Abstract-(a) Native PEC from the cyanobacterium, Mastigocladus laminosus, and its isolated asubunit show photoreversibly photochromic reactions with difference-maxima around 502 and 570 nm in the spectral region of the a-84 phycoviolobilin chromophore. (b) Native PEC and its P-subunit show little if any reversible photochemistry in the 600-620 nm region, where the phycocyanobilin chromophores on the P-subunit absorb maximally. (c) Reversible photochemistry is retained in ureadenatured PEC at pH = 7.0 or pH 6 3. The difference maxima are shifted to 510 and 600 nm, and the amplitudes are decreased. An irreversible absorbance increase occurs around 670 nm (pH < 3).(d) The amplitude of the reversible photoreaction difference spectrum is maximum in the presence of 4-5 M urea or 1 M KSCN, conditions known to dissociate phycobiliprotein aggregates into monomers.At the same time, the phycocyanobilin chromophore(s) are bleached irreversibly. (e) The amplitude becomes very small in high aggregates, e.g. in phycobilisomes. ( f ) In a reciprocal manner, the phototransformation of native PEC leads to a reversible shift of its aggregation equilibrium between trimer and monomer. The latter is favored by orange, the former by green light. (8) It is concluded that the phycoviolobilin chromophore of PEC is responsible for reversible photochemistry in PEC, and that there is not only an influence of aggregation state on photochemistry, but also vice versa an effect of the status of the chromophore on aggregation state. This could constitute a primary signal in the putative function as sensory pigment, either directly, or indirectly via the release of other polypeptides, via photodynamic effects, or the like.
C-phycocyanin from the cyanobacterium, Mastigocladus laminosus, and its subunits have been treated with ρ-chloromercuribenzenesulfonate (PCMS). A single reactive site was found on the 13- subunit, and assigned to the single free cystein-β109. The concomitant spectral changes (absorption, fluorescence, circular dichroism), together with the known close proximity of cys-β109 to chromophore β82, allowed an unambiguous assignment of the three spectrally, biochemically and functionally different chromophores to specific binding sites on the two peptide chains (α84: 616-618, β82: 622-624, β153: 598-600 nm).
Abstract-Two phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobacterium Westiellopsis proltfica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (a-84) to the cyanin @-84, 155) chromophores is efficient in the tnmer ((up),, it is impeded after dissociation to the monomer (a@). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol . 55, 1 19-1 24, 1992), exhibited the spectral properties similar to that of the a-subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained.
The transient absorption recovery induced in phycocyanin trimers by picosecond pulses of variable wavelength (570-620 nm) has been recorded and analyzed by applying a least-squares multi-exponential fit procedure.
The results suggest that in native PC trimers the chromophores exhibit a microheterogeneity with the effect that the derived apparent lifetimes are functions of excitation and probing wavelength. It is suggested that, due to strong excitonic coupling between a-84 and β-84 chromophores, the lifetime of the terminal acceptor state is reduced to about 900 ps; the apparent energy transfer time from chromophore β-155 to a-84 and β-84 chromophores varies between 20-50 ps depending on the actual chromophore-protein arrangement (microheterogeneity)
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