Action Spectroscopy on Dense Samples of Photosynthetic Reaction Centers of Rhodobacter sphaeroides WT Based on Nanosecond Laser-Flash 13C Photo-CIDNP MAS NMR

Abstract

Photochemically induced dynamic nuclear polarization magic-angle spinning nuclear magnetic resonance (photo-CIDNP MAS NMR) allows for the investigation of the electronic structure of the photochemical machinery of photosynthetic reaction centers (RCs) at atomic resolution. For such experiments, either continuous radiation from white xenon lamps or green laser pulses are applied to optically dense samples. In order to explore their optical properties, optically thick samples of isolated and quinone-removed RCs of the purple bacteria of Rhodobacter sphaeroides wild type are studied by nanosecond laser-flash 13C photo-CIDNP MAS NMR using excitation wavelengths between 720 and 940 nm. Action spectra of both the transient nuclear polarization as well as the nuclear hyperpolarization, remaining in the electronic ground state at the end of the photocycle, are obtained. It is shown that the signal intensity is limited by the amount of accessible RCs and that the different mechanisms of the photo-CIDNP production rely on the same photophysical origin, which is the photocycle induced by one single photon.

Photo-CIDNP in the Reaction Center of the Diatom Cyclotella meneghiniana Observed by 13C MAS NMR

“…[3][4] The underlying spin-dynamics (Scheme 1) of the well-known radical pair mechanisms (RPM) [5][6] [7] leads to transient light-induced nuclear polarization which can be observed in time-resolved experiments. [8] Under continuous illumination, up to three spin-chemical mechanisms running in parallel have been revealed contributing to the photo-chemically induced dynamic polarization (photo-CIDNP) in the solid state in photosynthetic RCs: the three spin mixing (TSM), the differential decay (DD) and the differential relaxation (DR) mechanisms. [9][10] [11] [12] Photosynthesis is a photochemical process that is initiated by the light-induced electron transfer in a photosynthetic reaction center (RC).…”