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

Anita Župčanová^1,2, Juan B. Arellano³, Jakub Pšenčík^2,4 and František Vácha^1,2

¹Institute of Plant Molecular Biology, Academy of Sciences of the Czech Republic,

Branisovska 31, 370 05 Ceske Budejovice, Czech Republic.

²Institute of Physical Biology, University of South Bohemia, Zamek 136, 373 33 Nove

Hrady, Czech Republic.

³Instituto de Recursos Naturales y Agrobiología (IRNASA-CSIC), Apdo. 257, 37071

Salamanca, Spain.

⁴Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16

Prague, Czech Republic.

Keywords: Bacteriochlorophyll aggregates, chlorosomes, esterifying alcohol, hydrophobic interactions.

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 (C1, C4, C8 and C12). Their aggregation behaviour was studied both in polar (aqueous buffer) and non-polar (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 alcohol 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 molecule, 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 aqueous environment, while in non-polar solvents it is the hydrophilic interaction. The data also suggest that BChl dimers are the building blocks of BChl c aggregates.