Department of Advanced Interdisciplinary Studies, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo153-8904, Japan.
Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo113-8654, Japan.
J Phys Chem B. 2023 Jan 19;127(2):505-513. doi: 10.1021/acs.jpcb.2c07232. Epub 2023 Jan 6.
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.
硅藻可以利用蓝绿光区的光，因为它们在光捕获天线蛋白中有叶绿素（Chl）、岩藻黄素和叶绿素/结合蛋白（FCP）。Chl 在卟啉环上有一个可质子化的丙烯酰基（-CH ═ CH-COOH/COO）。由于 Chl 丙烯酰基的质子化会改变其吸收波长，因此 Chl 是硅藻光保护的候选组成部分，它根据光强度在光捕获和能量耗散模式之间切换 FCP 功能。在这里，我们使用量子力学/分子力学方法研究了由于丙烯酰基的质子化状态变化导致 Chl 吸收波长变化的机制。质子化 Chl 的 Soret 带的计算吸收波长比去质子化 Chl 长约 25nm，这是由于最低（LUMO）和第二低（LUMO+1）未占据分子轨道向丙烯酰基的离域。这些结果表明，在高光条件下，腔侧 pH 值降低会导致 Chl 质子化，从而导致吸收波长红移。
