R. Buvet

Formation of prebiochemical compounds in models of the primitive Earth's atmosphere

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N2 or NH3) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N2 mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.

Formation of prebiochemical compounds in models of the primitive earth's atmosphere. I: CH4 NH3 and CH4 N2 atmospheres.

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N2 or NH3) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N2 mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.

Conditions of occurrence for primeval processes of transphosphorylations.

Model reactions of the phosphate group transfer through hydrolysis-condensation processesw without enzymic catalysts are studied in dilute aqueous solutions. Tripolyphosphate and acetylphosphate are used as energy and phosphate donors to perform syntheses of pyrophosphate. Results are discussed in terms of the variation versus pH of the overall standard free enthalpy change.

Significance of the Zero-Current Potential of the NAD+-NADH system☆

Abstract Since the first electrochemical reduction step of NAD÷ and the electrochemical oxidation step of NADH lie respectively at —0.69 and 0.5–0.9 V (N.H.E.). no direct measure of the formal potential of NAD÷-NADH system may be attained potentiometrically. We examine the significance of the previous potentiometric studies realized in the presence of a mediator and an enzyme. A reliable zer-current potential of NAD÷ and NADH solution is only obtained when small amounts of benzyl-viologen (BV 2÷ ) and xanthine oxidase (XO) are added. The various electrochemical reduction steps of 10 to 1000 μ M BV 2÷ aqueous solutions are studied at pH 9.8 on dropping mercury and rotating platinum disk electrodes. The formal potential of the BV 2÷ -BV − system, equal to —0.360 V (N.H.E.), may be measured on a platinum electrode, while on a mercury electrode a strong adsorption interferes and gives a polarographic prewave. When XO is present, the addition of NADH decreases the BV 2÷ reduction waves: an anodic wave corresponding to BV 2÷ oxidation appears at the same potential as the first BV 2÷ reduction wave. The zero-current potential measured in NAD÷, NADH, BV 2÷ , BV 2÷ and XO solutions is actually fixed by the BV 2÷ -BV÷ system, which equilibrates through chemical oxido-reduction reactions with the NAD÷-NADH system.

Propriétés électrochimiques de la nicotinamide et de ses dérivés en solution aqueuse : III. Influence du pH de la solution et de la concentration de nicotinamide sur la réduction polarographique de ce composé

The mode of reduction of nicotinamide has been studied by drop-time controlled polarography. Over the pH range 0–10, a consistent wave(I) has been observed. Its half-wave potential shifts — 75 mV per pH unit. In the pH range 0–4 this wave (I) is followed by a second wave (II) of abnormally high limiting current. Concentration is a fundamental factor in the control of the mode of reduction of nicotinamide. Wave I splits into two waves of different height when this concentration is higher than a given pH-dependent level. Data obtained in the electrochemical reduction of very dilute solutions of N1-methylnicotinamide and nicotinamide are compared. Quaternisation of the pyridine nitrogen considerably promotes the fixation of a first electron on the heterocycle ring but does not further change the electron energetics.

Electrochemical energetics of biochemical hydroxylations and of their non-enzymic models

Abstract The biochemical and physiological importance of hydroxylations, which permit the primary oxidation of hydrogenerated carbon atoms using oxygen partly reduced by bielectronic reducing agents without mediation of the electron transfer chain, is reviewed. The energetic and mechanistic data obtained from electrochemical studies of oxygen reduction permit to discuss the mechanisms of these hydroxylations. Artificial oxidations at room temperature in aqueous or organic solvents by so-called activated oxygen molecules or Fenton reagents involving hydrogen peroxide must be considered as non-enzymic analogs of biochemical hydroxylations. The energetics and kinetics of the reduction of oxygen to hydrogen peroxide by hydroquinone, followed by degradative oxidation of the produced quinone, representing a non-enzymic model of the biochemical oxidation of homogentisate, was experimentally studied and discussed from available data related to the energetics of redox couples involving hydrogen peroxide and superoxide ion radical. The field of application in chemistry and in physiology and medicine of such non-enzymic analogs of biochemical hydroxylations performed either chemically or electrochemically, is briefly reviewed.

Compared energetics of primordial and biological metabolisms.

In model experiments of chemical evolution, the accumulation of redox energy has been achieved up to now only within gaseous phase. Such experiments lead to atmospheric precursors and involve an accumulation of large quantities of free enthalpy. A part of this energy can be released after dissolution of the precursors in aqueous media, either by simple multiple bond hydration, or by addition of heteroatomic reagents more nucleophilic than water, or by addition of carbonaceous nucleophiles. Energy balances of such processes are discussed.The non-enzymic photochemical accumulation of redox energy in aqueous phase appears later on feasible, but the main unsolved problem lies in the understanding of the primordial processes which made the conversion of redox energy into energy available from hydrolysis possible in aqueous media in the earliest stage of chemical evolution.In this respect, chemiosmotic or configurational interpretations of oxidative phosphorylations cannot be taken into consideration because they require complex structures which cannot be allowed for at this early stage.On the contrary, a discussion of the energetics and kinetics of electron transfers to and from substrates makes understandable the basic principles involved in the energy storage processes by means of the chemical hypothesis, as well as their likely occurrence, even in a non-enzymic form, from the very earliest stages of chemical evolution.

A simple analogical model for the selection of optical activity and of the most efficient catalysts in the course of molecular evolution

Analogical models, based on the use of simple hydrodynamic devices are proposed to discuss qualitatively and easily the properties of chemical networks of reactions implying a limited supply of a reagent usable in several parallel autocatalytic reaction pathways.This discussion shows that the racemic state can be unstable at the steady state in such systems and that the pathways involving the most efficient catalysts are selected in the course of their chemical evolution.

Characterization and rapid evaluation of electrochemically oxidizable components in normal and pathological urine

Abstract An electrochemical technique consisting in measuring the electrolytical current through a capillary graphite paste electrode impregnated with the urines to be studied and subject to a linear sweep of potential has been developed to characterize and quantify the electrochemically oxidizable compounds in urine. The curves obtained exhibit series of peaks more or less isolated and of various amplitudes in general proportional to concentrations. They show immediately and visually, without previous separation, the presence of urinary electroactive components such as uric acid, oxalic acid, xanthurenic acid, 5HIAA, VMA, HVA, tyrosine and tryptophan, at potentials characterizing each of them. Exploitation of this kind of rapid, low cost, polyvalent and automatizable analytical technique seems to be very valuable for the detection of diseases and for therapeutic follow-up. This analytical method was designed for medical consultants as well as for the therapeutic follow-up of patients suffering from leukaemia and subject to total body irradiation.

Electrochemical energetics of biochemical processes implying storage of redox energy into hydrolysable compounds

Abstract The effects of the pH on the energetics of overall redox processes and condensation reactions are recalled. Typical data related to the excess of energy implied in the kinetics of redox processes are analysed. The mechanisms of biochemical energy—storage processes are discussed from these data.