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

Diatoms can use light in the blue-green region because they have chlorophyll (Chl) in light-harvesting antenna proteins, fucoxanthin and chlorophyll /-binding protein (FCP). Chl has a protonatable acrylate group (-CH═CH-COOH/COO) conjugated to the porphyrin ring. As the absorption wavelength of Chl changes upon the protonation of the acrylate group, Chl 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 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 is ∼25 nm longer than that of deprotonated Chl, 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 and thereby a red shift in the absorption wavelength.