A set-up to study photochemically induced dynamic nuclear polarization in photosynthetic reaction centres by solid-state NMR

Recently, solid-state NMR spectroscopy became a viable method to investigat e photosynthetic reaction centres (RCs) on the atomic level. To study the e lectronic structure of the radical cation stare of the RC, occurring after the electron emission, solid-state NMR using an illumination set-up can be exploited. This paper describes the illumination set-up we designed for a s tandard Bruker wide-bore MAS NMR probe. In addition we demonstrate its appl ication to get information from the active site in photosynthetic reaction centres of Rhodobacter sphaeroides R-26 by photochemically induced dynamic nuclear polarization (photo-CIDNP). Solid-state NMR spectra of natural abun dance C-13 in detergent solubilized quinone depleted photosynthetic reactio n centres under continuous illumination showed exceptionally strong nuclear spin polarization in NMR lines. Both enhanced-absorptive and emissive pola rization were seen in the carbon spectrum which could be assigned to a bact eriochlorophyll a (BChl a) cofactor, presumably the special pair BChl a. Th e sign and intensities of the C-13 NMR signals provide information about th e electron spin density distribution of the transiently formed radical P.on the atomic level.