Department of Chemistry, The University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, USA.
J Phys Chem A. 2011 Mar 31;115(12):2471-82. doi: 10.1021/jp111720a. Epub 2011 Mar 7.
Energy transport mechanisms in R-Phycoerythrin (RPE), a light harvesting protein located at the top of the phycobilisome antenna in red algae, are investigated using nonlinear optical spectroscopies and theoretical models. The RPE hexamer possesses a total of 30 bilin pigments, which can be subdivided into three classes based on their molecular structures and electronic resonance frequencies. Of particular interest to this study is the influence of exciton delocalization on the real-space paths traversed by photoexcitations as they concentrate on the lowest energy pigment sites. Transient grating measurements show that significant nuclear relaxation occurs at delay times less than 100 fs, whereas energy transport spans a wide range of time scales depending on the proximity of the initial and final states involved in the process. The fastest energy transport dynamics within the RPE complex are close to 1 ps; however, evidence for sub-100 fs exciton self-trapping is also obtained. In addition, photon echo experiments reveal vibronic interactions with overdamped and underdamped nuclear modes. To establish signatures of exciton delocalization, energy transport is simulated using both modified Redfield and Förster theories, which respectively employ delocalized and localized basis states. We conclude that exciton delocalization occurs between six pairs of phycoerythrobilin pigments (i.e., dimers) within the protein hexamer. It is interesting that these dimers are bound in locations analogous to the well-studied phycocyanobilin dimers of cyanobacterial allophycocyanin and c-phycocyanin in which wave function delocalization is also known to take hold. Strong conclusions regarding the electronic structures of the remaining pigments cannot be drawn based on the present experiments and simulations due to overlapping resonances and broad spectroscopic line widths, which prevent the resolution of dynamics at particular pigment sites.
使用非线性光学光谱学和理论模型研究了位于红藻藻胆体天线顶端的光捕获蛋白 R-藻红蛋白（RPE）中的能量传递机制。RPE 六聚体总共拥有 30 个 biliverdin 色素，根据其分子结构和电子共振频率可将其细分为三类。特别值得关注的是激子离域对光激发在集中于最低能量色素位置时所经历的实空间路径的影响。瞬态光栅测量表明，在小于 100 fs 的延迟时间内会发生明显的核弛豫，而能量传递则跨越广泛的时间尺度，具体取决于参与该过程的初始和最终状态的接近程度。RPE 复合物中最快的能量传递动力学接近 1 ps；然而，也获得了亚 100 fs 激子自捕获的证据。此外，光子回波实验揭示了与过阻尼和欠阻尼核模式的振动态相互作用。为了确定激子离域的特征，使用修改后的 Redfield 和 Förster 理论分别采用离域和局域基态对能量传递进行了模拟。我们得出结论，激子离域发生在蛋白六聚体中的六个藻红胆素对（即二聚体）之间。有趣的是，这些二聚体在类似于已研究充分的蓝藻藻蓝蛋白和 c-藻蓝蛋白中的藻蓝胆素二聚体的位置结合，在这些位置中，波函数离域也被认为存在。由于重叠的共振和较宽的光谱线宽，无法在特定色素位置解析动态，因此无法根据目前的实验和模拟得出关于其余色素的电子结构的强烈结论。
