Discover the secret behind its amazing efficiency in converting light into energy

During photosynthesis, each photon collected by the so-called “antennae” proteins generates energy with incredibly high efficiency. The researchers made the startling discovery that the apparent disorder in the arrangement of these proteins is the secret to this ability, known as “near-unity quantum efficiency.”

Photosynthesis allows plants and some bacteria to convert light energy into chemical energy, with quantum efficiency close to unity. This efficiency is achieved by a network of antennae proteins that collect photons and transfer the resulting energy to the photosynthetic reaction centers. Once this energy reaches its destination, it is converted into electrons to fuel the production of glucose and oxygen molecules.

While energy transfer at the scale of individual antennae proteins has been studied extensively, this is not the case at the scale of interstitial proteins. However, energy transfer over large nanometer distances depends on exchanges between these protein complexes. However, studying these exchanges is particularly challenging because of the number of proteins involved, their heterogeneous organization and their overlapping spectral characteristics. The result is a misunderstanding of how photosynthesis can be managed to achieve quantum efficiency close to unity—photon energy is thought to decrease over long distances and at reaction centers 25–200 nanometers apart (compared to antennas).

Researchers from the Massachusetts Institute of Technology (MIT) have proposed an explanation for this phenomenon, in a new study published in the journal. PNAS. By measuring for the first time the energy transferred between the antennae proteins, they discovered that their random arrangement greatly increases the efficiency of transferring this energy.

Gabriella Schlaw-Cohen, professor of chemistry at MIT and lead author of the new study, Explain that : ” For this antenna to work, power transmission over long distances is required. Our main finding is that perturbed regulation of light-harvesting proteins improves the efficiency of this long-range energy transfer. “.

Clutter enhances energy efficiency

In their study, Schlau Cohen and his team focused on photosynthetic violet bacteria, which are commonly used as models to study photosynthesis. Living in aquatic environments lacking in oxygen, they carry out anoxic photosynthesis through a single reaction center. These properties make them ideal for laboratory observations, not to mention their distinctive spectral properties.

In these bacteria, the collected photons pass through a network of antennae made up of proteins and pigments such as chlorophyll. Previously, scientists have used ultrafast spectroscopy to monitor how energy moves at each aerobic protein. With ultrafast laser pulses, this technology makes it possible to monitor events that occur on time scales ranging from femtoseconds to nanoseconds. However, monitoring this energy transfer at the level of proteins becomes more challenging, as it involves precise knowledge of the position of each complex.

In order to make it easier to monitor the energy transfer between the antennae proteins, the researchers designed nanometer-scale synthetic membranes, with a composition similar to that of bacteria. Specifically, purple bacteria have two types of antennae proteins, depending on the environment in which they live. Under normal light conditions, they express a protein called LH2 (which absorbs wavelengths from 800 to 850 nanometers), while a variant called LH3 is more expressed in low light conditions. In their study, the research team incorporated both versions of the protein into their nanodiscs.

By controlling the size of the membrane nanodisks, the distance between proteins can be accurately assessed. Through electron microscopy observations, the team was able to see that the light-harvesting proteins were spaced 2.5 to 3 nanometers apart — a distance roughly similar to that found in a normal bacterial membrane.

For the bound proteins, the researchers found that the movement of energy from one to the other takes about 6 picoseconds. On the other hand, this transfer time increases to 15 picoseconds between spaced ones. It has been found that as travel time is reduced, energy transfer becomes more efficient, as less energy is lost during transmission. ” When a photon is absorbed, there is only a short time left before the energy is lost through unwanted processes such as non-radiative decay. So the faster it converts, the more effective it is. says Shlaw Cohen.

By arranging the proteins in a lattice-like arrangement, the MIT team found that energy transfer was much less efficient. As a result, the random ordering observed in bacteria and most plant cells would make it possible to reach the famous quantum efficiency close to unity. This finding suggests that the heterogeneity that characterizes organisms in general could be an evolutionary advantage.

As a next step, the team plans to monitor the evolution of this energy transfer mechanism between antennae proteins and photosynthetic reaction center proteins. This phenomenon will also be studied in other, more complex photosynthetic organisms, in particular plants.

source : PNAS