Strand directionality affects cation binding and movement within tetramolecular G-quadruplexes

Nuclear magnetic resonance study of G-quadruplex structures formed by d(TG3T) and its modified analogs containing a 50-50 or 30-30 inversion of polarity sites, namely d(30TG50-50G2T30), d(30T50- 50G3T30) and d(50TG30-30G2T5’) demonstrates formation of G-quadruplex structures with tetrameric topology and distinct cation-binding preferences. All oligonucleotides are able to form quadruplex structures with two binding sites, although the modified oligonucleotides also form, in variable amounts, quadruplex structures with only one bound cation. The inter-quartet cavities at the inversion of polarity sites bind ammonium ions less tightly than a naturally occurring 50-30 backbone. Exchange of 15NH+ 4 ions between G-quadruplex and bulk solution is faster at the 30-end in comparison to the 50-end. In addition to strand directionality, cation movement is influenced by formation of an all-syn G-quartet. Formation of such quartet has been observed also for the parent d(TG3T) that besides the canonical quadruplex with only all-anti G-quartets, forms a tetramolecular parallel quadruplex containing one all-syn G-quartet, never observed before in unmodified quadruplex structures

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. I. P-31-NMR Spectroscopy

Build-up of the energized state of thylakoid membranes and the synthesis of ATP are warranted by organizing their bulk lipids into a bilayer. However, the major lipid species of these membranes, monogalactosyldiacylglycerol, is a non-bilayer lipid. It has also been documented that fully functional thylakoid membranes, in addition to the bilayer, contain an inverted hexagonal (H-II) phase and two isotropic phases. To shed light on the origin of these non-lamellar phases, we performed P-31-NMR spectroscopy experiments on sub-chloroplast particles of spinach: stacked, granum and unstacked, stroma thylakoid membranes. These membranes exhibited similar lipid polymorphism as the whole thylakoids. Saturation transfer experiments, applying saturating pulses at characteristic frequencies at 5 degrees C, provided evidence for distinct lipid phases-with component spectra very similar to those derived from mathematical deconvolution of the P-31-NMR spectra. Wheat-germ lipase treatment of samples selectively eliminated the phases exhibiting sharp isotropic peaks, suggesting easier accessibility of these lipids compared to the bilayer and the H-II phases. Gradually increasing lipid exchanges were observed between the bilayer and the two isotropic phases upon gradually elevating the temperature from 5 to 35 degrees C, suggesting close connections between these lipid phases. Data concerning the identity and structural and functional roles of different lipid phases will be presented in the accompanying paper