Dj. Hirsh et Gw. Brudvig, LONG-RANGE ELECTRON-SPIN SPIN INTERACTIONS IN THE BACTERIAL PHOTOSYNTHETIC REACTION-CENTER, Journal of physical chemistry, 97(50), 1993, pp. 13216-13222
The electron spin-lattice relaxation behavior of the oxidized bacterio
chlorophyll a dimer in reaction centers from Rhodobacter sphaeroides h
as been examined by the method of saturation-recovery EPR over the tem
perature range 3.8 K less-than-or-equal-to T less-than-or-equal-to 22
K. Its spin-lattice relaxation is nonexponential due to an orientation
-dependent dipolar interaction with the non-heme Fe(II) of the reactio
n center. The saturation-recovery EPR traces were fit by using an equa
tion which models the recovery in terms of a sum of isotropic (scalar)
and orientation-dependent (dipolar) rate constants. The center-to-cen
ter distance between the bacteriochlorophyll a dimer and the non-heme
Fe(II) is 28 angstrom and it is found that the Heisenberg exchange int
eraction is too small to make a measurable contribution to the scalar
relaxation rate of the oxidized bacteriochlorophyll a dimer. The, scal
ar relaxation rates for the oxidized bacteriochlorophyll a dimer show
a T1 temperature dependence which differs significantly from that of m
odel porphyrin radicals. It appears that the unusually rigid protein e
nvironment surrounding the bacteriochlorophyll a dimer produces a stro
ng coupling between the spin transitions of the radical and the low-fr
equency vibrational modes of the lattice. The dipolar mte constants of
the oxidized bacteriochlorophyll a dimer and those of the stable tyro
sine radical, Y(D)., in Mn-depleted photosystem II show the same tempe
rature dependence. This confirms the assignment of the non-heme Fe(II)
as the source of relaxation enhancement for Y(D). in Mn-depleted phot
osystem II and shows that the spin relaxation properties of the non-he
me Fe(II) species in the two proteins are very similar. Using the rela
tive magnitudes of the dipolar rate constants in the two proteins and
the distance between the bacteriochlorophyll a dimer and the non-heme
Fe(II) in the bacterial reaction center, we calculate a Y(D).-Fe(II) d
istance of 37 +/- 5 angstrom in photosystem II. This agrees well with
the distance predicted from the structure of the bacterial reaction ce
nter.