The photosynthetic reaction center of photosystem I (PS I) contains a
phylloquinone molecule (A(1)) which acts as a transient electron accep
tor. Tn PS I from the cyanobacterium Synechocystis PCC 6803 under redu
cing conditions, we have photoaccumulated an EPR signal assigned to th
e phylloquinone radical anion. The phyllosemiquinone EPR spectrum has
been studied in oriented multilayers of PS I using EPR at 9 GHz. In ad
dition, the phyllosemiquinone spectrum has been obtained at 283 GHz us
ing high-field, high-frequency EPR spectroscopy, From the orientation
dependence of the spectrum at 9 GHz and the resolved g values obtained
at 283 GHz, the phyllosemiquinone ring plane was determined to be alm
ost perpendicular to the membrane (76 degrees) while the oxygen-oxygen
(O-O) axis of the quinone was found to make an approximate 63 degrees
angle to the membrane plane. The orientation of the ring plane is sim
ilar to that determined for the quinone electron acceptor (Q(A)) in th
e purple bacterial reaction center, while the orientation of the O-O a
xis is significantly different. The new orientation information, when
taken with data in the literature, allows the position of the phylloqu
inone in the reaction center to be better defined.