Lubitz, Wolfgang; Lendzian, F.; Scheer, Hugo; Gottstein, J.; Plato, M.; Möbius, K.
Structural studies of the primary donor cation radical P-870+ in reaction centers of Rhodospirillum rubrum by electron-nuclear double resonance in solution.
In: Proceedings of the National Academy of Science of the USA, Vol. 81: S. 1401-1405
The light-induced cation radical of the primary
electron donor, Ps70, in photosynthetic reaction centers
from Rhodospirillum rubrum G-9, has been investigated by
electron-nuclear double resonance (ENDOR) in liquid aqueous
solution. The measured hyperfine coupling constants are
assigned to specific molecular positions by partial deuteration.
Comparison with the bacteriochlorophyll a cation radical
shows different reduction factors of the individual coupling
constants deviating from the value 2.0 reported in earlier investigations
in frozen solutions. The average of the coupling
constants is, however, reduced by a factor very close to 2.0.
EPR simulations using the ENDOR coupling constants support
a dimer model for PsO with C2 symmetry, where the two macrocycles
are close enough to form a supermolecular orbital resulting
in a different distribution of the unpaired electron,
compared with the monomeric bacteriochlorophyll a cation
radical. Molecular orbital calculations were used to obtain
structural information about this dimer.