For the first time, chemists from the Technological Institute of
Massachusetts (MIT), in the United States, have measured the transfer of energy
between light-harvesting photosynthetic proteins and have discovered that the
disorganized arrangement of light-harvesting proteins increases the
efficiency of energy transduction, as published in the journal Proceedings
of the National Academy of Sciences.
When photosynthetic cells absorb sunlight,
packets of energy called photons jump between a series of proteins
light-capturing cells until they reach the photosynthetic reaction center. There, the
cells convert energy into electrons, which ultimately drive production
of sugar molecules.
This transfer of energy through the complex of
Light gathering occurs with extremely high efficiency: almost every
absorbed photon of light generates an electron, a phenomenon known as efficiency
quantum near unity.
New study from MIT chemists offers a possible
explanation of how the proteins of the light-harvesting complex, also called
antenna, achieve that high efficiency. For the first time, researchers
were able to measure energy transfer between proteins
light collectors, which allowed them to discover that the arrangement
disorganization of these proteins enhances the efficiency of transduction of
power.
“For the antenna to work, you need a
long-distance energy transduction. Our key finding is that the
disordered organization of light-harvesting proteins increases efficiency
of this long-distance transduction of energy”, says Gabriela
Schlau-Cohen, associate professor of chemistry at MIT and lead author of the
new study.
For this study, the MIT team focused on the
purple bacteria, which are often found in aquatic environments poor in
oxygen and are often used as a model to study photosynthetic uptake
of light.
Within these cells, the captured photons travel at
through light-harvesting complexes made up of proteins and pigments that
absorb light, like chlorophyll.
Thanks to ultrafast spectroscopy, a technique that
uses extremely short laser pulses to study events that occur in
time scales from femtoseconds to nanoseconds, scientists have been able to
study how energy moves within a single one of these proteins. Without
However, studying how energy travels between these proteins has proved
much more difficult because it requires placing several proteins in a controlled way.
To create an experimental setup that would allow them to measure
how energy travels between two proteins, the MIT team designed membranes
nanoscale synthetics with a composition similar to that of membranes
natural cells. By controlling the size of these membranes, known as
nanodiscs, they were able to control the distance between two embedded proteins
inside the discs.
For this study, the researchers embedded into their
nanodiscs two versions of the main light-harvesting protein of
purple bacteria, known as LH2 and LH3. LH2 is the protein that is
present under normal light conditions, and LH3 is a variant that usually
express themselves only in low light conditions.
Using the facility’s cryoelectron microscope
MIT.nano, the researchers were able to image their proteins
embedded in the membrane and show that they were placed at distances
similar to those observed in the native membrane. They were also able to measure
distances between the light-harvesting proteins, which were on the order of 2.5 to 3
nanometers.
Since the LH2 and LH3 proteins absorb wavelengths
of slightly different light, it is possible to use spectroscopy
ultrafast to observe the energy transfer between them. If
of proteins in close proximity to each other, the researchers discovered that a
photon of energy takes about 6 picoseconds to travel between them.
For proteins farther away, the transfer
it takes up to 15 PS. A faster ride translates into a
more efficient energy transfer, since the longer the path, the
more energy is lost during the transfer.
“When a photon is absorbed, only one photon is available
time before that energy is lost through unwanted processes such as
non-radiative decay, so the faster you can
convert, the more efficient it will be,” says Schlau-Cohen.
Researchers also discovered that proteinsarranged in a lattice structure showed a transfer of energy
less efficient than proteins arranged in organized structures to the
random, as they often are in living cells.
“Orderly organization is actually less
efficient than the disorderly organization of biology, which seems to us
really interesting because biology tends to be messy. This
finding tells us that it may not be just an unavoidable drawback of the
biology, but rather that organisms may have evolved to
take advantage of it,” says Schlau-Cohen.
Now that they have established the ability to measure the
energy transfer between proteins, the researchers plan to explore
energy transfer between other proteins, such as the transfer between
antenna proteins and reaction center proteins. Also have
planned to study the energy transfer between antenna proteins that
are found in organisms other than purple bacteria, such as
green plants.
Europa Press
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