Mechanism of Absorption Wavelength Shift Depending on the Protonation State of the Acrylate Group in Chlorophyll c .

Diatoms can use light in the blue-green region because they have chlorophyll c (Chl c ) in light-harvesting antenna proteins, fucoxanthin and chlorophyll a / c -binding protein (FCP). Chl c has a protonatable acrylate group (-CH═CH-COOH/COO - ) conjugated to the porphyrin ring. As the absorption wavelength of Chl c changes upon the protonation of the acrylate group, Chl c is a candidate component that is responsible for photoprotection in diatoms, which switches the FCP function between light-harvesting and energy-dissipation modes depending on the light intensity. Here, we investigate the mechanism by which the absorption wavelength of Chl c changes owing to the change in the protonation state of the acrylate group, using a quantum mechanical/molecular mechanical approach. The calculated absorption wavelength of the Soret band of protonated Chl c is ∼25 nm longer than that of deprotonated Chl c , which is due to the delocalization of the lowest (LUMO) and second lowest (LUMO+1) unoccupied molecular orbitals toward the acrylate group. These results suggest that in FCP, the decrease in pH on the lumenal side under high-light conditions leads to protonation of Chl c and thereby a red shift in the absorption wavelength.