Theoretical Investigation of the Electronic Asymmetry of the Special Pair Cation Radical in the Photosynthetic Type-II Reaction Center

Analysis of the Electronic Structure of the Special Pair of a Bacterial Photosynthetic Reaction Center by 13C Photochemically Induced Dynamic Nuclear Polarization Magic‐Angle Spinning NMR Using a Double‐Quantum Axis

“…These patterns provide an experimental key to the supermolecular orbital structure. Until now, unfortunately, theoretical efforts mainly addressed the electronic structure of the radical‐cation state , probably due to a lack of empirical ground‐state data.…”

“…Hence, the ground‐state electronic symmetry is already broken . As reason for such symmetry break, not far‐reaching Coulomb effects, no difference on the acceptor site but local effects from side‐chain folding and assumable particular macrocycle foldings are responsible. On the primary electron acceptor cofactor, both upfield and downfield shifts of a negligible magnitude (∆ σ < 2.6) imply an electronic structure similar to that of a free bacteriopheophytin is acetone .…”

Bacteriopheophytin a in the Active Branch of the Reaction Center of Rhodobacter sphaeroides Is Not Disturbed by the Protein Matrix as Shown by 13C Photo-CIDNP MAS NMR

“…61−63 The similarity between the NMR shifts for the Φ A and monomeric BPhe a contrasts with the differences in chemical shifts for the two BChl a cofactors in the Special Pair (P L and P M ). 58 Since the Φ A is involved in the cascade of charge separation events in the RC and symmetry breaking is related to structural distortions in the ground state, 2,40,64,65 the data contribute to converging evidence that the primary source of the symmetry breaking is in the deformation of P, whereas the extent of protein induced deformation of the BPhe a molecule appears to be much less. Apparently there is very little tuning of the BPhe of the active branch by its surroundings and it represents an electron sink due to its redox potential that is different from P. A similar conclusion has been made for the next cofactor in the electron conversion chain, the quinone acceptor Q A in RCs, which is thought to remain in the same orientation upon illumination of the RC with light.…”

“…Recently, MAS NMR in the dark and with photo-CIDNP enhancement has provided a detailed view, with atomic selectivity, of P in its ground state. The analysis of 13 C shifts for the carbons in the aromatic ring has revealed packing effects on the two BChl a cofactors of P, induced by the folding and self-assembly of the RC complex. , There is converging evidence that the packing leads to electronic asymmetry of the form P L δ− P M ε– between the two BChl a of the P, due to variation of local side chain interactions, protein-induced macrocycle distortion, and an asymmetric electronic environment. ,− In addition to this time-averaged asymmetric electronic offset, a coupling with two collective low-frequency vibrational modes is thought to lead to dynamics of the asymmetry with time-dependent localization of the highest occupied orbital and variation of the orbital energy levels …”

Study of Electronic Structures and Pigment–Protein Interactions in the Reaction Center of Thermochromatium tepidum with a Dynamic Environment

“…Tremendous experimental 100−113 and theoretical 74,97,114−127 efforts have been devoted to constructing the electronic structures of P and to explaining the directionality of CS and ET. Asymmetric charge density distributions on the two halves of P have been revealed as an intrinsic property of P, 117,119 and observed in the ground and the excited state of P 74,[100][101][102][103][104][105][106][107][108][109][110][111][115][116][117][118][119][120][121]125 as well as in the cation radical of P. 112,113,116,117 The symmetry breaking of the charge density distribution in P L and P M could result from various factors, e.g., different orientations of the side groups of two pigments 105,116,117,119 and tuning effects of the surrounding protein residues. 103,104,115,117,120 For the RCs from Rb.…”