Antenna excited state decay kinetics establish primary electron transfer in reaction centers as heterogeneous

  • Laible P
  • Greenfield S
  • Wasielewski M
 et al. 
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The decay of the excited primary electron donor P* in bacterial photosynthetic
reaction centers (both membrane-bound and detergent-isolated) has
been observed to be nonexponential on a time scale of some tens of
picoseconds. Although the multipicosecond nonexponentiality of P*
has been ascribed to heterogeneity in the rate of primary electron
transfer (PET), the decay kinetics can be interpreted equally well
using homogeneous models. To address this ambiguity, we studied the
decay of excited bacteriochlorophyll (Bchl) in the membrane-bound
core antenna/reaction center complexes of wild-type and mutant reaction
center strains of Rhodobacter capsulatus. Reaction centers isolated
from these same strains display a range of multiexponentiality in
primary charge separation. The mutant strains carry substitutions
of amino acids residing near the monomeric Bchl on the active and/or
inactive sides of the reaction center. Transient absorption measurements
monitoring the Q(y) bleach of antenna Bchls require at least two
exponential components to fit all decays. The wild type was fitted
with equal-amplitude components whose lifetimes are 24 and 65 ps.
The shortest-lived component is relatively insensitive to mutation,
in contrast to the longer-lived component(s) whose amplitude and
magnitude were dramatically perturbed by amino acid substitutions.
Unlike the situation with isolated reaction centers, here the only
kinetic models consistent with the data are those in which the primary
electron-transfer rate constant is heterogeneous suggesting at least
two structural populations of RCs. PET in the population with the
shortest-lived antenna decay causes the kinetics to be transfer-to-trap-limited,
whereas the kinetics in the other population(s)-having longer-lived
antenna decays-are limited by the rate of PET. Observation of both
types of kinetic limitation within a single light-harvesting system
is unexpected and complicates any discussion of the rate-limiting
step of light energy utilization in photosynthesis.

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  • P D Laible

  • S R Greenfield

  • M R Wasielewski

  • D K Hanson

  • R M Pearlstein

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