The length of esterifying alcohol determines the aggregation properties of chlorosomal bacteriochlorophylls
Anita Župčanová1,2, Juan B.
Arellano3, Jakub Pšenčík2,4 and František Vácha1,2
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.