Infrared laser photolysis: A new tool for the study of prebiotic chemistry

Infrared laser induced dielectric breakdown and multiphoton absorption experiments on CH₄/NH₃ atmospheres are described. It is found that HCN, a central intermediate in prebiotic chemistry, is a principle product. This, combined with the fact that dielectric breakdown appears to have much in common with ordinary electric sparks, suggests that the laser could be a useful tool in studies of prebiotic chemistry. Several possible experiments in this vein are suggested.     

Dust in the universe: Implications for terrestrial prebiotic chemistry

In the present review we analyze the available literature on the distribution of dust in the Universe, methods of its observation and determination of the chemical composition, and the roles for terrestrial prebiotic chemistry. The most plausible natural sources of dust on the Earth in the prebiotic era are sedimentation of interplanetary dust, meteoritic and cometary impacts, volcanic eruptions, and soil microparticulates; the interplanetary medium being among the most powerful supplier of the dust matter. Two fundamental roles of dust particles for the origins of life are considered: (1) catalytic formation of prebiotic compounds; and (2) delivery of organic matter to the Earth by space dust particles. Due to the fact that there is only approximate information on the chemical composition and properties of interstellar, circumstellar, and major part of interplanetary dust, even the simulating experiments are difficult to perform. Until these gaps are filled, it seems reasonable to focus efforts of the scientists dealing with dust-driven catalytic formation of prebiotically important compounds on the volcanic and meteoritic/cometary impact environments.     

Emergence of homochirality in far-from-equilibrium systems: mechanisms and role in prebiotic chemistry

Since the model proposed by Frank (Frank FC, Biochem Biophys Acta 1953;11:459-463), several alternative models have been developed to explain how an asymmetric non-racemic steady state can be reached by a chirally symmetric chemical reactive system. This paper explains how a stable non-racemic regime can be obtained as a symmetry breaking occurring in a far-from-equilibrium reactive system initiated with an initial imbalance. Departing from the variations around the original Frank's model that are commonly described in the literature, i.e. open-flow systems of direct autocatalytic reactions, we discuss recent developments emphasizing both an active recycling of components and an autocatalytic network of simple reactions. We will present our APED model as the most natural realization of such thermodynamic openness and non-equilibrium, of recycling and of network autocatalysis, each of these in prebiotic conditions. The different experimental and theoretical models in the literature will be classified according to mechanism. The place and role of such self-structured networks responsible for the presence of homochirality in the primitive Earth will be detailed.     

A note on the prebiotic synthesis of organic acids in carbonaceous meteorities

Strong similarities between monocarboxylic and hydroxycarboxylic acids in the Murchison meteorite suggest corresponding similarities in their origins. However, various lines of evidence apparently implicate quite different precursor compounds in the synthesis of the different acids. These seeming inconsistencies can be resolved by postulating that the apparent precursors also share a related origin. Pervasive D enrichment indicates that this origin was in a presolar molecular cloud. The organic acids themselves were probably synthesised in an aqueous environment on an asteroidal parent body, the hydroxy (and amino) acids by means of the Strecker cyanohydrin reaction.     

The prebiotic synthesis of carbohydrates: A reassessment

Summary Questions concerning the significance of previous work on the formose reaction have led us to reexamine the question of the prebiotic synthesis of sugars. The results of new experiments lead to the following conclusions: The formose reaction is a geochemically plausible reaction which depends on neither basic conditions nor on the presence of trace amounts of carbohydrate impurities. However, this process is not a plausible source of ribose nor of any other individual sugar. In contrast to the nonspecific formation of complex mixtures of sugars via the formose reaction, the reduced sugar pentaerythritol is formed with great selectivity by the ultraviolet irradiation of 0.1 M formaldehyde. This compound may have played an important role in chemical evolution.Offprint requests to: A.W. Schwartz     

Prebiotic chemistry: chemical evolution of organics on the primitive Earth under simulated prebiotic conditions.

