Photo-CIDNP C-13 magic angle spinning NMR on bacterial reaction centres: Exploring the electronic structure of the special pair and its surroundings

Photochemically induced dynamic nuclear polarisation (photo-CIDNP) in intac t bacterial reaction centres has been observed by C-13-solid state NMR unde r continuous illumination with white light. Strong intensity enhancement of C-13 NMR signals of the aromatic rings allows probing the electronic groun d state of the two BChl cofactors of the special pair at the molecular scal e with atomic selectivity, Differences between the two BChl cofactors are d iscussed. Several aliphatic C-13 atoms of cofactors, as well as C-13 atoms of the imidazole ring of histidine residue(s), show nuclear-spin polarisati on to the same extent as the aromatic nuclei of the cofactors. Mechanisms a nd applications of polarisation transfer are discussed.