Stereoselective polymerization of gamma-benzyl glutamate NCA

The Asymmetry of Existence: Do We Owe Our Existence to Cold Dark Matter and the Weak Force?

A common theme throughout biology is homochirality, including its origin and especially implications. Homochirality has also intrigued scientists because of the hypothesis that life, as it currently exists, could not have occurred without it. In this review, we discuss several hypotheses regarding homochirality and their linkage to processes that range from subatomic in scale to processes that help define the structure of the universe. More importantly, this exploration begins with the knowledge that humans inhabit the universe in which there is an excess of normal matter over antimatter. It is a universe characterized by homochirality but is nonetheless contained in what is most easily described as a 3+1 dimensional spacetime wherein most laws of physics are invariant under spacetime transformations. This restriction on spacetime poses significant constraints on the processes that can be invoked to explain homochirality. However, in dealing with such restraints, including the total mass contained in the universe, the concepts of cold dark matter and dark energy can be incorporated into cosmological models with resultant behaviors and predictions very much in accord with the findings of the cosmic background surveys. Indeed, the introduction of cold dark matter and dark energy to solve problems relating to the mass found in the universe may provide a means for generating the needed asymmetry to allow homochirality to arise.

An experimental investigation of the evolution of chirality in a potential dynamic peptide system: N-terminal epimerization and degradation into diketopiperazine

The APED model (activation-polymerization-epimerization-depolymerization) is a unique example of a chemical system that allows symmetry breaking through a dynamic process involving indirect network autocatalysis. In its simplest version, the autocatalytic behavior of this model partly relies on the reproduction of local chiral centers in dipeptides through an epimerization process, with a thermodynamic preference for homochiral chains. We studied the reactivity of di- and tripeptides, containing a N-terminal phenylglycine (Phg) residue, as model compounds for the experimental determination of the kinetic and thermodynamic parameters related to the N-terminal epimerization process. Although the N-terminal residue is prone to spontaneous epimerization, catalysis was required for the epimerization to reach the equilibrium state in reasonable time. Unexpectedly, the observed equilibrium diastereoisomeric excesses have shown a general tendency for more stable heterochiral peptides, especially strong in the case of dipeptides. In parallel to this process, a stereoselective peptide cleavage through diketopiperazine formation was observed. Contrary to the N-terminal epimerization of peptides, the diketopiperazine formation did not need any catalyst, and heterochiral peptides were shown to be dynamically unstabilized, as they were cleaved faster than homochiral peptides. The validity of the extrapolation of these results to other residues and longer peptide chains is discussed, and some directions for future developments of the theoretical model are given.

On-line sample stacking of peptides in capillary electrophoresis for the study of prebiotic reactions between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides

The reaction between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides is slow leading to low concentrations of products (peptides). The detection by capillary electrophoresis of the analytes contained in such samples is therefore a challenging issue. In this work, on-line sample pre-concentration methods based on field-amplified sample stacking have been implemented and compared. Because of the high ionic strength present in the sample matrix, samples were diluted with an organic solvent prior to analysis to decrease the sample conductivity. Different modes of sample injection (field amplified sample injection (FASI), hydrodynamic normal sample stacking (NSS) or large volume sample stacking (LVSS)) were compared. Pre-concentration factors of 20 for FASI, about 30-40 for NSS and 60 for LVSS were obtained for the analysis of (l,l) dipeptide of valine in a large excess of isovaline and 0.2M of ionic strength. For LVSS application and resolution optimisation, a new non-covalent coating based on the partial modification of the capillary surface was used to tune the electroosmotic flow magnitude and to pump the sample matrix out of the capillary. This on-line sample pre-concentration step allowed confirming that oligopeptides including alpha,alpha-dialkylated amino acids are formed during the reaction between alpha,alpha-dialkylated amino acids and N-carboxyanhydride amino acids.

Beta-structures of polypeptides with L-and D-residues. Part II. Statistical analysis and enrichment in enantiomer

The possible formation of beta-structures from polypeptide chains with L-and D-Residues randomly distributed was statistically analyzed within the frame of two hypotheses. Firstly, only those segments containing residues of identical chirality can associate to form antiparallel beta-structures, and secondly these segments must have a minimum length. The influence of different factors was examined: initial ratio of the L-and D-monomer, minimum length required for the segments to be incorporated into beta-sheets, average length of the peptide molecules, and stereoselectivity in the course of the polymerization process. The results show that in all cases nuclei of beta-sheets surrounded by random coil segments are formed, the optical activity of which very increases to purity when the initial ratio of monomers deviates from the racemic mixture. This suggests experiments to enrich the system in one enantiomer. Comparison is made with the corresponding behavior and properties of the alpha-helical structure.