Green photosynthetic bacteria efficiently perform the light-harvesting process using giant antennas known as chlorosomes, which are formed through the self-aggregation of bacteriochlorophyll(BChl)- c or d molecules via noncovalent interactions, such as intermolecular coordination, hydrogen bonding, and π–π interactions. The chlorosomal cores consist of tubular and/or sheet-like supramolecular polymers in which the constituent BChl molecules are regularly arranged without supporting from any proteins. The stability of these supramolecular polymers is related not only on the aforementioned noncovalent interactions but also on the peripheral 17-propionate residue on the core cyclic tetrapyrrole. However, the effect of esterified branched hydrocarbon chains in the 17-propionate residue, including a farnesyl group on the supramolecular polymer structure remains to be elucidated. To investigate the morphology of supramolecular polymers bearing such branched hydrocarbon chains, the following model compounds were synthesized: zinc BChl- d analogs, Zn- 1 and Zn- 2 , with two different-length alkyl chains in the 17-propionate residue. In methylcyclohexane (MCH) as a low-polarity organic solvent containing a small amount of tetrahydrofuran (THF), three distinct supramolecular polymer morphologies, rope-, ring-, and sheet-like, were observed for doubly octadecylated Zn- 1 , that depend on the analog concentration and THF content. By contrast, Zn- 2 with a shorter branched alkyl chain, formed rod- and sheet-like supramolecular polymers that could be selectively prepared by changing its concentration in a mixture of MCH and THF. These results reveal that the steric bulkiness and conformational flexibility of branched alkyl chains govern the balance between one-dimensional stacking and lateral association, thereby controlling supramolecular polymorphism. This provides a molecular design principle for tuning the morphology and structural diversity of chlorophyll-based supramolecular polymers. • Supramolecular polymorphism was observed in alkyl esterified Zn-BChl- d analogs. • UV–Vis–NIR, CD, and AFM studies revealed their distinct J -aggregate structures. • Rope-, ring-, and sheet-like aggregates were built in branched alkyl chain analogs. • Linear alkyl chain analogs exclusively formed rod-shaped supramolecular polymers. • Peripheral alkyl chains controlled multiform supramolecular polymer morphologies.
Hara et al. (Sun,) studied this question.