A Method for Studying Pigment Organization in Photosynthetic Complexes

The use of polarized light spectroscopy is an important way to study pigment-organization in photosynthetic particles. For instance, linear dichroism (LD) measurements are often used to obtain information about the orientations of absorption transition moment vectors within these particles, which is interesting in view of understanding excitation energy transfer, but it may also lead to knowledge about the pigment organization. To perform LD-measurements, particles are often ordered in biaxially compressed polyacrylamide gels. Knowledge about the average orientation of the particles in the gel is required, to obtain quantitative information about the average transition moment directions. The orientation distribution is often badly known. Mathemetical expressions for rod-like and disc-like particles have been presented by Ganago et al. [1], which relate the amount of gel-compression to the average particle orientation. Recently, we showed that for rod-like chlorosomes of Chloroflexus aurantiacus, ordered in gels, the formalism described in [1] does indeed lead to a good estimation of the average orientation [2].Unfortunately, mathematical expressions are not available for particles, which are not rod- or disc-like.