ORIENTED MOLECULES AND THE STRUCTURE OF CHLOROPLASTS 



R. A. Olson 

 with the collaborative assistance of W . H. Jennings 



The molecular organization of chlorophyll within the chloroplast is sug- 

 gested by nnicroscopy of chloroplast structures and by the physical-chemical 

 properties of chlorophyll preparations. The orderly fine structure of chloro- 

 plasts revealed by electron microscopy is one of the most regular of any or- 

 ganelle. The anatomical implication of molecular organization of chlorophyll 

 within the chloroplast however, arises mainly from the presence of the pigment 

 at the site of an orderly, repetitive array common to systems, i.e. , crystals 

 and mesophases, where such organization is known to exist. 



Investigations of certain physical and chemical properties of chlorophyll 

 preparations have demonstrated the capacity of the molecule to form dimers at 

 sufficiently high concentrations and to organize monomolecular films at air- 

 water interfaces . '^ • "^J Adsorption of chlorophyll on mesophase preparations 

 yields pronounced anisotropic optical properties . (-^ > ^' Within the practical 

 limits of resolution, the several techniques for detecting optical anisotropy pro- 

 vide a sensitive method for studying molecular organization. Both bifluores- 

 cence (polarized emission) and dichroism (polarized absorption) have been ob- 

 served in preparations of chlorophyll adsorbed on ammonium oleate or on 

 lecithin mesophases. These observations indicate an orientation of the chloro- 

 phyll molecules which depends upon the molecular organization of the adsorbate 

 molecules constituting the mesophase. 



The study of the optical anisotropy of chlorophyll within the chloroplast 

 has been far less rewarding than the mesophase observations . Among the ear- 

 liest investigations, Menke (1938) used imbibition procedures to alter the bire- 

 fringence and the weak dichroism of Closterium chloroplasts . ' ' Considering 

 the chloroplast a uniaxial negatively birefringent body (the optical axis lies per- 

 pendicular to the chloroplast face), he concluded that the textural or morphic 

 anisotropy arose fronn a lamellar fine structure with repetitive planes parallel 

 to the flat face of the organelle. This hypothesis is now confirmed by visual 

 microscopy and has been further elaborated by electron microscopy. Continued 

 studies by Menke and others of chlorophyll dichroism and anomalous dispersion 

 of birefringence yielded evidence for a low intrinsic anisotropy in vivo.''*""' 

 Goedheer (1957), studying the ribbon shaped Mougeotia chloroplast, measured 

 a nnaximum dichroic ratio of 1 . 13 ± 0. 05 at 680 mfi. (4) He proposed that the 

 chlorophyll molecules were complexed with a globin frannework in such a way 

 that they were very slightly oriented. 



The bifluorescence of chloroplasts in Mesotaenium, Elodea, Mniunn, and 

 Oenothera was reported by Menke in 1958.(91 The red fluorescence from chloro- 

 plasts observed in profile aspect showed a partial linear polarization with the 

 E vector parallel to the lamellae. Menke attributed this effect to a statistical. 



545 



