Time-dependent self-assembly of 31-epimerically pure and mixed zinc methyl bacteriopheophorbides-d in an aqueous THF solution

Stereoselective self-aggregation of synthetic zinc 31-epimeric bacteriochlorophyll-d analogs possessing a methylene group at the 132-position as models of green photosynthetic bacterial chlorosomes

Zinc bacteriochlorophyll-d analogs possessing a methylene group at the 132-position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic 31-epimers were successfully separated by reverse phase HPLC to give diastereomerically pure samples. The stereochemistry of the chiral C31-center in the separated bacteriochlorophyll-d analogs was determined by HPLC analysis of the authentic stereoisomers prepared stereospecifically. Both the epimers were monomeric in tetrahydrofuran to give sharp absorption bands, while they self-aggregated to form chlorosomal oligomers with red-shifted bands in an aqueous Triton X-100 micelle solution. The resulting large oligomers deaggregated by addition of Triton X-100 to give monomeric species. Their aggregation and deaggregation were dependent on the 31-stereochemistry, indicating that each epimer produced self-aggregates that were supramolecularly different. The substitution with the 132-methylene group enhanced their self-aggregation abilities and the stability of their resulting self-aggregates.

Pathway complexity in the self-assembly of a zinc chlorin model system of natural bacteriochlorophyll J-aggregates

Self-assembly studies of a model compound of bacteriochlorophyll revealed the formation of nanoparticles as off-pathway and nanofibers as on-pathway products.

Whilst bacteriochlorophyll c, d, and e dyes self-assemble into the most efficient light harvesting J-aggregate systems found in nature, their supramolecular packing arrangements are still a matter of debate and a significant number of models have been suggested for their local and long-range ordering. Here we reveal for a synthetic model system based on a zinc chlorin (ZnChl) dye an intriguing interplay of two competing aggregation pathways by kinetic and thermodynamic studies in MeOH/water solvent mixtures: the formation of kinetically controlled off-pathway nanoparticles consisting of excitonically coupled J-dimers versus the formation of thermodynamically more stable one-dimensional helical fibers consisting of J-coupled extended aggregates. The higher order of the latter is evidenced by atomic force microscopy and a more narrow absorption spectrum of the J-aggregates. Based on a recently developed thermodynamic model that combines the cooperative K₂–K growth model with a competing dimerization model, an energy landscape could be derived that describes the pathway complexity of this biomimetic system. Our studies reveal that the kinetic stability of the off-pathway nanoparticles increases with increasing concentration of ZnChl or water content in a MeOH/water solvent mixture. For a water content >90% deeply trapped off-pathway nanoparticle products are formed that do not transform anymore to the more ordered thermodynamic product within reasonable time scales. Based on these observations, we hypothesize that out-of-equilibrium aggregate structures of natural BChl dyes may also exist in the natural chlorosomes of green bacteria.

Crystal structure and solvent-dependent behaviours of 3-amino-1,6-diethyl-2,5,7-trimethyl-4,4-di-phenyl-3a,4a-di-aza-4-bora-s-indacene

In the title compound (3-amino-4,4-diphenyl-BODIPY), C28H32BN3, the central six-membered ring has a flattened sofa conformation, with one of the N atoms deviating by 0.142 (4) Å from the mean plane of the other five atoms, which have an r.m.s. deviation of 0.015 Å. The dihedral angle between the two essentially planar outer five-membered rings is 8.0 (2)°. In the crystal, mol-ecules are linked via weak N-H⋯π inter-actions, forming chains along [010]. The com-pound displays solvent-dependent behaviours in both NMR and fluorescence spectroscopy. In the 1H NMR spectra, the aliphatic resonance signals virtually coalesce in solvents such as chloro-form, di-chloro-methane and di-bromo-ethane; however, they are fully resolved in solvents such as dimethyl sulfoxide (DMSO), methanol and toluene. The excitation and fluorescence intensities in chloro-form decreased significantly over time, while in DMSO the decrease is not so profound. In toluene, the excitation and fluorescent intensities are not time-dependent. This behaviour is presumably attributed to the assembly of 3-amino-4,4-diphenyl-BODIPY in solution that leads to the formation of noncovalent structures, while in polar or aromatic solvents, the formation of these assemblies is disrupted, leading to resolution of signals in the NMR spectra.

Stereoselective Self-Aggregation of 31 -Epimerically Pure Amino Analogs of Zinc Bacteriochlorophyll-d in an Aqueous Micelle Solution

Zinc bacteriochlorophyll-d analogs possessing an amino group instead of the original hydroxy group at the C3¹ position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic 3¹ -epimers were successfully separated by reverse phase HPLC to give diastereomerically pure samples. The stereochemistry of the chiral C3¹ -center in the separated amines was determined by NMR analysis of their diastereomeric amides as well as by their asymmetric synthesis from authentic stereoisomers. Both the epimers were monomeric in tetrahydrofuran to give sharp electronic absorption bands, while they self-aggregated to form chlorosomal oligomers with the redshifted bands in an aqueous Triton X-100 micelle solution (pH = 6.9). The resulting oligomers deaggregated by addition of p-toluenesulfonic acid to give monomeric N-protonated ammonium species. The aggregation and deaggregation were dependent on the 3¹ -stereochemistry, indicating that each epimer produced supramolecularly different self-aggregates.

The length of esterifying alcohol affects the aggregation properties of chlorosomal bacteriochlorophylls

Chlorosomes, the main light-harvesting complexes of green photosynthetic bacteria, contain bacteriochlorophyll (BChl) molecules in the form of self-assembling aggregates. To study the role of esterifying alcohols in BChl aggregation we have prepared a series of bacteriochlorophyllide c (BChlide c) derivatives differing in the length of the esterifying alcohol (C(1), C(4), C(8) and C(12)). Their aggregation behavior was studied both in polar (aqueous buffer) and nonpolar (hexane) environments and the esterifying alcohols were found to play an essential role. In aqueous buffer, hydrophobic interactions among esterifying alcohols drive BChlide c derivatives with longer chains into the formation of dimers, while this interaction is weak for BChlides with shorter esterifying alcohols and they remain mainly as monomers. All studied BChlide c derivatives form aggregates in hexane, but the process slows down with longer esterifying alcohols due to competing hydrophobic interactions with hexane molecules. In addition, the effect of the length of the solvent molecules (n-alkanes) was explored for BChl c aggregation. With an increasing length of n-alkane molecules, the hydrophobic interaction with the farnesyl chain becomes stronger, leading to a slower aggregation rate. The results show that the hydrophobic interaction is the driving force for the aggregation in an aqueous environment, while in nonpolar solvents it is the hydrophilic interaction.

Self-aggregation of synthetic zinc chlorophyll derivative possessing a perfluoroalkyl group in a fluorinated solvent

Zinc 3(1)-hydroxy-13(1)-oxo-chlorin possessing a perfluoroheptyl group on the 17-propionate was prepared by modifying chlorophyll a. The synthetic compound self-aggregated in 0.1% (v/v) THF and HCFC225cb (CClF2CF2CHClF) to give similar large oligomers to natural light-harvesting antennae of green photosynthetic bacteria and their models. Visible absorption, circular dichroism, and fluorescence emission spectra showed that F-F interaction between the perfluoroheptyl side chain and the fluorinated solvent (HCFC225cb) stabilized the supramolecular structure. The core part of the supramolecular structure was constructed by a special bond of Zn...O3(1)-H...O=C13(1) and pi-pi stacking, and the 17-propionates were at the peripheral part.