Interactions of chlorophyll a with synthesized peptide in aqueous solution.

The interactions between chlorophyll a and synthesized peptides have been studied using optical spectroscopy. Three 30-residue peptides are designed and synthesized: an amphiphilic peptide without histidine (L), an amphiphilic peptide with histidine (L/H) and a hydrophilic peptide (K/E). These peptide properties thereby allow us to examine the effect of the peptide hydrophobicity and/or histidine residue on pigment-peptide interactions. On mixing with peptides, chlorophyll a has a main absorption band in the Qy region with the maximum at 672 nm. For all three peptides, fluorescence patterns show that at a low concentration of the peptide (0.05 mM) in aqueous solution, the energy is transferred among various forms of the pigment. Only peptide L/H at high concentration (0.5 mM) in solution retains the Qy band of chlorophyll a at 672 nm, and the emission is that typically seen for the monomeric form of the pigment. The aggregation of chlorophyll a is suppressed most strongly in the presence of the peptides L/H. The results suggest that chlorophyll a is ligated to a histidine residue, located in the hydrophobic region of the peptides L/H, and in surrounded or shielded by the peptide alpha-helixes.

The influence of the presence of lipid on the aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c located in adsorbed layers and monolayers.

The photoacoustic spectra and time-resolved delayed luminescence spectra in the microsecond time range were measured for layers of 8,12-diethyl farnesyl bacteriochlorophyll c adsorbed on quartz supports by solvent evaporation and as Langmuir-Blodgett monolayers. Both types of model system were also investigated with the addition of lipid. The data showed a very strong influence of lipid addition on pigment aggregation. In samples with synthetic and natural lipid addition, the pigments were found to be predominantly in the monomeric and dimeric states, whereas in the same type of sample without lipid, the pigments were aggregated to a higher degree. The influence of the presence of lipid on the aggregation of bacteriochlorophyll c in monolayers and adsorbed layers may also suggest that the contact of various pigment molecules with the lipids surrounding the chlorosome may influence the formation of various pigment aggregates in vivo. The synthetic lipid L-alpha-phosphatidylcholine dipalmitoyl and the natural lipid L-alpha-phosphatidylcholine type IVS from soy beans were used. In the latter case, only adsorbed layers were investigated. Our interpretation is preliminary as only one 8,12-diethyl farnesyl bacteriochlorophyll c homologue was present in our systems.

Aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c at low temperature.

The effect of temperature on the aggregation of 3(l)R-8,12-diethyl farnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclohexane (1/1, v/v) was studied by means of absorption, circular dichroism and fluorescence spectroscopy. At room temperature essentially only two aggregate species, absorbing at 702 nm (A-702) and 719 nm (A-719), were present. Upon cooling to 219 K, A-702 was quantitatively converted to A-719. Further lowering of the temperature led to the stepwise formation of larger aggregates by the conversion of A-719 to aggregate species absorbing at 743 nm (A-743) and 755 nm (A-755). All absorption changes were reversible. A-719 was highly fluorescent (maximum at 192 K: 744 nm), while A-743 and especially A-755 were weakly fluorescent. Below 130 K the mixture solidified, and no major changes in the absorption spectrum were observed upon further cooling. At 45 K, however, a relatively strong emission at 775 nm was observed. Below 200 K, the absorption, fluorescence and circular dichroism spectra resembled that of the chlorosome. These results open up the possibility to study higher aggregates of BChl c as models for the chlorosome by various methods at low temperature, thus avoiding interference by thermal processes.

Fluorescence anisotropy of cyanobacterial phycobilisomes oriented in polyvinyl alcohol (PVA) films.

Polarized absorption (at 296 and 85 K), fluorescence, and photoacoustic (at 296 and 85 K) spectra of antenna complexes-phycobilisomes isolated from cyanobacteriaTolypothrix tenuis andOscillatoria and embedded in isotropic and anisotropic polyvinyl alcohol films-were measured. From the sets of polarized components of emission, the anisotropy of fluorescence for the pools of differently oriented molecules was calculated. On the basis of polarized photoacoustic and emission spectra, the competition between the process of thermal deactivation of excitation and excitation energy transfer in a chain of excitation donor and acceptor chromophores of phycobilisomes is discussed.