Physical and biophysical studies of membrane-membrane and membrane-protein interactions require well controlled model systems. We have developed a new method to obtain a single lipid bilayer floating in excess of water, in the vicinity of a fixed bilayer. It has been prepared in the gel phase, which is not biophysically relevant. The free bilayer has been precisely characterized by Neutron and X-Ray reflectivity. By increasing the temperature up to the melting transition of the lipid chains, one can observe a spectacular change in bilayer-bilayer distance. We attribute this swelling to the bilayer's interaction of Helfrich due to the bilayer fluctuations. Theoretical implications will be discussed, allowing us to measure the bending modulus of the membrane in a large range of temperatures. In order to characterize more precisely the fluctuations of the membrane we have developed X-Ray off-specular reflectivity leading to a direct measurement of the fluctuations spectrum of the membrane. As a conclusion we have obtained a free lipid bilayer, fluid and stable, biophysically relevant substrate for bilayer-bilayer or protein-bilayer interaction studies. This allows us to characterize the fluctuations spectrum of liquid membrane in a large range of temperatures. Enantioselective reactions starting from heterochiral mixtures of precursors might have been involved during the formation of homochiral biopolymers of life at prebiotic times. Since reactions in isotropic media would lead to heterochiral products, a way to obtain oligopeptides of homochiral sequences from racemic reactants would be through the assembly of the precursor molecules into ordered architectures followed by lattice-controlled reactions. Two-dimensional (2-D) self-assemblies of amphiphilic molecules formed on water surface provide an ideal medium. Racemates of reactive α-amino acid amphiphiles were designed to self-assemble, on water surface, into 2-D crystallites of three types: racemic compounds, enantiomorphous conglomerates and enantiomerically disordered solid-solutions. Such self-assemblies could undergo polymerization to yield mixtures of oligopeptides of enantiomeric composition controlled by the packing motif of the precursor crystalline phase. The 2-D self-assembly of various racemates and the reaction induced by an appropriate catalyst were studied, directly on water surface, by grazing incidence X-ray diffraction using synchrotron radiation. The enantiomeric composition of the oligopeptides collected from the interface was determined by matrix-assisted laser-desorption-ionization time-of-flight mass spectrometry using deuterium enantio-labeled monomers. Or results show that polymerization within racemic 2-D crystallites can lead to the enhanced generation of oligopeptides with homochiral sequences through reaction between translation-related rather than glide-related molecules. When the reaction occurred primarily between glide-related molecules, heterochiral oligopeptides were obtained. The latter polymerization was taken advantage of t...

The feasibility of generating oligopeptides with homochiral sequence via lattice-controlled polymerization of racemic mixtures of precursor molecules that undergo spontaneous segregation into two-dimensional (2-D) enantiomorphous domains at the air-aqueous solution interface was analyzed. For model systems, we studied the polymerization reaction within 2-D crystalline domains of mixtures of (R,S)-N(epsilon)-stearoyl-thio-lysine with approximately 10% (R,S)-N(epsilon)-stearoyl-lysine, and (R,S)-N(alpha)-carboxyanhydride of N(epsilon)-stearoyl-lysine. According to in situ grazing incidence X-ray diffraction (GIXD) measurements at the air-water interface, the molecules form 2-D crystallites packing by translation symmetry only. Oligopeptides 4-6 units long were obtained at the air-solution interface after injection of an appropriate catalyst into the subphase. The course of the chemical transformations was monitored by GIXD. The distribution of the diastereoisomeric oligopeptides was determined by matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF MS) mass spectrometry on samples prepared from precursor molecules enantioselectively labeled with deuterium. The experimental relative abundance of oligopeptides with homochiral sequence was found to be larger than that calculated for a theoretical random process, yielding an excess by a factor of 2.5-3.5 for the tetra- to hexapeptides. The present studies may be relevant for probing the role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions.

A possible role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated alpha-amino acids, in racemic and chiral non-racemic forms, which had self-assembled into two-dimensional (2D) ordered crystallites at the air-aqueous solution interface. As model systems we studied N(epsilon)-stearoyl-lysine thioethyl ester (C(18)-TE-Lys), gamma-stearyl-glutamic thioethyl ester (C(18)-TE-Glu), N(alpha)-carboxyanhydride of gamma-stearyl-glutamic acid (C(18)-Glu NCA) and gamma-stearyl-glutamic thioacid (C(18)-thio-Glu). According to in-situ grazing incidence X-ray diffraction measurements on the water surface, (R,S)-C(18)-TE-Lys, (R,S)-C(18)-TE-Glu, and (R,S)-C(18)-Glu-NCA amphiphiles self-assembled into ordered racemic 2D crystallites. Oligopeptides 2-12 units long were obtained at the air-aqueous solution interface after injection of appropriate catalysts into the water subphase. The experimental relative abundance of oligopeptides with homochiral sequence generated from (R,S)-C(18)-TE-Lys and (R,S)-C(18)-TE-Glu, as determined by mass spectrometry on enantioselectively deuterium-labeled samples, was found to be significantly larger than that obtained from (R,S) C(18)-thio-Glu which polymerizes randomly. An efficient chiral amplification was obtained in the polymerization of non-racemic mixtures of C(18)-Glu-NCA since the monomer molecules in the racemic 2D crystallites are oriented such that the reaction occurs between heterochiral molecules related by glide symmetry to yield heterochiral oligopeptides whereas the enantiomer in excess, in the enantiomorphous crystallites, yield oligopeptides of a single handedness.