Self-assembly of [Et,Et]-Bacteriochlorophyll cF on Highly Oriented Pyrolytic Graphite Revealed by Scanning Tunneling Microscopy

• Published in: Photochemistry and photobiology 2002-06, Vol.75 (6), p.619-626
• Main Authors: Möltgen, H., Kleinermanns, K., Jesorka, A., Schaffner, K., Holzwarth, A. R.
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Bacteriochlorophylls - chemistry; Disulfides - chemistry; Graphite; Hot Temperature; Microscopy, Scanning Tunneling - methods; Models, Molecular; Molecular Conformation; Molybdenum - chemistry; PHOTOSYNTHESIS AND BIO/CHEMILUMINESCENCE
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1562/0031-8655(2002)075<0619:SAOEEB>2.0.CO;2

The chlorosomal light-harvesting antennae of green phototrophic bacteria consist of large supramolecular aggregates of bacteriochlorophyll c (BChl c). The supramolecular structure of (31-R/S)-BChl c on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) has been investigated by scanning tunneling microscopy (STM). On MoS2, we observed single BChl c molecules, dimers or tetramers, depending on the polarity of the solvent. On HOPG, we observed extensive self-assembly of the dimers and tetramers. We propose C=O · · · H–O · · · Mg bonding networks for the observed dimer chains, in agreement with former ultraviolet–visible and infrared spectroscopic work. The BChl c moieties in the tetramers are probably linked by four C=O · · · H–O hydrogen bonds to form a circle and further stabilized by Mg · · · O–H bondings to underlying BChl c layers. The tetramers form highly ordered, distinct chains and extended two-dimensional networks. We investigated semisynthetic chlorins for comparison by STM but observed that only BChl c self-assembles to well-structured large aggregates on HOPG. The results on the synthetic chlorins support our structure proposition.