Neutron diffraction studies of oriented photosynthetic membranes

Abstract

The structures of photosynthetic membranes from chloroplasts of Euglena gracilis and spinach were studied by neutron diffraction from oriented specimens. Orientation of the membranes was achieved in two different ways. (1) Centrifugation and controlled drying; (2) by applying a magnetic field to aqueous suspensions. The low-resolution measurements with different H 2O/ 2H 2O mixtures (contrast variation) support a previously proposed model for the membrane structure, in which the proteins are associated with a compact lipid layer. Membrane arrangements within the chloroplasts, such as membrane pairing, were characterized for several membrane preparations and ionic environments, including one very close to that present “ in vivo”. These membrane arrangements correspond to the presence of grana structure (close contact between exterior faces of membrane sacs) and to different water contents of the sac interior. From the neutron scattering intensity, it is demonstrated that the partition region of the grana structure is hydrophilic. The pair correlation function, calculated directly from the scattering intensity in 2H 2O medium, clearly shows two different separations of membranes within the chloroplast, and gives the disorder of these separations. The scattering is predominantly lamellar, but there is a small contribution from the arrangement of proteins in the plane of the membrane. Degrees of membrane orientation in the magnetic field are measured and related (Appendix) to the optical dichroism measurements of Breton, Michel-Villay & Paillotin.