Reaction centers of wild-type Rhodobacter sphaeroides were selectively (13)C-isotope labeled in bacteriochlorophyll and bacteriopheophytin. (13)C solid-state CP/MAS NMR and photo-CIDNP were used to provide insight into the electronic structure of the primary electron donor and acceptor on the atomic scale. The first 2-dimensional photochemically induced dynamic nuclear polarization (photo-CIDNP) (13)C-(13)C solid-state MAS NMR spectra reveal that negative charging of the two BChl rings of the primary donor is involved in ground-state tuning of the oxidation potential of these cofactors in the protein via local electrostatic interactions. In particular, the (13)C shifts show moderate differences in the electronic structure between the two BChl molecules of the special pair in the electronic ground state, which can be attributed to hydrogen bonding of one of the BChl molecules. The major fraction of the electron spin density is strongly delocalized over the two BChl molecules of the special pair and the photochemically active BPhe. A small fraction of the pi-spin density is distributed over a fourth component, which is assigned to the accessory BChl. Comparison of the photo-CIDNP data with "dark" NMR spectra obtained in ultra high field indicates a rigid special pair environment upon photoreaction and suggests that structural changes of the aromatic macrocycles of the two BChl molecules of the special pair do not significantly contribute to the reorganization energy associated with the charge-transfer process.
Photochemically induced dynamic nuclear polarisation
(photoCIDNP) in intact bacterial reaction centres has been observed by 13Csolid
state NMR under continuous illumination with white light. Strong intensity
enhancement of 13C NMR signals of the aromatic rings allows probing the
electronic ground state of the two BChl cofactors of the special pair at the
molecular scale with atomic selectivity. Differences between the two BChl
cofactors are discussed. Several aliphatic 13C atoms of cofactors, as well as
13C atoms of the imidazole ring of histidine residue(s), show nuclearspin
polarisation to the same extent as the aromatic nuclei of the cofactors.
Mechanisms and applications of polarisation transfer are discussed.
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