Abstract
The electron spin-lattice relaxation behavior of the oxidized bacteriochlorophyll a dimer in reaction centers from Rhodobacter sphaeroides has been examined by the method of saturation-recovery EPR over the temperature range 3.8 K ≤ T ≤ 22 K. Its spin-lattice relaxation is nonexponential due to an orientation-dependent dipolar interaction with the non-heme Fe(II) of the reaction center. The saturation-recovery EPR traces were fit by using an equation which models the recovery in terms of a sum of isotropic (scalar) and orientation-dependent (dipolar) rate constants. The center-to-center distance between the bacteriochlorophyll a dimer and the non-heme Fe(II) is 28 Å and it is found that the Heisenberg exchange interaction is too small to make a measurable contribution to the scalar relaxation rate of the oxidized bacteriochlorophyll a dimer. The scalar relaxation rates for the oxidized bacteriochlorophyll a dimer show a T1 temperature dependence which differs significantly from that of model porphyrin radicals. It appears that the unusually rigid protein environment surrounding the bacteriochlorophyll a dimer produces a strong coupling between the spin transitions of the radical and the low-frequency vibrational modes of the lattice. The dipolar rate constants of the oxidized bacteriochlorophyll a dimer and those of the stable tyrosine radical, YD •, in Mn-depleted photosystem II show the same temperature dependence. This confirms the assignment of the non-heme Fe(II) as the source of relaxation enhancement for YD • in Mn-depleted photosystem II and shows that the spin relaxation properties of the non-heme Fe(II) species in the two proteins are very similar. Using the relative 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 YD •-Fe(II) distance of 37 ± 5 Å in photosystem II. This agrees well with the distance predicted from the structure of the bacterial reaction center.
| Original language | English |
|---|---|
| Pages (from-to) | 13216-13222 |
| Number of pages | 7 |
| Journal | Journal of Physical Chemistry |
| Volume | 97 |
| Issue number | 50 |
| Publication status | Published - 1993 |
Fingerprint
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
Cite this
Long-range electron spin-spin interactions in the bacterial photosynthetic reaction center. / Hirsh, Donald J.; Brudvig, Gary W.
In: Journal of Physical Chemistry, Vol. 97, No. 50, 1993, p. 13216-13222.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Long-range electron spin-spin interactions in the bacterial photosynthetic reaction center
AU - Hirsh, Donald J.
AU - Brudvig, Gary W
PY - 1993
Y1 - 1993
N2 - The electron spin-lattice relaxation behavior of the oxidized bacteriochlorophyll a dimer in reaction centers from Rhodobacter sphaeroides has been examined by the method of saturation-recovery EPR over the temperature range 3.8 K ≤ T ≤ 22 K. Its spin-lattice relaxation is nonexponential due to an orientation-dependent dipolar interaction with the non-heme Fe(II) of the reaction center. The saturation-recovery EPR traces were fit by using an equation which models the recovery in terms of a sum of isotropic (scalar) and orientation-dependent (dipolar) rate constants. The center-to-center distance between the bacteriochlorophyll a dimer and the non-heme Fe(II) is 28 Å and it is found that the Heisenberg exchange interaction is too small to make a measurable contribution to the scalar relaxation rate of the oxidized bacteriochlorophyll a dimer. The scalar relaxation rates for the oxidized bacteriochlorophyll a dimer show a T1 temperature dependence which differs significantly from that of model porphyrin radicals. It appears that the unusually rigid protein environment surrounding the bacteriochlorophyll a dimer produces a strong coupling between the spin transitions of the radical and the low-frequency vibrational modes of the lattice. The dipolar rate constants of the oxidized bacteriochlorophyll a dimer and those of the stable tyrosine radical, YD •, in Mn-depleted photosystem II show the same temperature dependence. This confirms the assignment of the non-heme Fe(II) as the source of relaxation enhancement for YD • in Mn-depleted photosystem II and shows that the spin relaxation properties of the non-heme Fe(II) species in the two proteins are very similar. Using the relative 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 YD •-Fe(II) distance of 37 ± 5 Å in photosystem II. This agrees well with the distance predicted from the structure of the bacterial reaction center.
AB - The electron spin-lattice relaxation behavior of the oxidized bacteriochlorophyll a dimer in reaction centers from Rhodobacter sphaeroides has been examined by the method of saturation-recovery EPR over the temperature range 3.8 K ≤ T ≤ 22 K. Its spin-lattice relaxation is nonexponential due to an orientation-dependent dipolar interaction with the non-heme Fe(II) of the reaction center. The saturation-recovery EPR traces were fit by using an equation which models the recovery in terms of a sum of isotropic (scalar) and orientation-dependent (dipolar) rate constants. The center-to-center distance between the bacteriochlorophyll a dimer and the non-heme Fe(II) is 28 Å and it is found that the Heisenberg exchange interaction is too small to make a measurable contribution to the scalar relaxation rate of the oxidized bacteriochlorophyll a dimer. The scalar relaxation rates for the oxidized bacteriochlorophyll a dimer show a T1 temperature dependence which differs significantly from that of model porphyrin radicals. It appears that the unusually rigid protein environment surrounding the bacteriochlorophyll a dimer produces a strong coupling between the spin transitions of the radical and the low-frequency vibrational modes of the lattice. The dipolar rate constants of the oxidized bacteriochlorophyll a dimer and those of the stable tyrosine radical, YD •, in Mn-depleted photosystem II show the same temperature dependence. This confirms the assignment of the non-heme Fe(II) as the source of relaxation enhancement for YD • in Mn-depleted photosystem II and shows that the spin relaxation properties of the non-heme Fe(II) species in the two proteins are very similar. Using the relative 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 YD •-Fe(II) distance of 37 ± 5 Å in photosystem II. This agrees well with the distance predicted from the structure of the bacterial reaction center.
UR - http://www.scopus.com/inward/record.url?scp=33751385929&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=33751385929&partnerID=8YFLogxK
M3 - Article
VL - 97
SP - 13216
EP - 13222
JO - Journal of Physical Chemistry
JF - Journal of Physical Chemistry
SN - 0022-3654
IS - 50
ER -