Determination of interaction forces between higher plant thylakoids and electron-density-profile evaluation using small-angle X-ray scattering

Osmotic dehydration of spinach thylakoids has enabled us to calculate the low-resolution (1.0-1.5 nm) electron density profiles of the thylakoids at Bragg periods between 16 and 23 nm. Electron density profile calculation was performed by a least-squares procedure and controlled by mapping the experimental scattering intensities on the continuous Fourier transform of the electron density profiles. The results show that the outsides of opposing thylakoids are appressed closely together, whereas the luminal extent of the thylakoid is a function of the applied osmotic pressure, thus determining the Bragg period. It could be shown that the distance of the inner surfaces of the thylakoid is established by the equilibrium between externally applied osmotic pressure and the electrostatic repulsion at the inner Ouminal) surface. The interaction can be modified by the luminal electrolyte concentration. The area per electric charge on the thylakoid inner surface was estimated to be 1.4 ± 0.2 nm•