A series of prebiotic mixtures of simple molecules, sources of C, H, N, and O, were examined under conditions that may have prevailed during the Hadean eon (4.6-3.8 billion years), namely an oxygen-free atmosphere and a significant UV radiation flux over a large wavelength range due to the absence of an ozone layer. Mixtures contained a C source (methanol, acetone or other ketones), a N source (ammonia or methylamine), and an O source (water) at various molar ratios of C : H : N : O. When subjected to UV light or heated for periods of 7 to 45 days under an argon atmosphere, they yielded a narrow product distribution of a few principal compounds. Different initial conditions produced different distributions. The nature of the products was ascertained by gas chromatographic-mass spectral analysis (GC-MS). UVC irradiation of an aqueous methanol-ammonia-water prebiotic mixture for 14 days under low UV dose (6 x 10(-2) Einstein) produced methylisourea, hexamethylenetetramine (HMT), methyl-HMT and hydroxy-HMT, whereas under high UV dose (45 days; 1.9 x 10(-1) Einstein) yielded only HMT. By contrast, the prebiotic mixture composed of acetone-ammonia-water produced five principal species with acetamide as the major component; thermally the same mixture produced a different product distribution of four principal species. UVC irradiation of the CH(3)CN-NH(3)-H(2)O prebiotic mixture for 7 days gave mostly trimethyl-s-triazine, whereas in the presence of two metal oxides (TiO(2) or Fe(2)O(3)) also produced some HMT; the thermal process yielded only acetamide.     

Catalytic reactions in the solar nebula: Implications for interstellar molecules and organic compounds in meteorites

Organic compounds in meteorites seem to have formed by Fischer-Tropsch-type, catalytic reactions of CO, H₂, and NH₃ in the solar nebula, at 360–400K and (4–10)×10^–6 atm. The onset of these reactions was triggered by the formation of catalytically active grains of magnetite and serpentine at these temperatures.Laboratory experiments show that the Fischer-Tropsch reaction gives a large kineticisotope fractionation of C¹²/C¹³, duplicating the hitherto unexplained fractionation in meteorites. All of the principal compound classes in meteorites are produced by this reaction, or a variant involving a brief excursion to higher temperatures. (1) normal, mono-, and dimethylalkanes (2)arenes andalkylarenes; (3) dimericisoprenoids from C₉ to C₁₄; (4)purines andpyrimidines, such as adenine, guanine, uracil, thymine, xanthine, etc.; (5)amino acids, including tyrosine and histidine; (6)porphyrin-like pigments; (7) aromaticpolymer with –OH and –COOH groups.These reactions may also have played a major role in the evolution of life: first, by converting carbon to a sufficiently non-volatile form to permit its accretion by the inner planets; second, by synthesizing organic compounds on the primitive planets whenever CO, H₂, NH₃, and clay or magnetite particles came together at the right temperature. Similar reactions in other solar nebulae may be the source of interstellar molecules, as first suggested by G. H. Herbig. Ten of the twelve polyatomic interstellar molecules have in fact been seen in these syntheses or in meteorites.This paper is a revised and abridged version of the author's article inScience182 (1973), 781.     

Alpha particle chemistry. On the formation of stable complexes between He2+ and other simple species: implications for atmospheric and interstellar chemistry

The possibility that stable complexes may be formed between alpha particles (He²⁺) and small molecules is investigated using QCISD quantum mechanical calculations. Implications for their presence in the terrestrial atmosphere and/or in interstellar space are discussed. Figure Optimized structure of a stable H₂OHe²⁺ complex     

