Reductive Coupling of Aldehydes by H2S in Aqueous Solutions,a C?C Bond Forming Reaction of Prebiotic Interest

We report here a novel reductive coupling reaction of conjugated, non‐ or poorly enolizable aldehydes induced by H₂S and operative in aqueous solutions under prebiotically relevant conditions. This reaction leads from retinal to β‐carotene, and from benzylic aldehydes to the corresponding diarylethylenes. This novel reaction also opens a new potentially prebiotic pathway leading from glyoxylic acid to various compounds that are involved in the reductive tricarboxylic acid cycle. This C? C bond forming reaction of prebiotic interest might have been operative, notably, in the sulfide‐rich environments of hydrothermal vents, which have been postulated as possible sites for the first steps of organic chemical evolution.     

Recreating ancient metabolic pathways before enzymes

The biochemistry of all living organisms uses complex, enzyme-catalyzed metabolic reaction networks. Yet, at life’s origins, enzymes had not yet evolved. Therefore, it has been postulated that non-enzymatic metabolic pathways predated their enzymatic counterparts. In this account article, we describe our recent work to evaluate whether two ancient carbon fixation pathways, the rTCA (reductive tricarboxylic acid) cycle and the reductive AcCoA (Wood-Ljungdahl) pathway, could have operated without enzymes and therefore have originated as prebiotic chemistry. We also describe the discovery of an Fe²⁺-promoted complex reaction network that may represent a prebiotic predecessor to the TCA and glyoxylate cycles. The collective results support the idea that most central metabolic pathways could have roots in prebiotic chemistry.     

Evolution of the biosynthesis of the branched-chain amino acids

The origin of the biosynthetic pathways for the branched-chain amino acids cannot be understood in terms of the backwards development of the present acetolactate pathway because it contains unstable intermediates. We propose that the first biosynthesis of the branched-chain amino acids was by the reductive carboxylation of short branched chain fatty acids giving keto acids which were then transaminated. Similar reaction sequences mediated by nonspecific enzymes would produce serine and threonine from the abundant prebiotic compounds glycolic and lactic acids. The aromatic amino acids may also have first been synthesized in this way, e.g. tryptophan from indole acetic acid. The next step would have been the biosynthesis of leucine from -ketoisovaleric acid. The acetolactate pathway developed subsequently. The first version of the Krebs cycle, which was used for amino acid biosynthesis, would have been assembled by making use of the reductive carboxylation and leucine biosynthesis enzymes, and completed with the development of a single new enzyme, succinate dehydrogenase. This evolutionary scheme suggests that there may be limitations to inferring the origins of metabolism by a simple back extrapolation of current pathways.     

Urea-acetylene dicarboxylic acid reaction: A likely pathway for prebiotic uracil formation

A number of routes have been suggested for the prebiotic synthesis of uracil involving the reaction of urea with malic acid, propiolic acid, cyanoacetylene and others. Cyanoacetylene has been detected in the interstellar medium as well as simulated prebiotic experiments. It is therefore plausible that dicyanoacetylene and its hydrolytic product acetylene dicarboxylic acid, (ADCA) may have played a role in chemical evolution. This aspect has been examined in the present work for the synthesis of uracil from ADCA and urea reaction.It was found that when ADCA reacted with urea, uracil was formed only in the presence of phosphoric acid and phosphates. Ammonium phosphates gave higher yields of uracil than other phosphates. In the absence of phosphoric acid or phosphates no uracil formation took place. This type of synthesis could have taken place in prebiotic oceans which contained ammonium phosphates and other salts.     

