Hughes Joseph L, Picorel Rafael, Seibert Michael, Krausz Elmars
Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia.
Biochemistry. 2006 Oct 10;45(40):12345-57. doi: 10.1021/bi0614683.
We have employed absorption, circular dichroism (CD), and persistent spectral hole-burning measurements at 1.7 K to study the photoconversion properties and exciton coupling of low-energy chlorophylls (Chls) in the CP43 proximal antenna light-harvesting subunit of photosystem II (PSII) isolated from spinach. These approximately 683 nm states act as traps for excitation energy in isolated CP43. They "bleach" at 683 nm upon illumination and photoconvert to a form absorbing in the range approximately 660-680 nm. We present new data that show the changes in the CD spectrum due to the photoconversion process. These changes occur in parallel with those in absorption, providing evidence that the feature undergoing the apparent bleach is a component of a weakly exciton-coupled system. From our photoconversion difference spectra, we assign four states in the Chl long-wavelength region of CP43, two of which are the known trap states and are both highly localized on single Chls. The other two states are associated with weak exciton coupling (maximally approximately 50 cm(-)(1)) to one of these traps. We propose a mechanism for photoconversion that involves Chl-protein hydrogen bonding. New hole-burning data are presented that indicate this mechanism is distinct to that for narrow-band spectral hole burning in CP43. We discuss the photophysical behavior of the Chl trap states in isolated CP43 compared to their behavior in intact PSII preparations. The latter represent a more intact, physiological complex, and we find no clear evidence that they exhibit the photoconversion process reported here.
我们利用1.7K下的吸收光谱、圆二色性（CD）光谱和持久光谱烧孔测量技术，研究了从菠菜中分离出的光系统II（PSII）的CP43近端天线光捕获亚基中低能叶绿素（Chls）的光转换特性和激子耦合。这些约683nm的状态在分离的CP43中充当激发能的陷阱。光照下它们在683nm处“漂白”，并光转换为在约660 - 680nm范围内吸收的形式。我们展示了新的数据，这些数据显示了由于光转换过程导致的CD光谱变化。这些变化与吸收光谱的变化同时发生，这表明经历明显漂白的特征是弱激子耦合系统的一个组成部分。从我们的光转换差光谱中，我们在CP43的Chl长波长区域确定了四个状态，其中两个是已知的陷阱状态，且都高度定域在单个Chls上。另外两个状态与其中一个陷阱存在弱激子耦合（最大约50cm⁻¹）。我们提出了一种涉及Chl - 蛋白质氢键的光转换机制。给出的新的光谱烧孔数据表明该机制与CP43中的窄带光谱烧孔机制不同。我们讨论了分离的CP43中Chl陷阱状态的光物理行为，并将其与它们在完整PSII制剂中的行为进行比较。后者代表了更完整的生理复合物，我们没有发现明确的证据表明它们表现出这里报道的光转换过程。
