| Popis: |
Spinach thylakoids and chromatophores from the photosynthetic bacterium Rhodopseudomonas capsulata were investigated by means of time-resolved infrared spectroscopy, using thin water-containing membrane films which fully maintained their photochemical activity. Upon flash excitation, reversible infrared absorbance changes were obtained and their difference spectra were recorded. In spinach thylakoids, these transient signals could be described by a sum of two exponential decay functions with half-times of about 2 and 30 ms, respectively. They were insensitive to the addition of benzyl viologen, ferricyanide or ferricyanide + DCMU. They are ascribed by their dependence on intensity and wavelength range of the actinic flash to processes in the antenna pigment-protein complexes. In chromatophores from photosynthetic bacteria, similar infrared signals in the millisecond time range were obtained. Their spectral distribution was investigated for three mutants of the photosynthetic bacterium and is different for membranes lacking carotenoids. Both signals, in thylakoids and chromatophores, reflect the proportion of absorbed flash energy which is neither channelled to the reaction center nor emitted as light, but is dissipated through radiationless decay. A common feature of the difference spectra from spinach thylakoids and bacterial chromatophores are bands identified by deuteration as being due to H 2 O. Some bands are interpreted in terms of water going transiently from the hydrogen-bonded to the free state. Other bands are assigned to the polypeptides of the light-harvesting complexes, and thus indicate their participation in energy dissipation. Membranes from photosynthetic bacteria containing a photochemical reaction center show a distinct slow signal component decaying in about 1 s. It saturates at low flash intensity and is abolished upon chemical oxidation of the primary electron donor. Two bands in the difference spectrum of this component are tentatively assigned to the ester C = O and keto C = O vibrations of photooxidized bacteriochlorophylls in the reaction center. The data suggest that chromophoric and non-chromophoric infrared absorbance changes contribute to the difference spectra, and thus may represent a clue to the processes at the active sites of polypeptides in photosynthesis. |
