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

Abstract

Recently, solid-state NMR spectroscopy became a viable method to investigate photosynthetic reaction centres (RCs) on t e atomic level. To study the electronic structure of the radical cation state 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 standard Bruker wide-bore MAS NMR probe. In addition we demonstrate its application to get information from the active site in photosynthetic reaction centres of <i>Rhodobacter spaeroides </i>R-26 by photochemically induced dynamic nuclear polarization (photo-CIDNP). Solid-state NMR spectra of natural abundance ¹³C in detergent solubilized quinone depleted photosynthetic reaction centres under continuous illumination showed exceptionally strong nuclear spin polarization in NMR lines. Both enhanced-absorptive and emissive polarization were seen in the carbon spectrum which could be assigned to a bacteriochlorophyll <i>a </i>(BChl <i>a) </i>cofactor, presumably the special pair BChl <i>a. </i>The sign and intensities of the ¹³C NMR signals provide information about the electron spin density distribution of the transiently formed radical P<b style="mso-bidi-font-weight:normal">^.</b>⁺on the atomic level.