Department of Physics and Astronomy, Aarhus University, Denmark.
Phys Chem Chem Phys. 2020 Sep 23;22(36):20331-20336. doi: 10.1039/d0cp03210g.
Chlorophyll (Chl) pigments are responsible for vital mechanisms in photosynthetic proteins: light harvesting, energy transfer and charge separation. A complex interplay between the Chl molecule and its microenvironment determines its transition energy. Interactions such as excitonic coupling with one or more pigments (Chls or carotenoids), axial ligation to the magnesium center, or electrostatic interactions between Chl and nearby amino-acid residues all influence the photophysical properties. Here we use time-resolved photodissociation action spectroscopy to determine transition energies of Chla/b complexes in vacuo to directly compare the impact of a negatively charged axial ligand (formate) to that of exciton coupling between two Chls. Experiments carried out at the electrostatic ion storage ring ELISA allow dissociation to be sampled on hundreds of milliseconds time scale. Absorption-band maxima of Chla-formate complexes are found at 433 ± 4 nm/2.86 ± 0.03 eV (Soret band) and in the region 654-675 nm/1.84-1.90 eV (Q band) and those of Chla dimers tagged by a quaternary ammonium ion at 419 ± 5 nm/2.96 ± 0.04 eV (Soret band) and 647 nm/1.92 eV (Q band). The axial ligand strongly affects the Chla transition energies causing redshifts of 0.21 eV of the Soret band and 0.04-0.1 eV of the Q band compared to Chla tagged by a quaternary ammonium. Slightly smaller shifts were found in case of Chlb. The redshifts are approximately twice that induced by excitonic coupling between two Chlas, also tagged by a quaternary ammonium ion. Axial ligation brings the absorption by isolated Chls very close to that of photosynthetic proteins.
叶绿素（Chl）色素负责光合作用蛋白中的重要机制：光捕获、能量转移和电荷分离。Chl 分子与其微环境之间的复杂相互作用决定了其跃迁能量。与一个或多个色素（Chl 或类胡萝卜素）的激子耦合、与镁中心的轴向配位、或 Chl 与附近氨基酸残基之间的静电相互作用等相互作用都会影响光物理性质。在这里，我们使用时间分辨光解作用光谱法来确定真空中 Chla/b 复合物的跃迁能量，以直接比较带负电荷的轴向配体（甲酸盐）与两个 Chl 之间的激子耦合的影响。在静电离子存储环 ELISA 上进行的实验允许在数百毫秒的时间尺度上采样离解。Chla-甲酸盐复合物的吸收带最大值位于 433 ± 4nm/2.86 ± 0.03eV（Soret 带）和 654-675nm/1.84-1.90eV（Q 带），而被季铵离子标记的 Chla 二聚体的吸收带最大值位于 419 ± 5nm/2.96 ± 0.04eV（Soret 带）和 647nm/1.92eV（Q 带）。轴向配体强烈影响 Chla 的跃迁能量，导致 Soret 带红移 0.21eV，Q 带红移 0.04-0.1eV，与被季铵离子标记的 Chla 相比。Chlb 的位移略小。这种红移大约是由两个 Chl 之间的激子耦合引起的两倍，这两个 Chl 也被季铵离子标记。轴向配位使孤立 Chl 的吸收非常接近光合作用蛋白的吸收。