Aldehyde metabolism and the aroma quality of stored golden delicious apples

Aldehyde production by intact apples was monitored by reversed phase HPLC of headspace concentrates, after reaction with 2,4-dinitrophenylhydrazine. Depending on the degree of maturity and their storage history, Golden Delicious apples showed a variable headspace composition, differences being mostly of a quantitative nature. Whereas the headspace of pre-climacteric fruits was particularly rich in C₁C₆ aldehydes, that of climacteric, ripening apples was greatly reduced, and some aldehydes were only present in trace amounts. Treatment of pre-climacteric or cold stored fruits with carboxylic acid vapours had a negligible effect on the aldehyde composition. Controlled atmospheric storage, however, led to a notable increase in the aldehydes derived from the added carboxylic acids or from those shortened by β-oxidation. This confirms the presence of a reductive path of carboxylic acids into aldehydes. Further results suggest that high carbon dioxide (CA-storage) interferes with carboxylic acid metabolism and alcohol dehydrogenase activity, leading to a deterioration of the aroma quality.     

3-substituted BINOL Schiff bases and their reductive products for catalytic asymmetric addition of diethylzinc to aldehydes

Three new chiral 3-substituted BINOL Schiff bases and their reductive 3-arylaminomethyl BINOL products have been synthesized and used for the asymmetric addition reaction of diethylzinc to aldehydes in the presence of titanium tetraisopropoxide. The reaction provided optically active secondary alcohols in high yield (86-100%) and good enantioselectivity (up to 92% ee).     

Reactions of aminomalononitrile with electrophiles

Aminomalononitrile (HCN trimer) reacts with electrophiles such as aldehydes and acrylonitrile under very mild conditions of temperature and pH to produce intermediates which, after acid hydrolysis, yield amino acids. The following amino acids have been identified and quantitated: glycine, D, L-erythro- and D, L-threo-beta - hydroxyaspartic acids, D, L glutamic acid, and D, L-threonine and allo-threonine. The mechanism of their formation and the possible significance of these reactions in prebiotic syntheses are discussed.     

On a hypothetical generational relationship between HCN and constituents of the reductive citric acid cycle

Encouraged by observations made on the course of reactions the HCN-tetramer can undergo with acetaldehyde, I delineate a constitutional and potentially generational relationship between HCN and those constituents of the reductive citric acid cycle that are direct precursors of amino acids in contemporary metabolism. In this context, the robustness postulate of classical prebiotic chemistry is questioned, and, by an analysis of the (hypothetical) reaction-tree of a stepwise hydrolysis of the HCN-tetramer, it is shown how such a non-robust chemical reaction platform could harbor the potential for the emergence of autocatalytic cycles. It is concluded that the chemistry of HCN should be revisited by focussing on its non-robust parts in order to demonstrate its full potential as one of the possible roots of prebiotic self-organizing chemical processes.     

Prebiotic synthesis of amino acids from formaldehyde and hydroxylamine in a modified sea medium

In the course of a study of a possible mechanism for chemical evolution in the primeval sea, we have found a novel reaction for peptide formation from glycine and urea in an aqueous solution. Glycine reacted with urea to give N-carbamylglycine, N-carbamylglycylglycine and glycylglycine. This reaction provides a new pathway for the prebiotic synthesis of peptides.     

Biocatalytic ketone reduction—a powerful tool for the production of chiral alcohols—part II: whole-cell reductions

The tricarboxylic acid (TCA) cycle is an energy-producing pathway for aerobic organisms. However, it is widely accepted that the phylogenetic origin of the TCA cycle is the reductive TCA cycle, which is a non-Calvin-type carbon-dioxide-fixing pathway. Most of the enzymes responsible for the oxidative and reductive TCA cycles are common to the two pathways, the difference being the direction in which the reactions operate. Because the reductive TCA cycle operates in an energetically unfavorable direction, some specific mechanisms are required for the reductive TCA-cycle-utilizing organisms. Recently, the molecular mechanism for the “citrate cleavage reaction” and the “reductive carboxylating reaction from 2-oxoglutarate to isocitrate” in Hydrogenobacter thermophilus have been demonstrated. Both of these reactions comprise two distinct consecutive reactions, each catalyzed by two novel enzymes. Sequence analyses of the newly discovered enzymes revealed phylogenetic and functional relationships between other TCA-cycle-related enzymes. The occurrence of novel enzymes involved in the citrate-cleaving reaction seems to be limited to the family Aquificaceae. In contrast, the key enzyme in the reductive carboxylation of 2-oxoglutarate appears to be more widely distributed in extant organisms. The four newly discovered enzymes have a number of potential biotechnological applications.     

