HUN-REN Biological Research Centre, Temesvári krt. 62, 6726 Szeged, Hungary.
Department of Physics and Astronomy and LaserLaB, Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
Int J Mol Sci. 2024 Nov 19;25(22):12430. doi: 10.3390/ijms252212430.
Photosystem I in most organisms contains long-wavelength or "Red" chlorophylls (Chls) absorbing light beyond 700 nm. At cryogenic temperatures, the Red Chls become quasi-traps for excitations as uphill energy transfer is blocked. One pathway for de-excitation of the Red Chls is via transfer to the oxidized RC (P700), which has broad absorption in the near-infrared region. This study investigates the excitation dynamics of Red Chls in Photosystem I from the cyanobacterium at cryogenic temperatures (77 K) and examines the role of the oxidized RC in modulating their fluorescence kinetics. Using time-resolved fluorescence spectroscopy, the kinetics of Red Chls were recorded for samples with open (neutral P700) and closed (P700) RCs. We found that emission lifetimes in the range of 710-720 nm remained unaffected by the RC state, while more red-shifted emissions (>730 nm) decayed significantly faster when the RC was closed. A kinetic model describing the quenching by the oxidized RC was constructed based on simultaneous fitting to the recorded fluorescence emission in Photosystem I with open and closed RCs. The analysis resolved multiple Red Chl forms and variable quenching efficiencies correlated with their spectral properties. Only the most red-shifted Chls, with emission beyond 730 nm, are efficiently quenched by P700, with rate constants of up to 6 ns. The modeling results support the notion that structural and energetic disorder in Photosystem I can have a comparable or larger effect on the excitation dynamics than the geometric arrangement of Chls.
在大多数生物体中，光系统 I 含有长波长或“红”叶绿素（Chls），可吸收超过 700nm 的光。在低温下，由于上坡能量转移受阻，红 Chls 成为激发态的准陷阱。红 Chls 去激发的一种途径是通过转移到氧化的 RC（P700），P700 在近红外区域具有广泛的吸收。本研究在低温（77 K）下研究了蓝藻光系统 I 中红 Chls 的激发动力学，并考察了氧化 RC 对其荧光动力学的调制作用。通过时间分辨荧光光谱法，记录了开放（中性 P700）和关闭（P700）RC 条件下样品中红 Chls 的动力学。我们发现，710-720nm 范围内的发射寿命不受 RC 状态的影响，而当 RC 关闭时，更红移的发射（>730nm）则显著更快地衰减。基于同时拟合光系统 I 中开放和关闭 RC 记录的荧光发射，构建了描述氧化 RC 猝灭的动力学模型。该分析确定了多个红 Chl 形式和可变猝灭效率，它们与光谱特性相关。只有发射超过 730nm 的最红移 Chls 可被 P700 有效猝灭，其速率常数高达 6ns。建模结果支持这样一种观点，即光系统 I 中的结构和能量无序可能对激发动力学的影响与 Chls 的几何排列相当，甚至更大。
