition
of the reaction centers appears to be highly conserved among all photosynthetic organisms
(Thornber, 1986). The basic pattern of the photochemical reactions, which is also conserved,
is schematically shown in Figure 4. Here, P is the reaction center pigment, D is the electron
donor to the reaction center, and A is the primary electron acceptor. Absorption or transfer
of light energy to P generates the first excited state p'. p' is a stronger reductant than P, and
a fast (3 x 10- 12 s) photochemical reaction generates the charge separated state D P+ A-. Now
P+, being a much stronger oxidant than P, removes an electron from D generating the state
D+ P A. Subsequent electron transport reactions reduce D+ and oxidize A to regenerate the
spectrum (when the latter is plotted as fraction of the incident light absorbed). For
photosynthetic systems, the fluorescence excitation spectrum probes the absorptions of all
pigments whose excited state energy is transferred to chI.
IN VIVO ORGANIZATION OF PHOTOSYNTHETIC PIGMENTS
Photosynthetic pigments can be broadly separated into two categories of pigment types
depending on their function: antenna and reaction center pigments. There is good evidence
to suggest that all functional photosynthetic pigments are bound in specific pigment-protein
complexes (Thornber, 1986). The antenna versus reaction center functions of pigments are
determined largely by pigment-pigment and pigment-protein interactions in the local binding
environment. Differences in these binding environments may also induce significant (5-90 nm)
spectral shifts in the lowest energy absorptions of pigments compared to their absorption in
organic solvents. The binding environment also provides new avenues for decay of excited
state energy among the pigments. Of principal importance are transfer of excited state energy
between neighboring antenna pigments and photochemistry on the reaction center pigment.
These new processes alter the interpretation of fluorescence yield changes from that of
isolated pigments described in the previous section. In addition, there are other structural
constraints and physiological processes that alter fluorescence yield in vivo, complicating the
interpretation of fluorescence and photochemical measurements.
Reaction centers and photochemistry. The reaction center is the pigment in which the energy
of absorbed sunlight in the form of an excited electronic state of chI is converted into
chemical energy in the form of oxidants and reductants. The pigment and protein composition
of the reaction centers appears to be highly conserved among all photosynthetic organisms
(Thornber, 1986). The basic pattern of the photochemical reactions, which is also conserved,
is schematically shown in Figure 4. Here, P is the reaction center pigment, D is the electron
donor to the reaction center, and A is the primary electron acceptor. Absorption or transfer
of light energy to P generates the first excited state p'. p' is a stronger reductant than P, and
a fast (3 x 10- 12 s) photochemical reaction generates the charge separated state D P+ A-. Now
P+, being a much stronger oxidant than P, removes an electron from D generating the state
D+ P A. Subsequent electron transport reactions reduce D+ and oxidize A to regenerate the