The Effects of pH and Ionic Strength on the Aggregation of Bacteriochlorophyll c in Aqueous Organic Media: The Possibility of Two Kinds of Aggregates

The influence of pH on charged porphyrins studied by fluorescence and photoacoustic spectroscopy

We have studied the influence of solvent acidity on the aggregation/protonation behaviour of two charged tetraphenylporphyrins: positively charged tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin (TAP) and negatively charged tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Spectroscopic measurements (absorption, fluorescence, excitation and photoacoustic) have been made to follow the radiative and nonradiative relaxation processes of excited dye molecules at various pH values. The ability of these porphyrins to exist in aggregated and protonated forms was also investigated. It has been shown that TPPS4 exists in at least three spectroscopic forms: monomer (M1), dication (D1) and aggregated dication (AD), whereas TAP exists in two forms: monomer (M2) and dication (D2). These forms are characterised by different absorption and fluorescence properties. The short wavelength forms of the dyes (M1 and M2) were assigned to the monomeric forms and D1 and D2 were assigned to the protonated forms of TPPS4 and TAP, respectively; AD was identified as the aggregate of the protonated form of TPPS4. Each form shows deactivation of its excited state by nonradiative pathways-particularly high thermal deactivation was observed for the aggregates of the protonated form. Energy transfer between the monomeric form and the dication form has been demonstrated and Förster radii have been estimated (R0 = 31 and 44 A for TAP and TPPS4, respectively). Since protonation and aggregation of photoreceptors can strongly affect their photosensitizing effects (e.g. cellular uptake, singlet oxygen production) the results presented here may be important in the study of the function of photosensitizers in tumour tissues because, as is already known, the microenvironment in cancerous tissue is more acidic than in healthy cells.

Regio- and stereoisomeric control of the aggregation of zinc-chlorins possessing inverted interactive hydroxyl and carbonyl groups

As models for a self-aggregative, naturally occurring magnesium-chlorin bacteriochlorophyll-d possessing 3(1)-secondary alcoholic hydroxyl and 13(1)-oxo groups, zinc-chlorins were synthesized with 3(1)-oxo and 13(1)-secondary (1) or tertiary hydroxyl groups (2). Compared to the monomers in a tetrahydrofuran solution, diastereomers 13(1)R-1R and 13(1)S-1S gave red-shifted absorption maxima (643 --> 674 nm in 1R and 708 nm in 1S) in 1 v/v% CH(2)Cl(2)-hexane solution, indicating their self-aggregation. Therefore, the positioning of the two groups at 3(1)/13(1) or 13(1)/3(1) on the N21-N23 molecular (Q(y)) axis is not necessarily important for the self-aggregation. The (1)H NMR and CD spectroscopic studies showed that the 674 nm absorbing species of 1R was characterized as a face-to-face "closed" dimer, while the 708 nm absorbing species of 1S was a large oligomer constructed with aggregation of head-to-tail "open" dimers. This diastereomeric control over the aggregation of 1R and 1S is more pronounced than that observed in the regioisomerically 3(1)-secondary alcoholic R/S-diastereomers 3R and 3S. The difference is ascribable to the conformational fixation of the 13(1)-hydroxyl group of the exo five-membered ring in 1. In contrast to self-aggregative 3(1)-tertiary alcoholic 4, both 13(1)-epimers of 13(1)-tertiary alcoholic 2 were monomeric even in nonpolar organic media: the additional 13(1)-methyl group (1 --> 2) drastically suppressed the self-aggregation due to the interference of the methyl group in intermolecular pi-pi interaction.

Pure and scrambled self-aggregates prepared with zinc analogues of bacteriochlorophylls c and d

Zinc analogues of bacteriochlorophylls c and d self-assembled in aqueous media with phospholipids. A methanol solution of zinc chlorin and alpha-lecithin was put in a cellulose tube and the inner methanol solvent was gradually replaced with water by dialysis to form the self-assembled oligomers. Visible absorption spectra of the aqueous solution showed that zinc chlorins formed J-aggregates within the hydrophobic core of alpha-lecithin assemblies and that the supramolecular structure of the aggregates depended upon the stereochemistry at the 3(1)-position and the alkyl substituents at the 8-, 12-, and 17(4)-positions of the zinc chlorin. When the aqueous aggregates were prepared with a mixture of 3(1)-epimers and/or 8-, 12-, or 17(4)-homologues of zinc 3(1)-hydroxy-13(1)-oxochlorins, the structurally distinct components coaggregated to make scrambled oligomers. However, during the dialysis, zinc 3(1)-hydroxy- and 7(1)-hydroxy-13(1)-oxochlorins slowly individually aggregated to give two structurally different oligomer units in the cellulose tube. In contrast, if the two zinc chlorin components rapidly self-assembled in an aqueous medium, these components coaggregated to form scrambled oligomers. The present study shows that both the molecular structure of the pigments and the speed of the oligomerization determine the molecular arrangement in chlorosome-type self-assembled oligomers.

The effects of epimerization at the 3(1)-position of bacteriochlorophylls c on their aggregation in chlorosomes of green sulfur bacteria. Control of the ratio of 3(1) epimers by light intensity

R- and S-epimerization at the 3(1) position of bacteriochlorophyll (BChl) c and the formation of rod-like aggregates in chlorosomes of green sulfur bacteria were markedly affected in Chlorobium (Cb.) tepidum and Cb. limicola by cultivation under various light intensities (photon fluence rate). The stronger the light, the higher the ratio of the S-epimer to the R-epimer for each homolog of BChl c in the bacteria. S[P,E] BChl cF and S[I,E] BChl cF were found to be the major S-epimers in Cb. tepidum and Cb. limicola, respectively. R[P,E] BChl cF decreased markedly compared to R[E,E] BChl cF in Cb. tepidum, whereas no observable change in the ratio of R[P,E]/R[E,E] was detected for Cb. limicola. With increase in light intensity the Qy absorption maximum of the bacteria shifted to shorter wavelengths. In vitro spectroscopic studies of the aggregates showed a marked difference in the formation of aggregates from R- and S-epimers of BChl c; the S-epimers formed aggregates much more slowly than did the R-epimers. These results suggest that the ratio of the epimers of BChl c might significantly affect the aggregation of BChl in the chlorosome. We propose different roles for the R- and S-epimers in chlorosomes of Cb. limicola and Cb. tepidum.