Fluorescence-ODMR of Light Harvesting Pigments of Photosynthetic Bacteria

The triplet state of photosynthetic pigments has been widely used in the past as an internal nonde­ structive probe for the geometric and electronic structure of the pigment-protein complexes of photo­ synthetic bacteria. Especially the reaction center complex (RC) has been intensively studied by means of ESR in high field and zero field [1—7]. Less is known about antenna complexes. So far only the light harvesting (LH) antenna complexes of the m u­ tant A l a + pho~ from Rhodopseudomonas (Rps) capsulata [9] have been investigated by fluorescence detected magnetic resonance in zero magnetic field (F-ODM R) [8]. The LH antenna system of Rps. sphaeroides R 26.1 contains two subunits which are characterized by their different absorption, B 850 and B 870, which absorb at 850 nm and at 870 nm, respectively [10]. The minimal units of both types contain two different polypeptides each, the sequences of which have been analyzed recently [11], It seems that ohne Bacteriochlorophyll molecule is bound to each polypep­ tide [12]. The LH antenna system of Rps. capsulata A l a + consists of one B 870 complex whose minimal unit is likewise formed by two polypeptides, the amino-acid sequences of which have been deter­ mined [13, 14]. In vivo these polypeptides aggregate with each other and with the reaction centers to form photo­ synthetic units in the chromatophore membranes. The red shift of the antenna bacteriochlorophyll ab­ sorption from 780 nm (in vitro) to 850 and 870 nm