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.

Reversible photochemistry in a chlorophyll model system

A reversible, endothermic photochemical redox reaction, sensitized by chlorophyll a and related compounds, has been demonstrated in a heterogeneous particulate system. The oxidant, 5,5'-dithiobis(2-nitrobenzoate) (DTNB), is photoreduced to the thiolate which remains primarily in an aqueous phase. Reductants are trisubstituted hydrazines, capable of oxidation to tetrazanes in the hydrocarbon particle phase. In the course of three days in the dark, thiolate and tetrazane react to regenerate DTNB in yields approaching 100%. A novelty of the present system is that photoreaction often takes place in discrete rate regimes, which are related to the presence of spectrally identifiable associations of chlorophyll pigments, Mg-containing and free bases. Among the associations that promote photochemical activity are those of chlorophyll and pheophytin with themselves and with each other. Perhaps more active are associations of a Mg rhodochlorin allomerization product of chlorophyll with its free base. Contributing to the associations is the stabilizing presence of amphiphiles that both ligate the Mg of chlorophyll strongly and hydrogen-bond to carbonyls: 2-tridecylimidazole, 2-tridecylimidazoline, and (2-aminoethyl)myristamide. Results of this work demonstrate the possibility of generating reaction center models in an artificial heterogeneous system, and of conducting reversible photochemical reactions with them.

Dodecylpyridinium alkanoates stabilize dispersed chlorophyll

In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared-absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomeric and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen-bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.