Dynamics of Primary Charge Separation in Bacterial
Photosynthesis
Plants and photosynthetic bacteria store energy through a series of
electron transfer reactions initiated by photoexcitation of a special chlorophyll
dimer, leading eventually to reduction of a quinone on the opposite side
of the membrane. In spite of extensive experimental and theoretical efforts
in the last decade, the role of the accessory bacteriochlorophyll in the
early stages of the process has been the subject of significant controversy.
We have performed accurate quantum mechanical simulations of the primary
charge transfer in photosynthetic reaction centers. The process was modeled
by three coupled electronic states corresponding to the photoexcited special
chlorophyll pair (donor), the reduced bacteriopheophytin (acceptor) and
the reduced accessory chlorophyll (bridge) that interact with a dissipative
medium of protein and solvent degrees of freedom whose spectral density
was obtained from classical simulations. Fixing the energies of the donor
and acceptor states at the experimentally known values and varying the
free energy of the bridge we have performed long-time simulations of the
charge transfer dynamics and compared to experimental results on wild-type
and modified reaction centers.
The time evolution of these three electronic state populations in wild-type
and modified reaction centers was followed over 17 ps using our iterative
path integral scheme. Agreement with experimental observations of significant
long-lived population in certain modified reaction centers is observed
if the bridge free energy is about 400 wavenumbers lower than that
of the excited special pair. The bacteriochlorophyll monomer appears to
be involved as a true intermediate in a two-step process which is well
approximated by kinetic equations. This picture reproduces the observed
temperature dependence and is consistent with kinetic data on various mutants.
We have verified that the observed population kinetics are qualitatively
insensitive with respect to reasonable variation of the coupling parameters.
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Coherence in the energy transfer of photosynthetic light-harvesting
complexes
The process of energy transduction in the membranes of photosynthetic
bacteria begins with the absorption of visible light by light harvesting
antennas which then funnel the energy into the reaction cen-ter. The light
harvesting complexes are molecular aggregates composed of several units
that contain peptides, chlorophyll molecules and carotenoids. These building
blocks are organized in symmetric structures that assume the shape of a
ring. The light harvesting complex I (LH-I) immediately surrounds the reaction
center, while a second type of light harvesting complex (LH-II) channels
energy to the reaction center through LH-I. The structure and electronic
arrangement of Rps. acidophila and Rs. molischianum were determined in
recent experimental and theoretical studies. The basic unit of LH-II is
a heterodimer consisting of two small protein subunits. The heterodimers
bind three bacteriochlorophyll (BChl) molecules, two of which are in close
contact. These dimers form a ring which absorbs around 850 nm. The third
BChl of the structural unit is located about 19 Angstroms away on an outer
ring whose absorption maximum lies at 800 nm. Carotenoid molecules in close
proximity to the outer BChl ring harvest light in a different spectral
range and also prevent photooxidation of the chlorophylls. The LH-II of
the above species is composed of eight or nine such units.
We employed a simple model for the electronic excitations and the exciton-vibration
coupling characterizing the B850 ring of the light harvesting complex in
photosynthetic bacteria to investigate the possibility of coherence in
the energy transfer within the system. The structure of the equilibrium
density matrix was studied using the path integral formulation of quantum
statistical mechanics. The calculated mean coherence length was computed
from the average root-mean-square deviation of closed imaginary time paths
which are sampled via a Monte Carlo procedure. This procedure allows simultaneous
examination of the effects of thermal averaging, dynamic and static disorder
in a single calculation. The mean coherence length was found to be of the
order of two to three chlorophyll monomers at room temperature. The principal
factor responsible for this localization is thermal averaging, although
static and dynamic disorder further destabilize extended states. At low
temperatures the circular arrangement of the pigments favors coherence
with respect to the situation in a linear aggregate. Visual inspection
of typical paths offers an intuitive picture of the extent of coherent
energy delocalization in biological antenna systems.
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