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Abstract:
Photochemically induced dynamic nuclear polarisation has been observed from entire photosynthetic units (PSUs) bound to chromatophore membrane (membrane-bound PSU) of the purple bacteria Rhodobacter sphaeroides, which have been selectively 13C-isotope enriched at all BChl and BPheo cofactors. These 1.5 MDa membrane-bound protein complexes comprise reaction centres as well as the antenna systems called light harvesting complexes I and II. Due to light-induced enhancement of nuclear polarisation, the 13C magic-angle spinning (MAS) NMR spectrum shows absorptive lines originating from the cofactors involved into the photochemical machinery and allowing the determination of the electronic ground state structure at atomic resolution. Addition of detergent released intact PSU from the chromatophore membrane (so called detergent solubilised PSU) and caused significant changes in the sign and intensity pattern of the light-induced MAS NMR spectrum. In contrast, detergent solubilised PSU and detergent solubilised bacterial reaction centres with the same isotope label pattern exhibit essentially the same chemical shifts with only minor differences in the intensity pattern. The pronounced differences between intact membrane-bound and detergent solubilised PSU are tentatively explained by the loss of self-orientation of the membrane-bound samples by solubilisation. This interpretation suggests that the theoretically predicted anisotropy of the light-induced nuclear polarisation has been observed for the first time.
