Reversible changes in the circular dichroism of phytochrome during photoisomerisation of the pigment

Abstract

This chapter discusses the circular dichroism (CD) and optical rotatory dispersion (ORD) of photosynthetic organelles and their component pigments. The measurement of optical rotation, particularly CD, spectra can provide information relevant to a number of interesting molecular properties of chlorophylls. These include (1) determination of the absolute configurations at asymmetric carbon atoms of isolated chlorophylls; (2) characterization of the geometries of chlorophylls in dimers and higher aggregates; (3) evidence for the presence, number, and structures of associated chlorophylls in photosynthetic lamellae and information about changes in interaction on activation by light; and (4) evidence bearing on the interactions of asymmetric environments, such as the protein matrix of the naturally occurring chlorophyll complexes. The CD and magnetic circular dichroism (MCD) spectral changes associated with the conversion of protochlorophyll to chlorophyll following etiolation in higher plants are investigated in the chapter. There are important changes that occur as chloroplasts synthesize more chlorophyll during maturation. By using absorption, fluorescence, and structural measurements, the CD measurements provide a valuable tool in investigating the nature of pigment interactions in photosynthetic organelles.