45518-G4
Two-Dimensional Fourier Transform Spectroscopy of Resonance Energy Transfer in Biological Molecules
How biological molecules harness and control light energy is fundamental to life. In photosynthesis, elaborate antenna arrays gather solar energy and transfer it to photochemical reaction centers with 95% efficiency. Through cleverly selecting the molecular components and manipulating their relative structural arrangements and physical environments, nature has designed robust multichromophoric structures that achieve diverse goals. An essential part of understanding how these goals are met is making the connection between structure and dynamics. While X-ray structures reveal the spatial configuration of biomolecules, static structure cannot capture the dynamical changes in nuclear configurations and electronic coupling that govern energy transfer. Recent demonstrations of two-dimensional electronic spectroscopy (2DES) at visible wavelengths, suggest that 2DES can provide a detailed picture of energy transport, revealing electronic coupling mechanisms on the relevant ultrafast (femtosecond-picosecond) timescales.
Most nonlinear spectroscopies that have been used to study energy transfer are inherently one dimensional and as such cannot determine whether features in the linear absorption spectrum arise from electronic coupling between chromophores, or from inhomogeneous effects such as chromophore-protein interactions. While single molecule experiments have been essential in characterizing inhomogeneity, they lack the time resolution necessary to follow ultrafast processes such as energy transfer. The techniques of multidimensional Fourier transform spectroscopy have revolutionized nuclear magnetic resonance (NMR), making it an invaluable tool for determining high resolution structures of complicated biomolecules. NMR can also study dynamics, but on limited time scales. Extending the techniques of multidimensional NMR into the infrared regime, one can now monitor the time-evolving structures of small peptides with unprecedented time resolution. We have been pursuing this goal at optical frequencies to examine energy and charge transfer in photosynthesis with both high time resolution and the ability to discern homogeneous and inhomogeneous contributions. In the past funding year we have met several primary goals towards applying 2DES to the study of light-harvesting systems: 1) we have developed a simplified 2DES experiment based on pulse-shaping technology, 2) we have obtained data on several simple dye systems and have started to model the data in preparation for more complex modeling of the light-harvesting systems 3) we have obtained natural photosynthetic samples.
Despite the rich chemical information available from 2DES, the relative difficulty of implementing the experiment has limited the degree to which this method has been utilized. We recently implemented a method that employs an acousto-optic pulse-shaper used in a pump-probe geometry to obtain absorptive spectra in a simple dye system. This approach takes advantage of the high degree of precision with which a pulse-shaper can create phase-locked pulse pairs and automatically retrieves absorptive 2D spectra. One significant advantage of the pulse-shaping method is that it allows automatic determination of the absorptive spectra, without the need for a phasing procedure. This approach is experimentally simpler to implement and may increase the access of the 2D method to many other groups. We have recently written up these results, which are the first to demonstrate 2D spectroscopy with a pulse-shaper at visible wavelengths and in a 2-color experiment. More recently we have extended our 2D method to use a continuum probe source. This will allow us to probe a broader range of electronic transitions. We have demonstrated the method on a dye system, where we have been able to observe the dynamics of intramolecular modes of the dye molecule. We also studied a simple energy transfer system consisting of a donor and acceptor pair attached to a rigid DNA spacer. This system was designed to allow us to observe resonant energy transfer. We have also been working to model this data, and are currently writing up the results for publication.
In the past year we have worked in collaboration with Charles Yocum in the Department of Molecular, Cellular and Developmental Biology at the University of Michigan to isolate natural photosynthetic complexes of interest. We have isolated reaction center complexes from Photosystem II (PSII) from spinach. Our goal is to learn about the energy and charge transfer pathways in PSII, which we will study as a function of temperature. We have purchased a cryostat that will allow us to study PSII over a 77K-293K temperature range. Leading up to the low temperature PSII experiments, we are studying J-aggregates that exhibit energy transfer dynamics on similar timescales to PSII. We are currently working on models to successfully reproduce the J-aggregate 2D spectra. These models will be extended to allow us to describe the more complex PSII system.
