Linear dichroism of chlorosomes from chloroflexus aurantiacus in compressed gels and electric fields

The linear dichroism of chlorosomes from Chloroflexus aurantiacus was measured between 250 and 800 nm. To orient the chlorosomes we used a new way of compressing polyacrylamide gels, where the dimension of the gel along the measuring light-beam is kept constant. The press required for such a way of compressing is relatively easy to construct. A theoretical description is given to interpret the measured linear dichroism in terms of the orientation of the transition moments. The results obtained with the polyacrylamide gels are compared with the linear dichroism measurements for chlorosomes oriented in electric fields. Both the spectral features as well as the absolute size of the linear dichroism signals are in reasonable agreement. We find that the transition moment corresponding to the 741 nm bacteriochlorophyll c (Bchl c) absorption band makes an angle of 20 degrees with the long axis of the chlorosome. For the 461 nm Bchl c band an angle of 30 degrees is found. Both angles are significantly lower than the values reported so far in literature and they imply that Bchl c is highly organized in the chlorosomes.