The Length of Esterifying Alcohol Affects the Aggregation Properties of Chlorosomal Bacteriochlorophylls

Abstract

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.

Synthesis of Zinc 3-Hydroxymethyl-porphyrins Possessing Carbonyl Groups at the 13- and/or 15-Positions for Models of Self-Aggregative Chlorophylls in Green Photosynthetic Bacteria

“…It is well-known that metal complexes of cyclic tetrapyrroles are useful for preparation of functional supramolecules, for example, excitonically delocalized J -aggregates in main light-harvesting antenna systems of green photosynthetic bacteria (called chlorosomes). , Major composite molecules in chlorosomes are bacteriochlorophyll(BChl)s- c / d / e , which have hydroxy and carbonyl groups as well as a coordinative magnesium metal on the y axis (a line through N21 and N23, left drawing in Figure ). − These three moieties are essentially required for making chlorosome-like self-aggregates based on coordination bonding (3 1 -O···Mg), hydrogen bonding (3 1 -O−H···OC-13) and strong π−π stacking. Many synthetic studies on clarifying the structural requirements of chlorosomal self-aggregation have been reported, and three types of π-conjugated systems, porphyrin, − chlorin, − and bacteriochlorin, − have been found acceptable for this self-aggregation. We focused on a porphyrin π-system in this report, because the fully conjugated π-system is significantly stable, and the intense Soret band of the aggregated porphyrin situated at around 400−550 nm is well-matched with the sunlight spectrum.…”

The lamellar spacing in self-assembling bacteriochlorophyll aggregates is proportional to the length of the esterifying alcohol

“…The three main scattering peaks of BChl c yielded a unit cell for the aggregate arrangement and the corresponding cell dimensions are compatible with a dimer of BChl c molecules in the asymmetric unit ). Self-assembly experiments with BChl c and BChlide c molecules revealed an important role of the esterifying alcohols, and favor structural models where alcohol tails extend to both sides of the lamellar layer (Klinger et al 2004;Zupcanova et al 2008). Such an arrangement facilitates the hydrophobic interaction between esterifying alcohols from the neighboring layers and provides plausible explanation for the formation of lamellae.…”

Temperature-dependent spectral changes of self-aggregates of zinc chlorophylls esterified by different linear alcohols at the 17-propionate

“…Linear alkyl chains of the semi-synthetic magnesium chlorins also little changed the visible absorption spectra and sizes of their self-aggregates in Triton X-100 micelles, although they would affect deaggregation behaviour by additive 1-hexanol (17). Recently, Zupcanova et al (18) also reported self-assembly of BChl c esterified by several alcohols in aqueous buffer and nonpolar alkanes, where the length of esterifying alcohols affected Q y absorption bands in their visible absorption spectra. Both in vivo and in vitro studies suggested that esterifying alcohols at the 17-propionate had slight influence on BChl organisation in chlorosomal self-aggregates.…”