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Electron Spin Density Distribution in the Special Pair Triplet of <i>Rhodobacter sphaeroides</i> R26 Revealed by Magnetic Field Dependence of the Solid-State Photo-CIDNP Effect
Photo-CIDNP (photochemically induced dynamic nuclear
polarization)
can be observed in frozen and quinone-blocked photosynthetic reaction
centers (RCs) as modification of magic-angle spinning (MAS) NMR signal
intensity under illumination. Studying the carotenoidless mutant strain
R26 of <i>Rhodobacter sphaeroides</i>, we demonstrate by
experiment and theory that contributions to the nuclear spin polarization
from the three-spin mixing and differential decay mechanism can be
separated from polarization generated by the radical pair mechanism,
which is partially maintained due to differential relaxation (DR)
in the singlet and triplet branch. At a magnetic field of 1.4 T, the
latter contribution leads to dramatic signal enhancement of about
80 000 and dominates over the two other mechanisms. The DR
mechanism encodes information on the spin density distribution in
the donor triplet state. Relative peak intensities in the photo-CIDNP
spectra provide a critical test for triplet spin densities computed
for different model chemistries and conformations. The unpaired electrons
are distributed almost evenly over the two moieties of the special
pair of bacteriochlorophylls, with only slight excess in the L branch