PARENT SESSION

Symposium S3A Bacteriochlorophyll based antenna systems
Tuesday August 31st, 2004 10:20 AM-12:20 PM Room 511D
Chair: Neil Hunter
Co-Chair: Richard Cogdell

Self-organization of BChl c in chlorosomes studied by MAS NMR of aggregated Cd-chlorins. Ido de Boer^*,1, Joerg Matysik¹, Shin-ich Sasaki², Tomohiro Miyatake², Shiki Yagai², Hitoshi Tamiaki², Alfred Holzwarth³, Huub de Groot¹, ¹ Biophysical Organic Chemistry, Leiden, the Netherlands² Department of Bioscience and Biotechnology, Kusatsu, Japan³ Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany

ABSTRACT- Magic Angle Spinning (MAS) NMR has been used to investigate the molecular mechanisms behind the self-organization of BChl c in chlorosomal light-harvesting antennae. Two ¹³C and ¹⁵N enriched aggregated bacteriochlorophyll c analogues were studied. The chlorin model molecules differ from the natural BChl c in the central metal, the 3-, 12- and 20-side chains. One model system has the farnesyl tail replaced by a methyl, while the other has a stearyl tail. Detailed structural information of these model aggregates was obtained using MAS NMR. An essential element in the process of self-organization is OH---Cd coordination, very similar to the OH---Mg coordination in the natural system. Anomalously large ring current shifts up to 10 ppm reveal a very dense stacking of the molecules in planar layers, different from the chlorosomes. From this dense stacking it is deduced that the bulkiness of the 3- and 20-side chains determine the overall density of the chlorophyll stacking. In addition, our model structures confirm and validate the essential role of the [3¹ R ]- and [3¹ S]-stereoisomers in the formation of the chlorosomal antennae, since tubular structures are not formed without this chirality. With the tail truncated by methyl, a microcrystalline solid is formed with favorable interactions between the planar sheets in a head-to-tail orientation. The stearyl tails lead to a more disordered aggregate consisting of both syn and anti layers similar to the chlorosomes, as indicated by a doubling of the N-D signal. These results provide fundamental insight into the self-assembly of BChl c in the chlorosomes and reveal a balance between relatively strong local interactions and contributions to the free energy of the system associated with a longer distance scale.

KEY WORDS: chlorosomes, MAS NMR, bacteriochlorophyll, self-assembly