Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

Andrew Niedringhaus , Veronica R. Policht , Riley Sechrist , Arkaprabha Konar , Philip D. Laible , David F. Bocian, Dewey Holten , Christine Kirmaier , and Jennifer P. Ogilvie

Screen Shot 2020-08-09 at 8.49.43 PM.png

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an impor- tant model system for probing the structure–function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial re- action centers (BRCs) from two mutants of the purple bacterium Rho- dobacter capsulatus, spanning the Qy absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two- step sequential reaction scheme in which charge separation proceeds +− bacteriochlorophyll BA*