Prebiotic synthesis of amino acids from formaldehyde and hydroxylamine in a modified sea medium

In the course of a study of a possible mechanism for chemical evolution in the primeval sea, we have found a novel reaction for peptide formation from glycine and urea in an aqueous solution. Glycine reacted with urea to give N-carbamylglycine, N-carbamylglycylglycine and glycylglycine. This reaction provides a new pathway for the prebiotic synthesis of peptides.     

Combinatorial synthesis and biological evaluation of isoxazole-based libraries as antithrombotic agents

The 3-substituted phenyl-5-isoxazolecarboxaldehydes have been identified as activated aldehydes for the generation of isoxazole-based combinatorial libraries on solid phase through automation. Three highly functionalized isoxazole-based libraries comprising of 32, 96 and 45 compounds each have been synthesized in parallel format using Baylis Hillman reaction, Michael addition, reductive amination and alkylation reactions. With an objective of lead generation all the three libraries were evaluated for their antithrombin activity in vivo.     

The design, synthesis, and evaluation of two universal doxorubicin-linkers: preparation of conjugates that retain topoisomerase II activity

The design, synthesis, and evaluation of two N-alkylmaleimide aldehydes have been achieved, which upon reductive alkylation with the C3'-amino group of doxorubicin (DOX) permits the preparation of DOX conjugates via Michael addition of thiol-containing vectors. This method enables the mild, facile, and high-throughput preparation of DOX conjugates that retain the basic C3'-nitrogen, a pre-requisite for topoisomerase II inhibition. Seven DOX-amino acid conjugates were prepared, each displaying similar inhibitory activity as the parent drug.     

Synthesis of New Chiral Peptide Nucleic Acid (PNA) Monomers by a Simplified Reductive Animation Method

Abstract

The aim of this work was the preparation of four new peptide nucleic acid (PNA) monomer backbone by reductive animation of N^α-Boc-protected chiral amino aldehydes, derived from Leu, Phe, Tyr(Bzl), and Thr(Bzl), with methyl glycinate. To the crude 2-substituted methyl N-(2-Boc-aminoethyl)glycinates obtained, thymin-1-ylacetic acid was coupled using TBTU procedure in a one-pot reaction. PNA monomers were isolated and characterized.     

Synthesis of chiral alpha-amino aldehydes linked by their amine function to solid support.

The anchoring of an alpha-amino-acid derivative by its amine function on to a solid support allows some chemical reactions starting from the carboxylic acid function. This paper describes the preparation of alpha-amino aldehydes linked to the support by their amine function. This was performed by reduction with LiAlH4 of the corresponding Weinreb amide linked to the resin. The aldehydes obtained were then involved in Wittig or reductive amination reactions. In addition, the linked Weinreb amide was reacted with methylmagnesium bromide to yield the corresponding ketone. After cleavage from the support, the compounds were obtained in good to excellent yields and characterized.     

The synthesis of amino acids and sugars on an inorganic template from constituents of the prebiotic atmosphere

Inelastic Electron Tunnelling Spectroscopy (IETS) has been used to identify the reaction products present on an alumina surface when it is exposed to likely components of the earth's prebiotic atmosphere. The alumina barrier of Al-AlO _x -Pb tunnelling junctions have been exposed to water; aqueous ammonia; wet carbon monoxide gas and to aqueous formaldehyde vapour under normal atmospheric conditions at room temperature. The water spectrum shows strong coincidence with that of a genuine sample of formic acid. It is proposed that atmospheric CO₂ is involved in this surface catalyzed reaction. The aqueous ammonia spectrum is assigned as an amino acid species produced from ammonia, water and atmospheric carbon dioxide. This spectrum compares very closely with the tunnelling spectrum of a genuine sample of glycine. The wet carbon monoxide spectrum and the aqueous formaldehyde spectrum have been produced by an infusion doping process. These spectra of CO and aqueous formaldehyde are assigned as a sugar like polymer or a sugar formed on the alumina surface. A tunnelling spectrum of D(–) fructose has been produced to aid this assignment. The role of an inorganic template such as alumina in the original prebiotic synthesis of amino acids and sugars is considered.     

