13C MAS NMR and Photo-CIDNP Reveal a Pronounced Asymmetry in the Electronic Ground State of the Special Pair of Rhodobacter sphaeroides Reaction Centers

• Published in: Biochemistry (Easton) 2002-07, Vol.41 (27), p.8708-8717
• Main Authors: Schulten, Els A. M, Matysik, Jörg, Alia, Kiihne, Suzanne, Raap, Jan, Lugtenburg, Johan, Gast, Peter, Hoff, Arnold J, de Groot, Huub J. M
• Format: Article
• Language: English
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi025608u

Reaction centers of wild-type Rhodobacter sphaeroides were selectively 13C-isotope labeled in bacteriochlorophyll and bacteriopheophytin. 13C 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) 13C−13C 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 13C 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 π-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.