The Nonbilayer Lipid MGDG and the Major Light-Harvesting Complex (LHCII) Promote Membrane Stacking in Supported Lipid Bilayers

The thylakoid membrane of algae and land plants is characterized by its intricate architecture, comprising tightly appressed membrane stacks termed grana. The contributions of individual components to grana stack formation are not yet fully elucidated. As an <i>in vitro</i> model, we use supported lipid bilayers made of thylakoid lipid mixtures to study the effect of major light-harvesting complex (LHCII), different lipids, and ions on membrane stacking, seen as elevated structures forming on top of the planar membrane surface in the presence of LHCII protein. These structures were examined by confocal laser scanning microscopy, atomic force microscopy, and fluorescence recovery after photobleaching, revealing multilamellar LHCII–membrane stacks composed of connected lipid bilayers. Both native-like and non-native interactions between the LHCII complexes may contribute to membrane appression in the supported bilayers. However, applying <i>in vivo</i>-like salt conditions to uncharged glycolipid membranes drastically increased the level of stack formation due to enforced LHCII–LHCII interactions, which is in line with recent crystallographic and cryo-electron microscopic data [Wan, T., et al. (2014) <i>Mol. Plant 7</i>, 916–919; Albanese, P., et al. (2017) <i>Sci. Rep. 7</i>, 10067–10083]. Furthermore, we observed the nonbilayer lipid MGDG to strongly promote membrane stacking, pointing to the long-term proposed function of MGDG in stabilizing the inner membrane leaflet of highly curved margins in the periphery of each grana disc because of its negative intrinsic curvature [Murphy, D. J. (1982) <i>FEBS Lett. 150</i>, 19–26]

A Versatile Dinucleating Ligand Containing Sulfonamide Groups

Copper, iron, and gallium coordination chemistries of the new pentadentate bis-sulfonamide ligand 2,6-bis­(<i>N</i>-2-pyridylmethylsulfonamido)-4-methylphenol (psmpH₃) were investigated. PsmpH₃ is capable of varying degrees of deprotonation, and notably, complexes containing the fully trideprotonated ligand can be prepared in aqueous solutions using only divalent metal ions. Two of the copper­(II) complexes, [Cu₂(psmp)­(OH)] and [Cu₂(psmp)­(OAc)₂]⁻, demonstrate the anticipated 1:2 ligand/metal stoichiometry and show that the dimetallic binding site created for exogenous ligands possesses high inherent flexibility since additional one- and three-atom bridging ligands bridge the two copper­(II) ions in each complex, respectively. This gives rise to a difference of 0.4 Å in the Cu···Cu distances. Complexes with 2:3 and 2:1 ligand/metal stoichiometries for the divalent and trivalent metal ions, respectively, were observed in [Cu₃(psmp)₂­(H₂O)] and [M­(psmpH)­(psmpH₂)], where M = Ga^III, Fe^III. The deprotonated tridentate <i>N</i>-2-pyridylsulfonylmethylphenolato moieties chelate the metal ions in a meridional fashion, whereas in [Cu₃(psmp)₂­(H₂O)] the rare μ₂-<i>N</i>-sulfonamido bridging coordination mode is observed. In the bis-ligand mononuclear complexes, one picolyl arm of each ligand is protonated and uncoordinated. Magnetic susceptibility measurements on the doubly and triply bridged dicopper­(II) complexes indicate strong and medium strength antiferromagnetic coupling interactions, with <i>J</i> = 234 cm^–1 and 115 cm^–1 for [Cu₂(psmp)­(OH)] and [Cu₂(psmp)­(OAc)₂]⁻, respectively (in H_HDvV =...+<i>JS</i>₁<i>S</i>₂ convention). The trinuclear [Cu₃(psmp)₂­(H₂O)], in which the central copper ion is linked to two flanking copper atoms by two μ₂-<i>N</i>-sulfonamido bridges and two phenoxide bridges shows an overall magnetic behavior of antiferromagnetic coupling. This is corroborated computationally by broken-symmetry density functional theory, which for isotropic modeling of the coupling predicts an antiferromagnetic coupling strength of <i>J</i> = 70.5 cm^–1