Department of Physics and Astronomy, Aarhus University, Aarhus 8000C, Denmark.
Phys Chem Chem Phys. 2021 Dec 22;24(1):149-155. doi: 10.1039/d1cp04356k.
Excited-state dynamics plays a key role for light harvesting and energy transport in photosynthetic proteins but it is nontrivial to separate the intrinsic photophysics of the light-absorbers (chlorophylls) from interactions with the protein matrix. Here we study chlorophyll (4-coordinate complex) and axially ligated chlorophyll (5-coordinate complex) isolated applying mass spectrometry to shed light on the intrinsic dynamics in the absence of nearby chlorophylls, carotenoids, amino acids, and water molecules. The 4-coordinate complexes are tagged by quaternary ammonium ions while the charge is provided by a formate ligand in the case of 5-coordinate complexes. Regardless of excitation to the Soret band or the Q band, a fast ps decay is observed, which is ascribed to the decay of the lowest excited singlet state either by intersystem crossing (ISC) to nearby triplet states or by excited-state relaxation on the excited-state potential-energy surface. The lifetime of the first excited state is 15 ps with Mg at the chlorophyll center, but only 1.7 ps when formate is attached to Mg. When the Soret band is excited, an initial sup-ps relaxation is observed which is ascribed to fast internal conversion to the first excited state. With respect to ISC, two factors seem to play a role for the reduced lifetime of the formate-chlorophyll complex: (i) The Mg ion is pulled out of the porphyrin plane thus reducing the symmetry of the chromophore, and (ii) the first excited state (Q band) and T are tuned almost into resonance by the ligand, which increases the singlet-triplet mixing.
激发态动力学在光合作用蛋白的光捕获和能量传递中起着关键作用，但要将光吸收剂（叶绿素）的固有光物理与蛋白质基质的相互作用分开并不容易。在这里，我们通过质谱研究了叶绿素（四配位络合物）和轴向配位的叶绿素（五配位络合物），在没有附近的叶绿素、类胡萝卜素、氨基酸和水分子的情况下，研究其固有动力学。四配位络合物通过季铵离子标记，而五配位络合物的电荷由甲酸盐配体提供。无论激发到 Soret 带还是 Q 带，都观察到快速的 ps 衰减，这归因于最低激发单线态的衰减，要么通过系间窜越（ISC）到附近的三重态，要么通过在激发态势能面上的激发态松弛。以镁为中心的叶绿素的第一激发态寿命为 15 ps，但当甲酸盐与镁结合时，寿命仅为 1.7 ps。当 Soret 带被激发时，观察到初始超 ps 弛豫，这归因于快速的内转换到第一激发态。就 ISC 而言，有两个因素似乎对甲酸盐-叶绿素络合物的寿命缩短起作用：（i）镁离子被拉出卟啉平面，从而降低了发色团的对称性，（ii）配体将第一激发态（Q 带）和 T 调谐到几乎共振，从而增加了单线态-三线态混合。
