From whole cells towards photosynthetic reaction centres: "functional" and intrinsic dynamic properties.

The core of the reaction center protein is dominated by the D1/D2 heterodimer hosting all the redox cofactor involved in charge separation and electron transfer processes. The D1 protein is the subject of intense research being either the main actor in Photosystem II (PSII) assembly and repair cycle. Several studies demonstrated that even single point mutations in the D1 primary structure could affect PSII photochemistry and the physiological performance of the hosting organisms. Here, we address the question if there is a "functional" dynamics in addition to the intrinsic dynamical behaviour common to all proteins and how do they couple. In particular, understanding if "rigidity" is essential for the charge transfer process and if this property is shared by all the photosynthetic systems and how this information can be apply to design high performant bio-sensors. To this end a comparison between Chlamydomonas cells carrying both native and mutated D1 protein (hosted in the PSII of the cell) has been undertaken using neutron scattering experiment. Mutation were located in the functionally important regions D1 protein. All the mutants had a lower chlorophyll content indicating a possible modified antenna size. However, the mutation's type and localization impacted photosynthetic performance in a different manner. Mutants displayed reduced electron transport efficiency in physiological conditions, and increased photosynthetic performance stability and oxygen evolution capacity in stressful high-light conditions. Results show that point genetic mutations may notably affect not only the biochemical proterties but also the T dependence of the whole complex dynamics in particular suggesting the wild type more rigid than mutants. We highlight non negligible differences at longer time scale, rather than short, and large scale. We defined an intrinsic soft matter dynamics and a "functional dynamics". We bring to light for the first time that hydration water collective density fluctuations can provide also a measurement of "functional dynamics". Finally, delay and prompt fluorescence results also suggest a different behavior between mutants and wild type samples.