Chiral Amplification of Oligopeptides in Two-Dimensional Crystalline Self-Assemblies on Water

Abstract

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...

Mineral Surfaces, Geochemical Complexities, and the Origins of Life

“…These natural chiral surface environments occur in both left-and right-handed variants in approximately equal proportions (Frondel 1978;Evgenii and Wolfram 1978). Nevertheless, the widespread occurrence of local chiral environments provided the prebiotic Earth with innumerable sites for experiments in chiral selection and organizationexperiments that may have led, through a process of chiral amplification, to a fortuitous, selfreplicating homochiral entity (Bonner 1991(Bonner , 1995Lippmann and Dix 1999;Zepik et al 2002;Klussman et al 2006Klussman et al , 2007Noorduin et al 2008). …”

Molecular chirality at surfaces

“…By tuning the B-field such that the prochiral molecules are oriented in an oblique angle with respect to a highly symmetric substrate lattice during adsorption, chiral symmetry breaking is induced, because adsorption into a one-mirror domain is preferred over the other. Finally, it is noteworthy that chiral amplification by the formation of homochiral oligopeptides has been observed, starting from racemic 2D crystalline selfassembled monolayer of amino acids on water [168].…”

Two-Step Enantio-Selective Optical Switch

“…Asymmetric synthesis [1] and chiral purification [2,3,4] of a ("racemic") mixture of enantiomers (chiral molecules and their mirror images) are among the most important and difficult tasks in chemistry. The possibility of achieving purification solely by optical means has been also theorized [5,6,7,8].…”