N-Carbamoyl Amino Acid Solid–Gas Nitrosation by NO/NO x : A New Route to Oligopeptides via α-Amino Acid N-Carboxyanhydride. Prebiotic Implications

α-N-Carbamoyl amino acid (CAA), whose conditions of formation in a prebiotic hydrosphere have been described previously (Taillades et al. 1998), could have been an important intermediate in prebiotic peptide synthesis through reaction with atmospheric NO _x . Nitrosation of solid CAA (glycine or valine derivative) by a 4/1 NO/O₂ gaseous mixture (1 atm) yields N-carboxyanhydride (NCA) quantitatively in less than 1 h at room temperature. The crude solid NCA undergoes quantitative oligomerization (from trimer to nonamer under the conditions we used) when treated with a (bi)carbonate aqueous buffer at pH 9. We therefore suggest that part of the prebiotic amino acid activation/polymerization process may have taken place in a dry phase (``drying-lagoon' scenario). Received: 23 June 1998 / Accepted: 7 December 1998     

Strategies for the analysis of chlorinated lipids in biological systems

Myeloperoxidase-derived HOCl reacts with the vinyl ether bond of plasmalogens yielding α-chlorofatty aldehydes. These chlorinated aldehydes can be purified using thin-layer chromatography, which is essential for subsequent analysis of extracts from some tissues such as myocardium. The α-chlorofatty aldehyde 2-chlorohexadecanal (2-ClHDA) is quantified after conversion to its pentafluorobenzyl oxime derivative using gas chromatography–mass spectrometry and negative-ion chemical ionization detection. 2-ClHDA accumulates in activated human neutrophils and monocytes, as well as in atherosclerotic lesions and infarcted myocardium. Metabolites of 2-ClHDA have also been identified, including the oxidation product, 2-chlorohexadecanoic acid (2-ClHA), and the reduction product, 2-chlorohexadecanol. 2-ClHA can be quantified using LC–MS with selected reaction monitoring (SRM) detection. 2-ClHA can be ω-oxidized by hepatocytes and subsequently β-oxidized from the ω-end, leading to the production of the dicarboxylic acid, 2-chloroadipic acid. This dicarboxylic acid is excreted in the urine and can also be quantified using LC–MS methods with SRM detection. Quantitative analyses of these novel chlorinated lipids are essential to identify the role of these lipids in leukocyte-mediated injury and disease.     

Two distinct quinone-modulated modes of antimycin-sensitive cytochrome b reduction in the cytochrome bc1 complex

Reduction of cytochrome b-560 (analogous to cyt b-562 of mitochondria) via an antimycin-sensitive route has been revealed in chromatophores of the photosynthetic bacterium, Rhodopseudomonas sphaeroides Ga. Indeed, the results suggest that two reductive mechanisms can be operative. One is consistent with the idea that the quinol generated at the reaction center QB site enters the Q pool and, via the Qc site, equilibrates with cytochrome b-560. The other reductive mode circumvents redox equilibrium with the pool; we consider that this could result from a direct encounter of the reaction center with the bc1 complex perhaps involving a direct QB-Qc site interaction. This latter reaction is suppressed by occupancy of the Qc site, not only by antimycin but by ubiquinol and ubiquinone.     

Potentially prebiotic syntheses of condensed phosphates

In view of the importance of a prebiotic source of high energy phosphates, we have investigated a number of potentially prebiotic processes to produce condensed phosphates from orthophosphate and cyclic trimetaphosphate from tripolyphosphate. The reagents investigated include polymerizing nitriles, acid anhydrides, lactones, hexamethylene tetramine and carbon suboxide. A number of these processes give substantial yields of pyrophosphate from orthophosphate and trimetaphosphate from tripolyphosphate. Although these reactions may have been applicable in local areas, they are not sufficiently robust to have been of importance in the prebiotic open ocean.