5 research outputs found
Excitonic Energy Landscape of the Y16F Mutant of the <i>Chlorobium tepidum</i> Fenna–Matthews–Olson (FMO) Complex: High Resolution Spectroscopic and Modeling Studies
We report high-resolution
(low-temperature) absorption, emission,
and nonresonant/resonant hole-burned (HB) spectra and results of excitonic
calculations using a non-Markovian reduced density matrix theory (with
an improved algorithm for parameter optimization in heterogeneous
samples) obtained for the Y16F mutant of the Fenna–Matthews–Olson
(FMO) trimer from the green sulfur bacterium <i>Chlorobium tepidum</i>. We show that the Y16F mutant is a mixture of FMO complexes with
three independent low-energy traps (located near 817, 821, and 826
nm), in agreement with measured composite emission and HB spectra.
Two of these traps belong to mutated FMO subpopulations characterized
by significantly modified low-energy excitonic states. Hamiltonians
for the two major subpopulations (Sub<sub>821</sub> and Sub<sub>817</sub>) provide new insight into extensive changes induced by the single-point
mutation in the vicinity of BChl 3 (where tyrosine Y16 was replaced
with phenylalanine F16). The average decay time(s) from the higher
exciton state(s) in the Y16F mutant depends on frequency and occurs
on a picosecond time scale
The Protein Environment of the Bacteriopheophytin Anion Modulates Charge Separation and Charge Recombination in Bacterial Reaction Centers
The
kinetics and pathway of electron transfer has been explored
in a series of reaction center mutants from <i>Rhodobacter sphaeroides</i>, in which the leucine residue at M214 near the bacteriopheophytin
cofactor in the A-branch has been replaced with methionine, cysteine,
alanine, and glycine. These amino acids have substantially different
volumes, both from each other and, except for methionine, from the
native leucine. Though the mutation site of M214 is close to the bacteriopheophytin
cofactor, which is involved in the electron transfer, none of the
mutations alter the cofactor composition of the reaction center and
the primary charge separation reaction is essentially undisturbed.
However, the kinetics of electron transfer from H<sub>A</sub><sup>–</sup> → Q<sub>A</sub> becomes both slower and substantially
heterogeneous in three of the four mutants. The decreased H<sub>A</sub><sup>–</sup> → Q<sub>A</sub> electron transfer rate
allows charge recombination between P<sup>+</sup> and H<sub>A</sub><sup>–</sup> to compete with the forward reaction, resulting
in a drop in the overall yield of charge separation. Both the yield
change and the variation in kinetics correlate well with the volume
of the mutant amino acid side chains. Analysis of the kinetics suggests
that the introduction of a smaller side chain at M214 results in greater
protein structural heterogeneity and dynamics on multiple time scales,
resulting in perturbation of the electronic environment and its evolution
in the vicinity of the early charge-separated radical pair, P<sup>+</sup>H<sub>A</sub><sup>–</sup>, and the subsequent acceptor
Q<sub>A</sub>, affecting both the extent and time scale of dielectric
relaxation. It appears that the reaction center has been optimized
not only in terms of its static structure–function relationships,
but also finely tuned to favor particular reaction pathways on particular
time scales by adjusting protein dynamics
Native Mass Spectrometry Analysis of Oligomerization States of Fluorescence Recovery Protein and Orange Carotenoid Protein: Two Proteins Involved in the Cyanobacterial Photoprotection Cycle
The
orange carotenoid protein (OCP) and fluorescence recovery protein
(FRP) are present in many cyanobacteria and regulate an essential
photoprotection cycle in an antagonistic manner as a function of light
intensity. We characterized the oligomerization states of OCP and
FRP by using native mass spectrometry, a technique that has the capability
of studying native proteins under a wide range of protein concentrations
and molecular masses. We found that dimeric FRP is the predominant
state at protein concentrations ranging from 3 to 180 ÎĽM and
that higher-order oligomers gradually form at protein concentrations
above this range. The OCP, however, demonstrates significantly different
oligomerization behavior. Monomeric OCP (mOCP) dominates at low protein
concentrations, with an observable population of dimeric OCP (dOCP).
The ratio of dOCP to mOCP, however, increases proportionally with
protein concentration. Higher-order OCP oligomers form at protein
concentrations beyond 10 ÎĽM. Additionally, native mass spectrometry
coupled with ion mobility allowed us to measure protein collisional
cross sections and interrogate the unfolding of different FRP and
OCP oligomers. We found that monomeric FRP exhibits a one-stage unfolding
process, which could be correlated with its C-terminal bent crystal
structure. The structural domain compositions of FRP and OCP are compared
and discussed
Redox Conditions Affect Ultrafast Exciton Transport in Photosynthetic Pigment–Protein Complexes
Pigment–protein
complexes in photosynthetic antennae can
suffer oxidative damage from reactive oxygen species generated during
solar light harvesting. How the redox environment of a pigment–protein
complex affects energy transport on the ultrafast light-harvesting
time scale remains poorly understood. Using two-dimensional electronic
spectroscopy, we observe differences in femtosecond energy-transfer
processes in the Fenna–Matthews–Olson (FMO) antenna
complex under different redox conditions. We attribute these differences
in the ultrafast dynamics to changes to the system–bath coupling
around specific chromophores, and we identify a highly conserved tyrosine/tryptophan
chain near the chromophores showing the largest changes. We discuss
how the mechanism of tyrosine/tryptophan chain oxidation may contribute
to these differences in ultrafast dynamics that can moderate energy
transfer to downstream complexes where reactive oxygen species are
formed. These results highlight the importance of redox conditions
on the ultrafast transport of energy in photosynthesis. Tailoring
the redox environment may enable energy transport engineering in synthetic
light-harvesting systems