TRIPLET PROPERTIES AND INTERACTIONS OF THE PRIMARY ELECTRON-DONOR ANDANTENNA CHROMOPHORES IN MEMBRANES OF HELIOBACTERIUM-CHLORUM, STUDIED WITH ADMR SPECTROSCOPY

The tripler states of antenna and reaction center bacteriochlorophyll (BChl) g in membranes of Heliobacterium chlorum were studied by optica lly detected magnetic resonance in zero magnetic field, using absorban ce detection. A variety of tripler states was detected, which were all localized on single BChl g chromophores as concluded from a compariso n with the triplet state of monomeric BChl g in organic solvents. With the aid of the microwave-induced absorbance difference spectra, we as sign a tripler state with zero-field splitting parameters \D\ = 727.5 and \E\ = 254.5 MHz to that of the primary donor. The low \E\ value in dicates that the BChls of the primary donor are monoligated. The inten sities of the zero-field transitions were strongly dependent on the re dox state of the secondary electron accepters. A tripler state with \D \ = 690-705 MHz and \E\ = 230 MHz, present under all redox conditions, is associated with antenna BChl g absorbing at 814 nm. Its tripler yi eld was independent of the redox conditions; we conclude therefore tha t the antenna chromophores absorbing at 814 nm are not connected with the reaction center at cryogenic temperatures (1.2 K). In addition, re latively strong signals were detected belonging to tripler states with \D\ and \E\ of 663-680 and 220-227 MHz, respectively, whose amplitude s were dependent on the redox conditions. Triplet states with these ze ro-field splitting parameters are located on antenna chromophores abso rbing between 798-814 nm; their zero-field transitions and absorbance difference spectra indicate a considerable heterogeneity. The concentr ation of tripler states of antenna chromophores absorbing around 800 n m decreased markedly upon prolonged excitation at 1.2 K. This phenomen on is attributed to quenching of excitations on antenna pigments by st able charge separation in the closely connected reaction center, possi bly involving a low-quantum yield menaquinone electron acceptor.