Photosynthesis begins with the absorption of light energy by the chlorophyll pigments of the light-harvesting antennae (Light Harvesting Complexes – LHC – composed of proteins and chlorophyll and carotenoid pigments). This absorption creates excitation energy (change from a ground electronic state to an excited state of the collecting chlorophyll), which is transferred from one chlorophyll to the next to the photosynthetic reaction centre where it is converted into chemical potential energy (by charge separation). This energy conversion process is extremely efficient. So efficient, in fact, that it can cause a potentially deleterious over-excitation of the system. The plant sets up mechanisms to protect itself against this: from the macroscopic scale, through the movement of its leaves, to the molecular scale, through a mechanism that allows the dissipation of energy as heat. This last mechanism, which is multifactorial, is known as the non-photochemical quenching of chlorophyll fluorescence. 

Whether in vivo or in vitro, the Laboratory of Bioenergetics, Metalloproteins and Stress (I2BC department), led by Bruno Robert, has shown that this quenching is linked to a rearrangement of the proteins and pigments that build the LHC, which creates energy traps. Excited chlorophyll pigments transfer their energy to carotenoid pigments, which immediately dissipate it as heat. In the case of LHCII, the main collecting antenna of higher plants, in vitro spectroscopy experiments conducted on the aggregated complex (in the absence of the detergent conventionally used to solubilise it) establish that this transfer occurs between chlorophyll a and a lutein (a carotenoid). The extent of the quenching appears to be correlated with conformational changes (torsion) affecting lutein and another carotenoid, neoxanthin. 

To further describe and understand these changes, Bruno Robert's team studied the structure of LHCII in different environments that influence its electronic properties. Remarkably, the team succeeded in isolating for the first time a state of LHCII, obtained using the detergent n-dodecyl-α-D-maltoside, and in characterising the spectroscopic properties of its pigments. Their study, published in JBC, shows that, in this state, all the changes associated with non-photochemical quenching (changes in protein-chlorophyll interactions, neoxanthin torsion) are present except for lutein torsion, while no quenching is associated with this state. This state of LHCII should be considered as an intermediate state that would allow the transition from an unquenched state, capable of absorbing and converting light energy into chemical energy, to a "quenched" state through non-photochemical quenching. The neoxanthine torsion would be an indicator of large-scale conformational changes in LHCII that would precede smaller-scale changes directly responsible for quenching and revealed by the lutein torsion. This unquenched LHCII intermediate, described here for the first time, provides insight into the molecular mechanism of quenching.