Understanding natural light-harvesting systems is fundamental for the design of high-efficiency materials for solar energy capture and storage. Excess light conditions are potentially harmful for photosynthetic organisms. Consequently, they have developed strategies to dissipate energy and prevent damage. The Orange Carotenoid Protein (OCP) is a two-domain soluble protein involved in photoprotective processes in cyanobacteria via nonphotochemical quenching (NPQ). Upon photoactivation by blue-green light absorption, OCP undergoes profound conformational changes including a 12 Å displacement of the ketocarotenoid inside the cavity, followed by domain separation. Major aspects of this mechanism are still elusive. By using a combination of molecular dynamics, metadynamics, and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations, we were able to mimic the translocation of the carotenoid starting from the inactive OCPO and obtaining metastable red-shifted states in the photoactivation mechanism, replicating the λmax values of reference experimental spectra. A model of an intermediate photoproduct is the starting point to elucidate the subsequent rearrangements of the protein that lead to the active form, OCPR.

Carotenoid-protein dynamics in photosynthetic organisms: The case of OCP

ACS Fall 2021

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