Prebiotic Origin Research Articles

Stereoselective decarboxylation of amino acids in the solid state, with special reference to chiral discrimination in prebiotic evolution

The decarboxylations of sublimated solidd- andl-leucine by nonpolarized γ-rays give quite different quantum yields, indicating significant selection. The G(CO2) value for thed-isomer is higher than that for thel-isomer by a factor of 2 within a dose range of 103–105 rads. The G value for thedl-racemate is close to that of thed-isomer. The effect vanishes if instead of sublimation, crystallization from aqueous solution is the last preparation step. Our results on sublimated leucine agree well with those reported for γ-induced decarboxylation of solid β-phenylalanine prepared similarly by sublimation. The asymmetry increases with longer cooling periods after irradiation. An intrinsic energy difference due to parity nonconservation between enantiomers is discussed as a possible stereoselective mechanism, with special reference to the prebiotic origin of asymmetry in living matter. Other possible sources of the observed effects are also discussed.

Complementary carrier peptide synthesis: general strategy and implications for prebiotic origin of peptide synthesis.

A method for peptide synthesis is proposed based on a template-directed scheme that parallels that of the native ribosomal mechanism. In this procedure, peptide bond formation is facilitated by the juxtaposition of aminoacyl and peptidyl oligonucleotide carriers bound adjacent to one another on an oligonucleotide template. The general strategy of the synthesis and relevant model studies are described. The scheme provides an intrinsic mechanism by which oligonucleotides can direct the synthesis of polypeptides in the absence of protein or ribosomal machinery and, as such, suggests a model for the origin of prebiotic protein synthesis.

Liposomes from ionic, single-chain amphiphiles.

In studies of the minimum physiochemical requirements for lipid membrane formation, we have made liposomes from dilute, aqueous dispersions of C(8)-C(18) single-chain amphiphiles. In general, membrane formation from ionic soaps and detergents requires the presence of uncharged amphiphiles. Vesicles were characterized by phase-contrast microscopy, by trapping of ionic dyes, as well as by negativestain and freez-frature electron microscopy. They were typically heterogeneous in size, but the average diameter could be experimentally varied in some cases over the range of 1 to 100 micrometer. Uni-, oligo-, and multilamellar vesicles were observed. Membrane permeability to various solutes was determined in part by a new technique which utilized phase-contract microscopy; when impermeable vesciles exclude added solutes such as sucrose, refractive index differences are created between vesicle contents and surrounding medium, so that the vesicles appear bright in the phase microscope. Permeant solutes do not produce this effect. Spectrophotometric permeability determinations confirmed the results of this technique and provided quantitative measures of permeability. Monoalkyl liposomes have potential uses as models of biomembranes and in drug delivery. They are also relevant to the prebiotic origin of biomembranes.

Asymmetric adsorption by quartz: A model for the prebiotic origin of optical activity

One mechanism previously proposed for the abiotic accumulation of molecules of one chirality in nature is asymmetric adsorption on the chiral surfaces of optically active quartz crystals. Earlier literature in this field is reviewed, with the conclusion that previous investigations of this phenomenon, using optical rotation criteria, have afforded ambiguous results. We now have studied the adsorption of radioactive D- and L-alanine on powdered d- and l-quartz, using change in radioactivity level as a criterion for both gross and differential adsorption. d-Quartz preferentially adsorbed D-alanine from anhydrous dimethyl-formamide solution, and l-quartz L-alanine. The differential adsorption varied between 1.0 and 1.8%. The implications of these observations are discussed from the viewpoint of early chemical evolution and the origin of optically active organic compounds in nature.

Coacervate-like microspheres from lysine-rich proteinoid

Microspheres form isothermally from lysine-rich proteinoid when the ionic strength of the solution is increased with NaCl or other salts. Studies with different monovalent anions and with polymers of different amino acid composition indicate that charge neutralization and hydrophobic bonding contribute to microsphere formation. The particles also form in sea water, especially if heated or made slightly alkaline. The microspheres differ from those made from acidic proteinoid but resemble coacervate droplets in some ways (isothermal formation, limited stability, stabilization by quinone, uptake of dyes). Because the constituent lysine-rich proteinoid is of simulated prebiotic origin, the study is interpreted to add emphasis to and suggest an evolutionary continuity for coacervation phenomena.

Effect of ionizing radiation and U.V. light on N-acetylglycine in the presence of ammonia.

Aqueous solutions of 1-14C-N-acetylglycine and ammonia were irradiated with 7.5 MeV fast electrons,60Co γ-rays, and u.v. light (low pressure mercury lamp). The products and their hydrolyzates were separated by electrophoresis, paper chromatography, and ion exchange chromatography. Among the main reactions are: decarboxylations, aminations yielding diglycine, dimerizations giving rise to diaminosuccinic acid, succinic acid, aspartic acid, β-aspartylglycine, β-alanylglycine, and the N-acetyl derivatives of these compounds. Methylations yield α-alanine, and a reductive transacetylation yields threonine. The results are briefly discussed with respect to the effect of different concentrations and the use of different qualities of radiation. The results also point to a possible pathway to the prebiotic origin of a number of biologically important micromolecules, including oligopeptides. The results indicate, however, that a prebiotic formation of polypeptides or protein-like macromolecules by the action of ionizing radiations or u.v. light upon aqueous solutions of monomers is unlikely under the conditions of this investigation.

Preferential polymerization and adsorption of L-optical isomers of amino acids relative to D-optical isomers on kaolinite templates

Experiments on the polymerization of the L- and D-optical isomers of aspartic acid and serine using kaolinite as a catalyst showed that the L-optical isomers were polymerized at a much higher rate than the D-optical isomers; racemic (DL-) mixtures were polymerized at an intermediate rate. The peptides formed from the L-monomers were preferentially adsorbed by the clay. In the absence of kaolinite, no significant or consistent difference in the behavior of the L- and D-optical isomers was observed. In experiments on the adsorption of L- and D-phenylalanine by kaolinite, the L-optical isomer was preferentially adsorbed. Adsorption of L-phenylalanine was sensitive to pH, whereas adsorption of D-phenylalanine was not. The experimental results indicate that catalytically active faces of the kaolinite crystals acted as specific templates which preferentially adsorbed and polymerized molecules with the L-configuration. The observed discrimination between the L- and D-optical isomers of the amino acids evidently occurred on the enantiomorphous edge ( hk) faces of the kaolinite. These findings may have significance for the pre-biotic origin of proteins.