The prebiotic role of adenine: A critical analysis

Adenine plays an essential role in replication in all known living systems today, and is prominent in many other aspects of biochemistry. It occurs among the products of oligomerization of HCN. These circumstances have stimulated the idea that adenine was a component in a replication system that was present at the start of life. Such replicators have included not only RNA, but also a number of simpler RNA-like alternatives which utilize a simpler backbone.Despite these encouraging indicators, a consideration of the chemical properties of adenine reveals reasons that disfavor its participation in such a role. These properties include the following: (1) Adenine synthesis requires HCN concentrations of at least 0.01 M. Such concentrations would be expected only in unique circumstances on the early Earth. Adenine yields are low in prebiotic simulations, and if a subsequent high-temperature hydrolysis step is omitted, the reported yield does not represent adenine itself, but 8-substituted adenines and other derivatives. (2) Adenine is susceptibile to hydrolysis (the half life for deamination at 37 °C, pH 7, is about 80 years), and to reaction with a variety of simple electrophiles, forming a multiplicity of products. Its accumulation would not be expected over a geological time scale, and its regioselective incorporation into a replicator appears implausible. (3) The adenine-uracil interaction, which involves two hydrogen bonds (rather than three, as in guanine-cytosine pairing) is weak and nonspecific. Pairing of adenine with many other partners has been observed with monomers, synthetic oligonucleotides and in RNA. The hydrogen-bonding properties of adenine appear inadequate for it to function in any specific recognition scheme under the chaotic conditions of a prebiotic soup.New and fundamental discoveries in the chemistry of adenine would be needed to reverse this perception. An alternative and attractive possibility is that some other replicator preceeded RNA (or RNA-like substances) in the origin of life.     

Quantum chemical study of relative reactivities of a series of amines and nitriles: Relevance to prebiotic chemistry

Using the Iterative Extended Hucken Theory (IEHT) calculations of the elctron distribution and orbital energies of a series of thirteen amines, nitriles and amino-nitriles relevant to prebiotic and cosmo-chemistry have been carried out. Ground state properties such as the energy and nature of the highest occupied (HOMO) and lowest empty (LEMO) molecular orbitals, net atomic charges and number of non-bonding electrons have been identified as criteria for correlating the relative nucleophilicity of amine and nitrile nitrogens and the electrophilicity of nitrile and other unsaturated carbon atoms. The results of such correlations can be partially verified by known chemical behavior of these compounds and are used to predict and understand their role in prebiotic organic synthesis.     

Models on prebiotic polymer competition: A deterministic approach

Two simple models are proposed and analysed, in which it is shown that the formation of a new polymer, resulting from an “error” in the template action mechanism of production of an old polymer, may compromise the stability of the initial system under specific conditions, in the context of prebiotic evolution. Autocatalysis is shown to be a “selective advantage”, enabling the “mutant” to dominate in concentration and even replace the initial polymer. The addition of a third molecule playing the role of a catalyst causes hysteresis effects.     

Superoxide organic chemistry within the liposomal bilayer, part II: a correlation between location and chemistry

Coumarin ester derivatives 1, substituted at C-4 and/or C-12 with alkyl chains, were synthesized and intercalated within DMPC liposomal bilayers. By correlating the 13C chemical shift with medium polarity [E(T)(30)], the relative location of these substrates within the liposomal bilayer was determined. The length of the alkyl chain substituents clearly influences the lipophilicity of the substrates and their location and orientation within the liposome: Superoxide readily saponifies the C-12 esteric linkage of 1, when this reaction site lies in a polar region of the liposome (E(T)(30) > 45 kcal/mol), to give the corresponding 7-hydroxy coumarin derivatives 2. However, when C-12 lies deeper and is hence less available to O(2)(*-), the lactonic carbon C-2, which lies in a shallower region (E(T)(30) = 43-49), is the preferred site for superoxide-mediated cleavage. When coumarin 1 is disubstituted with long chains at both C-12 and C-4, these derivatives lie deep within the bilayer and react only slowly with O(2)(*-). These results indicate there is indeed a correlation between location within the bilayer and substrate reactivity. Contrary to the suggestion of Dix and Aikens (Chem. Res. Toxicol.6:2-18; 1993) superoxide can penetrate deep within the liposomal bilayer. Nevertheless, its concentration drops precipitously (to approximately 16% of what it is near the interface) below E(T) values of 38, thereby precluding substantial reaction with many highly lipophilic substrates. This work also confirms the findings of others that reactions of small oxy-radicals occur within cellular membranes and appear to be of significant biological importance.     

