Reconstitution of electrochromically active pigment-protein complexes from Rhodobacter sphaeroides into liposomes

Photosynthetic reaction center functionalized nano-composite films: effective strategies for probing and exploiting the photo-induced electron transfer of photosensitive membrane protein

Photosynthetic reaction center (RC), a robust transmembrane pigment-protein complex, works as the crucial component participating the primary event of the photo-electrochemical conversion in bacteria. Sparked by the high photo-induced charge separation yield (ca. 100%) of RC, great interests have been aroused to fabricate versatile RC-functionalized nano-composite films for exploring the initial photosynthetic electron transfer (ET) of RC, and thus exploiting well-designed bio-photoelectric converters. In this review, we classify and summarize the current status about the concepts and methods of constructing RC-immobilized nano-composite films or devices for probing the photo-induced ET, and applying to novel bioelectronics if it is possible.

Electrochromic responses of carotenoid absorbance bands in purified light-harvesting complexes from Rhodobacter capsulatus reconstituted into liposomes

Light-Harvesting Complexes I and II (LHI and LHII) were extracted from chromatophores of Rhodobacter capsulatus, purified in Triton X-100 and reconstituted into phospholipid vesicles. Application of membrane potentials (K+ diffusion potentials) to LHII proteoliposomes led to absorbance changes in the carotenoid bands which were spectrally similar to those in chromatophores. These (electrochromic) absorbance changes were linear with the applied membrane potential between -107 mV and +105 mV. The data were consistent with the existence of two forms of carotenoid in LHII. One form, comprising 2/3 of the total and with a long wavelength absorbance maximum at 510 nm, was not significantly affected by membrane potential. The other component, comprising 1/3 of the total and with a long wavelength absorbance maximum at 516.5 nm, was shifted by approx. 1.6 nm to the red by a membrane potential of 105 mV. Reduction of the B800 bacteriochlorophyll in LHII with NaBH4 before reconstitution did not affect the absorbance spectrum of the carotenoids and it did not affect their response to applied membrane potentials in proteoliposomes. Although the electrochromically-sensitive carotenoids might be associated with B800, interactions with the bacteriochlorophyll are perhaps not the cause of the polarisation of the carotenoid that is responsible for the linearity of the response. The carotenoids in reconstituted LHI complexes were not detectably electrochromic. The electrochromic absorbance changes of carotenoids in LHII could be useful for membrane potential measurement in liposomes containing ion-translocating proteins.

Effects of phospholipase A2 digestion on the carotenoid and bacteriochlorophyll components of the light-harvesting complexes in Rhodobacter sphaeroides chromatophores

The instantaneous electrochromic response of carotenoids associated with the B800-850 light-harvesting complex of Rhodobacter sphaeroides has been used widely as an intrinsic probe of membrane potential. In the present study, the structural basis for this phenomenon was examined by phospholipase A2 digestion of chromatophores from R. sphaeroides strain NF57G, containing B800-850 as the sole pigment-protein complex. The major phospholipase-induced alterations of the overall carotenoid absorption spectrum were characterized by an absorbance loss and a blue shift that were accompanied by a decrease in absorbance at 800 nm and a red shift in the B850 absorbance band. In wild-type chromatophores, the electrochromic carotenoid response induced by both flash illumination and a K+ diffusion potential was diminished by approximately 60% after 1 h of digestion. The initial loss of the carotenoid response was correlated specifically to the hydrolysis of phosphatidylethanolamine, and was shown to arise from effects exerted directly upon the electrochromically active carotenoid pool, possibly by alterations in the spatial relationship between the field-sensitive carotenoids and the polarizing permanent field. In phospholipase A2-digested NF57G preparations in which the B800 band was diminished by nearly half and the carotenoid response was abolished, no significant changes in the efficiency of energy transfer from carotenoids to bacteriochlorophyll were detected at 77 K, suggesting that the electrochromically active carotenoids are not energetically linked to B800 bacteriochlorophyll.

Spectral identification of the electrochromically active carotenoids of Rhodobacter sphaeroides in chromatophores and reconstituted liposomes

Reaction centers with both light harvesting complexes I and II (B875 and B800/850; i.e., RCLHILHII complexes) have been isolated from Rhodobacter sphaeroides. These complexes have been incorporated into liposomes made from lipids purified from Escherichia coli. The electrochromic bandshift of carotenoids, present in these reconstituted complexes, shows shifted minima and maxima with respect to a similar spectrum in chromatophores of Rb. sphaeroides in a potassium diffusion potential induced difference spectrum (see also Crielaard, W., Hellingwerf, K.J. and Konings, W.N. (1989) Biochim. Biophys. Acta 973, 205-211). The absorbance spectrum, at room temperature or at 77 K, of both membrane preparations did not, however, reveal differences in the carotenoid region. The long-wavelength carotenoid peak in both preparations is located at 513 nm (77 K). A small difference could be observed between the 77 K excitation spectra of the B850 fluorescence. Reconstituted complexes show a carotenoid peak at 513 nm, whereas in chromatophores this peak is located at 514.5 nm. When fluorescence was recorded at 805 nm, to detect B800 excitation, there was a marked difference between both preparations. In liposomes the long wavelength B800-associated carotenoid peak is located at 512.5 nm, whereas in chromatophores this peak is located at 516 nm. These results explain the shifted minima and maxima in a potassium diffusion induced difference spectrum in proteoliposomes. The prediction of two carotenoid pools in chromatophores (De Grooth, B.G. and Amesz, J. (1977) Biochim. Biophys. Acta 462, 247-258) is confirmed, and the field sensitive carotenoids are identified as the pool that is associated with the B800 band (Kramer, H.J.M., Van Grondelle, R., Hunter, C.N., Westerhuis, W.H.J. and Amesz, J. (1984) Biochim. Biophys. Acta 765, 156-165).

