Experimental and theoretical considerations of mechanisms controlling cation effects on thylakoid membrane stacking...

The roles of specific cation binding, charge neutralization and electrostatic screening mechanisms in controlling salt-induced stacking and chlorophyll fluorescence changes in thylakoid membranes are examined in the light of new experimental evidence and theoretical calculations of the forces between membrane surfaces. A comparison of the biphasic stacking and fluorescence phenomena generated by organic mono- and divalent cations known sterically to inhibit specific binding with the effects generated by inorganic mono- and divalent cations suggests that the observed salt-induced changes at pH greater than or equal to 7.5 are predominantly governed by the electrostatic screening mechanism in agreement with previous work (e.g. Barber, J., Mills, J.D. and Love, A. (1977) FEBS Lett. 74, 174-181). Detailed calculations of the coulombic double layer repulsive force between negatively charged membrane surfaces immersed in a mixed electrolyte of valence type Z1+/Z1-,Z2+/Z1- were performed both under the constraints of fixed surface charged density and fixed surface potential. From a close comparison of the theoretical results with new experimental data on salt-induced stacking and fluorescence changes and a consideration of the contributions of the 'hydration' repulsive force and the van der Waals attractive force, it is argued that a reduction in surface charge density alone by lateral diffusion is probably insufficient to realize membrane stacking and that an increase in the van der Waals attractive force is necessary to account for the experimental observations perhaps through the formation of protein rich domains. In view of the complexity of the thylakoid membranes, the conclusions are to be considered qualitative. Nevertheless, these calculations give support to a model in which the cation induced chlorophyll fluorescence and stacking changes can be explained by lateral diffusion of two types of pigment protein complexes in the lipid matrix of the membrane. Such diffusion gives rise to changes in energy transfer between Photosystem II and Photosystem I and also to the creation of domains having low and high electrical surface charge density.