Read Elizabeth L, Lee Hohjai, Fleming Graham R
Department of Chemistry, University of California-Berkeley, Berkeley, CA 94720, USA.
Photosynth Res. 2009 Aug-Sep;101(2-3):233-43. doi: 10.1007/s11120-009-9464-9. Epub 2009 Jul 10.
The broad linewidths in absorption spectra of photosynthetic complexes obscure information related to their structure and function. Photon echo techniques represent a powerful class of time-resolved electronic spectroscopy that allow researchers to probe the interactions normally hidden under broad linewidths with sufficient time resolution to follow the fastest energy transfer events in light harvesting. Here, we outline the technical approach and applications of two types of photon echo experiments: the photon echo peak shift and two-dimensional (2D) Fourier transform photon echo spectroscopy. We review several extensions of these techniques to photosynthetic complexes. Photon echo peak shift spectroscopy can be used to determine the strength of coupling between a pigment and its surrounding environment including neighboring pigments and to quantify timescales of energy transfer. Two-dimensional spectroscopy yields a frequency-resolved map of absorption and emission processes, allowing coupling interactions and energy transfer pathways to be viewed directly. Furthermore, 2D spectroscopy reveals structural information such as the relative orientations of coupled transitions. Both classes of experiments can be used to probe the quantum mechanical nature of photosynthetic light-harvesting: peak shift experiments allow quantification of correlated energetic fluctuations between pigments, while 2D techniques measure quantum beating directly, both of which indicate the extent of quantum coherence over multiple pigment sites in the protein complex. The mechanistic and structural information obtained by these techniques reveals valuable insights into the design principles of photosynthetic light-harvesting complexes, and a multitude of variations on the methods outlined here.
光合复合物吸收光谱中的宽线宽模糊了与其结构和功能相关的信息。光子回波技术是一类强大的时间分辨电子光谱技术，使研究人员能够以足够的时间分辨率探测通常隐藏在宽线宽之下的相互作用，以追踪光捕获中最快的能量转移事件。在这里，我们概述了两种类型的光子回波实验的技术方法和应用：光子回波峰移和二维（2D）傅里叶变换光子回波光谱。我们回顾了这些技术在光合复合物方面的几种扩展。光子回波峰移光谱可用于确定色素与其周围环境（包括相邻色素）之间的耦合强度，并量化能量转移的时间尺度。二维光谱产生吸收和发射过程的频率分辨图谱，使耦合相互作用和能量转移途径能够直接观察到。此外，二维光谱揭示了诸如耦合跃迁的相对取向等结构信息。这两类实验都可用于探测光合光捕获的量子力学性质：峰移实验允许量化色素之间相关的能量涨落，而二维技术直接测量量子拍频，这两者都表明了蛋白质复合物中多个色素位点上量子相干的程度。通过这些技术获得的机理和结构信息揭示了对光合光捕获复合物设计原理的宝贵见解，以及此处概述方法的多种变体。
