Self‐Assemblies of Zinc Bacteriochlorophyll‐d Analogues Having Amide, Ester, and Urea Groups as Substituents at 17‐Position and Observation of Lamellar Supramolecular Nanostructures

Abstract

Chlorosomes are unique light-harvesting apparatuses in photosynthetic green bacteria. Single chlorosomes contain a large number of bacteriochlorophyll (BChl)-c, -d, -e, and -f molecules, which self-assemble without protein assistance. These BChl self-assemblies involving specific intermolecular interactions (Mg⋅⋅⋅O3 -H⋅⋅⋅O=C13 and π-π stacks of chlorin skeletons) in a chlorosome have been reported to be round-shaped rods (or tubes) with diameters of 5 or 10 nm, or lamellae with a layer spacing of approximately 2 nm. Herein, the self-assembly of synthetic zinc BChl-d analogues having ester, amide, and urea groups in the 17-substituent is reported. Spectroscopic analyses indicate that the zinc BChl-d analogues self-assemble in a nonpolar organic solvent in a similar manner to natural chlorosomal BChls with additional assistance by hydrogen-bonding of secondary amide (or urea) groups (CON-H⋅⋅⋅O=CNH). Microscopic analyses of the supramolecules of a zinc BChl-d analogue bearing amide and urea groups show round- or square-shaped rods with widths of about 65 nm. Cryogenic TEM shows a lamellar arrangement of the zinc chlorin with a layer spacing of 1.5 nm inside the rod. Similar thick rods are also visible in the micrographs of self-assemblies of zinc BChl-d analogues with one or two secondary amide moieties in the 17-substituent.

Bioinspired supramolecular nanosheets of zinc chlorophyll assemblies

“…This indicates the presence of the 2 1 screw axis along the c -axis. The 0.69-nm distance between adjacent zinc chlorophyll molecules was similar to previously reported results for microcrystalline of a cadmium BChl- d analog 26 as well as self-assemblies of Zn-3 25 (Fig. 1c, bottom), the 3 1 -hydroxylated analog of Zn-2 .…”

“…1c) 20–24 . Recently, we have reported that a zinc 3 1 -hydroxy-13 1 -oxo-chlorin having amide and urea groups in the 17-substituent self-assembled to form rod-like supramolecular nanostructures with a spacing of 1.4 nm 25 . In this study, we synthesized zinc 3 1 -methoxy-chlorophyll derivatives lacking the 3 1 -hydroxy group, which was requisite for chlorosomal J -aggregation through specific hydrogen bonding (3 1 -O–H∙∙∙O=C-13).…”

Recent Progress in Hydrogen-Bonded π-Conjugated Systems Displaying J-Type Aggregates

“…Some of these studies also presented the ester-containing analogues as control molecules, not showing the same supramolecular structures and strongly coupled J-aggregates, highlighting the important role of having directional H-bonding groups. 76…”

Multipronged Biomimetic Approach To Create Optically Tunable Nanoparticles

“…Firstly, chemical modifications to a chlorin dye (reduced porphyrin) were made to replicate the π–π stacking, hydrogen bonding, and metal–oxygen coordination observed in natural chlorosome supramolecular assemblies, thus enabling spectrally tunable photonic properties. Secondly, a lipophilic oleylamine moiety was conjugated to the chlorin at the 17‐position to anchor these dyes stably within the HDL interior, thus promoting the completion of ordered dye packing, and its sidechain amide group offered additional intermolecular amide hydrogen bonds . Thirdly, a phospholipid monolayer shell resembling the natural chlorosome's exterior lipid monolayer was coated on the hydrophobic porphyrin assembly and constrained with an apolipoprotein mimetic peptide, bestowing the controlled size and favorable biological behavior of HDL.…”