Linear and Circular Dichroism

Abstract

The efficiency of photosynthetic light energy conversion depends largely on the molecular architecture of the photosynthetic membranes. Linear and cicular dichroism (LD and CD) techniques have contributed significantly to our knowledge of the molecular organization of the pigment system in various complexes and membranes. Systematic LD studies have led to the recognition of an apparently universal property of pigment systems in vivo: all pigments in all photosynthetic organisms display non-random orientation with respect to each other, to the protein axes and to the membrane plane. This molecular organization plays an important role in the energy transfer between pigment molecules. CD spectroscopy is widely used for the detection of excitonic interactions, which have been found to occur in virtually all reaction center and antenna complexes. Excitonic CD carries information on the distances and orientation of the interacting pigment molecules. CD is also capable of revealing information about certain macro- organizational parameters in molecular aggregates with sizes commensurate with the wavelength of visible light. These non-invasive techniques can be used for systems in a wide range of structural complexity, from isolated pigment molecules to whole organelles. CD and LD techniques have been extended to the (sub)picosecond time range. Combined with the methods of quantitative evaluation of data, these techniques will certainly remain indispensable in elucidation of the structure and function of the photophysical and photochemical apparatus. The purpose of this chapter is to provide an introduction to the theory and practice of LD and CD methods in photosynthesis. The main emphasis will be placed on the underlying principles and the basics of the experimental procedures, complemented with a few illustrations of results. I would also like to draw attention to a few recently introduced polarization techniques which are ripe for application in photosynthesis.