Stark effect spectroscopy of carotenoids in photosynthetic antenna and reaction center complexes

The effects of electric fields on the absorption spectra of the carotenoids spheroidene and spheroidenone in photosynthetic antenna and reaction center complexes (wild-type and several mutants) from purple non-sulfur bacteria are compared with those for the isolated pigments in organic glasses. In general, the field effects are substantially larger for the carotenoid in the protein complexes than for the extracted pigments and larger for spheroidenone than spheroidene. Furthermore, the electrochromic effects for carotenoids in all complexes are much larger than those for the Qx transitions of the bacteriochlorophyll and bacteriopheophytin pigments which absorb in the 450-700 nm spectral region. The underlying mechanism responsible for the Stark effect spectra in the complexes is found to be dominated by a change in permanent dipole moment of the carotenoid upon excitation. The magnitude of this dipole moment change is found to be considerably larger in the B800-850 complex compared to the reaction center for spheroidene; it is approximately equivalent in the two complexes for spheroidenone. These results are discussed in terms of the effects of differences in the carotenoid functional groups, isomers and perturbations on the electronic structure from interactions with the organized environment in the proteins. these data provide a quantitative basis for the analysis of carotenoid bandshifts which are used to measure transmembrane potential, and they highlight some of the pitfalls in making such measurements on complex membranes containing multiple populations of carotenoids. The results for spheroidenone should be useful for studies of mutant proteins, since mutant strains are often grown semi-aerobically to minimize reversion.

Large protein-induced dipoles for a symmetric carotenoid in a photosynthetic antenna complex

Unusually large electric field effects have been measured for the absorption spectra of carotenoids (spheroidene) in the B800-850 light-harvesting complex from the photosynthetic bacterium Rhodobacter sphaeroides. Quantitative analysis shows that the difference in the permanent dipole moment between the ground state and excited states in this protein complex is substantially larger than for pure spheroidene extracted from the protein. The results demonstrate the presence of a large perturbation on the electronic structure of this nearly symmetric carotenoid due to the organized environment in the protein. This work also provides an explanation for the seemingly anomalous dependence of carotenoid band shifts on transmembrane potential and a generally useful approach for calibrating electric field-sensitive dyes that are widely used to probe potentials in biological systems.

Kinetics of photosynthetic electron transfer in artificial vesicles reconstituted with purified complexes from Rhodobacter capsulatus. II. Direct electron transfer between the reaction center and the bc1 complex and role of cytochrome c2

1. The cyclic photosynthetic chain of Rhodobacter capsulatus has been reconstituted incorporating into phospholipid liposomes containing ubiquinone-10 two multiprotein complexes: the reaction center and the ubiquinol-cytochrome-c2 reductase (or bc1 complex). 2. In the presence of cytochrome c2 added externally, at concentrations in the range 10-10(4) nM, a flash-induced cyclic electron transfer can be observed. In the presence of antimycin, an inhibitor of the quinone-reducing site of the bc1 complex, the reduction of cytochrome b561 is a consequence of the donation of electrons to the photo-oxidized reaction center. At low ionic strength (10 mM KCl) and at concentrations of cytochrome c2 lower than 1 microM, the rate of this reaction is limited by the concentration of cytochrome c2. At higher concentrations the reduction rate of cytochrome b561 is controlled by the concentration of quinol in the membrane, and, therefore, is increased when the ubiquinone pool is progressively reduced. At saturating concentrations of cytochrome c2 and optimal redox poise, the half-time for cytochrome b561 reduction is about 3 ms. 3. At high ionic stength (200 mM KCl), tenfold higher concentrations of cytochrome c2 are required for promoting equivalent rates of cytochrome-b561 reduction. If the absolute values of these rates are compared with those of the cytochrome-c2-reaction-center electron transfer, it can be concluded that the reaction of oxidized cytochrome c2 with the bc1 complex is rate-limiting and involves electrstatic interactions. 4. A significant rate of intercomplex electron transfer can be observed also in the absence of cytochrome c2; in this case the electron donor to the recation center is the cytochrome c1 of the oxidoreductase complex. The oxidation of cytochrome c1 triggers a normal electron transfer within the bc1 complex. The intercomplex reaction follows second-order kinetics and is slowed at high ionic strength, suggesting a collisional interaction facilitated by electrostatic attraction. From the second-order rate constant of this process, a minimal bidimensional diffusion coefficient for the complexes in the membrane equal to 3 X 10(-11) cm2 s-1 can be evaluated.