Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
J Phys Chem B. 2009 Dec 3;113(48):15771-82. doi: 10.1021/jp908093x.
The electronic structure and photoinduced relaxation dynamics of the cyanobacterial light harvesting protein, C-Phycocyanin (CPC), are examined using transient grating and two-dimensional (2D) photon echo spectroscopies possessing sub-20 fs time resolution. In combination with linear absorption and fluorescence measurements, these time-resolved experiments are used to constrain the parameters of a Frenkel exciton Hamiltonian. Particular emphasis is placed on elucidating the nature of excited states involving the alpha84 and beta84 phycocyanobilin pigment dimers of CPC. This paper obtains new experimental evidence suggesting that electronic relaxation proceeds by way of incoherent energy transfer between the alpha84 and beta84 pigment sites (i.e., the weak coupling limit of energy transfer). Transient absorption anisotropies simulated in the weak coupling limit agree well with measurements, whereas signals computed in an exciton basis possess short-lived (electronic) coherent components not present in the experimental data. In addition, 2D photon echo spectra for CPC show no sign of the interfering nonlinearities predicted by a theoretical model to be characteristic of exciton formation. Another important new observation is that the sub-100 fs dynamics in the transient absorption anisotropy are dominated by an impulsively excited hydrogen out-of-plane wagging mode similar to those observed in phytochrome and retinal. Detection of this 795 cm(-1) coherence is of particular interest because our recent study of a closely related protein, Allophycocyanin (APC), assigns a similar coordinate as a promoting mode enabling ultrafast internal conversion. Together, the experiments conducted for APC and CPC suggest that interactions between the pigments and environment are the key to understanding why electronic relaxation in CPC is more than three times slower than APC despite the nearly identical geometries of the pigment dimers. Most important in reaching this conclusion is the present finding that relaxation of the 2D photon echo line shapes of CPC is approximately two times faster than that measured for APC. Overall, the present results underscore the ability of phycobiliproteins to control light harvesting dynamics through solvation and variation in the conformations of open-chain tetrapyrrole chromophores.
藻蓝蛋白（CPC）的电子结构和光诱导弛豫动力学通过具有亚 20 fs 时间分辨率的瞬态光栅和二维（2D）光子回波光谱来研究。结合线性吸收和荧光测量，这些时间分辨实验用于约束弗伦克尔激子哈密顿量的参数。特别强调的是阐明涉及 CPC 的 alpha84 和 beta84 藻蓝胆素二聚体的激发态的性质。本文获得了新的实验证据，表明电子弛豫是通过 alpha84 和 beta84 色素位点之间的非相干能量转移进行的（即能量转移的弱耦合极限）。在弱耦合极限下模拟的瞬态吸收各向异性与测量结果吻合得很好，而在激子基下计算的信号则具有实验数据中不存在的短寿命（电子）相干分量。此外，CPC 的 2D 光子回波光谱没有表现出理论模型预测的与激子形成特征的干扰非线性的迹象。另一个重要的新观察结果是，瞬态吸收各向异性中小于 100 fs 的动力学由类似于在光敏色素和视网膜中观察到的瞬态激发的氢面外摇摆模式主导。检测到这个 795 cm(-1) 的相干性特别有趣，因为我们最近对一种密切相关的蛋白质，别藻蓝蛋白（APC）的研究将类似的坐标指定为促进模式，使超快内部转换成为可能。总的来说，对于 APC 和 CPC 进行的实验表明，色素和环境之间的相互作用是理解为什么尽管 CPC 的色素二聚体的几何形状几乎相同，但电子弛豫速度比 APC 慢三倍以上的关键。对于得出这个结论最重要的是，目前的发现是，CPC 的 2D 光子回波线形状的弛豫速度比 APC 测量的速度快大约两倍。总的来说，目前的结果强调了藻胆蛋白通过溶剂化和开链四吡咯发色团构象的变化来控制光捕获动力学的能力。
