Structure of photosynthetic membranes of Euglena using X-ray diffraction

Novel X-ray diffraction results of membranes from chloroplasts of Euglena are presented, together with freeze-etch images obtained concurrently. Conditions were found for sharp lamellar reflections corresponding to ordered stacking of thylakoids. The periodicity measured by diffraction agrees well with that observed by microscopy. Intensities of diffraction were analysed in order to calculate the electron density distributions across the membranes. Some arguments in favour of the preferred phases of the reflection are given. The distributions indicate firstly the presence of 25 Å-wide regions where the hydrocarbon chains of the membrane lipids are concentrated. This result is discussed in terms of structural models for the chloroplast membrane. Comparison with results of freeze-etching indicates where in the density distribution are the regons inside and outside the membrane sacs. Secondly, the density distributions show maxima on the outside of the membranes only, corresponding possibly to an asymmetrical distribution of lipids.