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Biomedical subjects

N Lahav

Publications and source records attributed to N Lahav.

25 records · Page 2Linked to original sources
PubMed1979

Prebiotic nucleotide oligomerization in a fluctuating environment: effects of kaolinite and cyanamide.

The day kaolinite was tested for its ability to promote nucleotide oligomerization in model prebiotic systems. Heterogeneous mixtures of clay, water and nucleotide were repeatedly evaporated to dryness at 60 degrees C and redissolved in water in cyclic fashion in the presence or absence of cyanamide and/or ammonium chloride. With or without cycling, kaolinite alone did not promote the oligomerization of nucleotides at detectable levels. Cycling of clay in combination with cyanamide, however, promoted high levels of condensation to a mixture of oligonucleotides and dinucleotide pyrophosphate without requiring ammonium chloride. Although cycling with clay favored synthesis of dinucleotide pyrophosphate, cycling without clay enhanced formation of oligonucleotides. These results support the hypothesis that the presence of clays in fluctuating environments would have influenced the -ourse of prebiotic condensation reactions.

Alkaline Phosphatase↗
PubMed1978

Peptide formation in the prebiotic era: thermal condensation of glycine in fluctuating clay environments.

As geologically relevant models of prebiotic environments, systems consisting of clay, water, and amino acids were subjected to cyclic variations in temperature and water content. Fluctuations of both variables produced longer oligopeptides in higher yields than were produced by temperature fluctuations alone. The results suggest that fluctuating environments provided a favorable geological setting in which the rate and extent of chemical evolution would have been determined by the number and frequency of cycles.

Bentonite↗
PubMed1977

Polymerization of alanine in the presence of a non-swelling montmorillonite.

Alanine, starting from alanine-adenylate, has been polymerized in the presence of non-swelling Al-montmorillonite. The yield of polymerization is much lower than that obtained in the presence of swelling Na-montmorillonite. The possibility that the changing interlayer spacing in Na-montmorillonite might be responsible for its catalytic properties, is discussed.

Alanine↗
PubMed1976

The possible role of solid surface area in condensation reactions during chemical evolution: reevaluation.

Published data on adsorption and condensation of amino acids, purine and pyrimidine bases, sugars, nucleosides, and nucleotides are analyzed in connection with Bernal's hypothesis that clays and other minerals may have provided the most likely surface for adsorption and condensation of these molecules in prebiotic times. Using surface concentration and reaction rate as the main criteria for the feasibility of condensation reactions, four types of prebiotic environments were analyzed: (1) an ocean-sediment system, (2) a dehydrated lagoon bed produced by evaporation, (3) the surface of a frozen sediment, and (4) a fluctuating system where hydration (rainstorms, tidal variations, flooding) and dehysration (evaporation) take place in a cyclic manner. With the possible exception of nucleotides, low adsorption of organomonomers on sediment surfaces of a prebiotic ocean (pH 8) is expected, and significant condensation is considered unlikely. In dehydrated and frozen systems, high surface concentrations are probable and condensation is more likely. In fluctuating environments, condensation rates will be enhanced and the size distribution of the oligomers formed during dehydration may be influenced by a "redistribution mechanism" in which adsorbed oligomers and monomers are desorbed and redistributed on the solid surface during the next hydration-dehydration cycle.

Adsorption↗
PubMed1975

Simultaneous existence of different enviroments in aqueous clay systems and its possible role in prebiotic synthesis.

The formation of packets of parallel oriented platelets and separating distances of several angstrom units in montmorillonite-water systems produces an intrinsic inhomogeneity with respect to the proton donating power of internal and external zones. Stable packets can be induced by both inorganic and organic molecules or ions, in suspensions or in drying-out systems. The coexistence of zones with different proton donating power was demonstrated by the pH-sensitive color reaction of benzidine, where stable packets of montmorillonite platelets were formed by the use of either paraquat or diquat. The close proximity of the two types of zones, which can be of the order of several angstroms, produces the conditions which were defined by Katchalsky as essential for the polymerization of amino acids. Since these enviromental conditions are quite common in nature, both at present and in prebiotic times, it is proposed that the inhomogeneity of clay-water systems with respect to proton donating power should be taken into account in both theoretical and experimental efforts to demonstrate the catalytic activity of clays in prebiotic synthesis.

Amino Acids↗
PubMed1972

Effect of several clay minerals and humic acid on the survival of Klebsiella aerogenes exposed to ultraviolet irradiation.

The effect of various clay minerals and humic acid on the survival of Klebsiella aerogenes exposed to ultraviolet (UV) irradiation was investigated. A protective effect was observed and found to depend on the specific light absorption and light scattering properties of the clay minerals and the humic acid used. The higher the specific absorption, the better was the survival of K. aerogenes after UV irradiation. Bacterial survival was lower in clays saturated with divalent cations (Ca, Zn) than in those homoionic to monovalent cations (K).

Absorption↗