The dialectics of socialization: A tentative formulation

Conclusion A cognitive anthropological approach helps us to understand that children's conceptions are antithetical to adult conceptions. Furthermore, socialization is a dialectical process involving an interplay of these antithetical child conceptions and adult conceptions. As indicated in the field observations, change in the child is internal but the transition from one stage of growth to another occurs only through interaction with the environment. One sees elements of adult conceptions created in the child but they are and remain child conceptions until such time as the Leap has occurred. Babs' teacher, for example, misunderstood this antithetical quality in Babs and assumed her mature behavior indicated she was really mature, really an adult. Qualitatively and quantitatively Babs was still a child and had to initiate action on her own part to prove that. She exhibited behavior that was misconstrued as adult-like, but because she was still a child, her conceptions of herself, her environment and correlatively, of her behavior remained child-like.Carol was obviously quite resourceful and adept at moving in and out of various behavioral patterns appropriate to social situations in which she found herself. This was necessary for survival, particularly when she was forced into situations where prejudicial people controlled her life and her future. The antithetical qualities of her conceptual framework and that of her teacher are overshadowed by the real historical circumstances of race relations in this society. But those antithetical conceptual qualities are still clearly evident. Adult conceptions from the outside intrude on Carol's life and her thinking. And, elements of adult conceptions are being created within her. Yet these autonomous creations will remain child conceptions as long as she remains a child. She responds to her environment, but she responds as a child, subjectified/objectified. She is still qualitatively and quantitatively a childClearly much of what children learn from the environment — from school — is harmful to their autonomy. Through current socialization processes, the dialectics of learning remain incomplete. The interaction whereby children's categories antithetical to adult categories are transformed into a new synthesis of adult categories is missing. Instead, the adult conceptions created in and by the children merely replicate the adult categories of their socializers. Therefore, the current socialization process is a reactionary one. Children are increasingly objectified as their adult categories are less and less new categories, but merely replications of already formalized adult categories. Because the dialectical process of socialization is left incomplete, incomplete adults reach physical maturity. Such people are incapable of governing their own lives, of living out the complexities and inconsistencies of adulthood.Reformists are correct in calling for change in educational methods but not with the methods they suggest. Children should be educated for democracy, but the methods utilized should not result in chaos and disorder because they lack discipline.In fact, although ideologically at odds with each other, methodologically conventional and emancipated teachers differ only in locus of control. Conventional teachers see socialization as a unidirectional process in which they provide the stimulus and the students provide the response. Emancipated teachers see socialization as a dialectical process and deploy that process to educate children for greater autonomy, social and self-understanding. But, in rejecting conventional modes of thought, unconventional teachers do not necessarily reject sensible pedagogical methods. The processes and methods of education are more complex than a simple liberal (soft)/conservative (strict) theoretical dichotomy of education would suggest. Children can still be respected even as their teachers realize their conceptual categories are incomplete and inferior. Relativists who insist, for example, that all speech patterns, because they exhibit internal consistency and structural regularity are inherently equal, fail to recognize or to uncover the true source of social inequality. Relativistic revelations only mystify that inequality and contribute nothing to social liberation.Education for liberation is a slow, arduous task and will not occur by using short-cuts, or pop-psych approaches. It will come from strict adherence to exacting and demanding methods. It will be facilitated through a recognition of the dialectical nature of the socialization process and a cognitive anthropological investigation into the nature of conceptual categories of children. And it will have to be part of a general transformation of this society, while, at the same time, remaining an index of the succes of any such transformation. This means, among other things, that children must be taught to govern their own lives and to govern others. It does not mean, of course, that every person would govern; only that every person should understand the mechanics of governing — that every person, as Gramsci held, could govern.Maribeth Durst is Assistant Professor of Sociology, at St. John's University, Staten Island, New York.

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