Photochemically induced dynamic nuclear polarization in bacterial photosynthetic reaction centres observed by ¹³C solid-state NMR

Abstract

Photosynthesis, the synthesis of organic compounds upon utilization of light energy, is one of the key reactions for life on earth. The first step of this complex process, a light induced electron transfer, occurs in photosynthetic reaction centre (RC) membrane proteins. The electron is emitted from a (bacterio)chlorophyll (BCh1) aggregate in its electronically excited state. In bacterial RCs, this aggregate is a strongly coupled BChl a dimer, the so-called “special pair” (P). When photoexcited to a higher electronic singlet state, P transfers an electron to the primary acceptor, a pheophytin molecule, within 3 ps. The electron is then transferred to Q_A in about 200 ps. Subsequently, in a much slower reaction taking about 100 µs, the electron is transferred to the ultimate electron acceptor Q_B. This light-induced electron transfer sequence is repeated after the special pair has been re-reduced by a cytochrome. Although the spatial structure and kinetics of several RCs are known to atomic resolution, there is no clear understanding of the process of electron emission from the electronically excited primary electron donor. In addition, a detailed picture of the molecular mechanism of the inhibition of the back reaction, which is probably due to the high exothermic reaction enthalpy pushing the system into the inverted Marcus region, is missing. The functionally crucial electronic structure can be probed by spectroscopic methods. Grosso modo, vibrational spectroscopy provides information about electron densities between nuclei, and NMR spectroscopy at the nuclei.