摘要:

Since the Swedish chemist Baron Jo¨ns Jacob Berzelius first analysed the Alais meteorite for organic molecules’ in 1834, attempts to forge a link between extraterrestrial organic materials and terrestrial life have remained alluring, but often deceptive. New studies reported in this and last week’s issues hold the promise of important advances in both endeavours. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.51255

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A SIGNIFICANT parties of prebiotic organic matter on the early Earth may have been introduced by carbonaceous asteroids and comets.1The distribution and stable‐isotope composition of individual organic compounds in carbonaceous meteorites, which are thought to be derived from asteroidal parent bodies, may therefore provide important information concerning mechanistic pathways for prebiotic synthesis2and the composition of organic matter on Earth before living systems developed.3Previous studies11,12have shown that meteorite amino acids are enriched in13C relatives to their terrestrial counterparts, but individual species were not distinguished. Here we report the13C contents of individual amino acids in the Murchison meteorite. The amino acids are enriched in13C, indicating an extraterrestrial origin. Alanine is not racemic, and the13C enrichment of its D‐ and L‐enantiomers implies that the excess of the L‐enantiomer is indigenous rather than terrestrial contamination, suggesting that optically active materials were present in the early Solar System before life began. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51236

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A dynamical mechanism is found whereby a dissymmetric molecule and its mirror image are ionized as different rates by longitudinally polarized electrons such as produced by nuclear &bgr; decay. An enhancement is predicted for molecules containing heavy atoms. Order‐of‐magnitude estimates indicate that the asymmetric effect of this mechanism may be detectable by current experiments on positronium formation. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51252

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

It has long been known that Earth’s biochemistry is overwhelmingly dissymmetric or chiral.1–4In model chemical systems5–7that spontaneously evolve to a state dominated by either theLor the D emantiomer parity violation in &bgr;‐decay and that attributable to weak neutral currents (WNC) in molecules8,9is thought to be too small to have any significant influence on the emergent chirality.10,11Other conceivable systematic chiral influences are generally even weaker.12–14We show here that there is a simple and extremely sensitive mechanism by which a minute but systematic chiral interaction, no stronger than the WNC interaction is amino acids, can, over a period of ∼15,000 yr, determine which emantiomer will dominate. Such a mechanism is especially interesting when considering the origins of terrestrial biochemistry, particularly in view of the work by Mason and Tranter,15who found that it is the terrestrially dominant L amino acids that are favoured by the WNC interaction. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51237

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The indispensable role played by homochirality and chiral homogeneity in the self‐replication of crucial biomolecules is stressed, with the conclusion that life could neither exist nor originate without these chiral molecular attributes. Hypotheses historically proposed for the origin of chiral molecules on Earth are reviewed, including biogenic theories as well as abiotic theories embracing both indeterminate and determinate mechanisms. Indeterminate mechanisms, including autocatalytic symmetry breaking, asymmetric adsorption on quartz and clay minerals, and asymmetric syntheses in chiral crystals, are discussed and evaluated in the context of the prebiotic environment. Abiotic determinate mechanisms based on electric, magnetic and gravitational fields, on circularly polarized light (CPL), and on parity violation effects are summarized, with the emphasis that only CPL has proved practicable experimentally, but that it would be implausible on the primitive Earth. Mechanisms for the amplification of small, indigenous enantiomeric excesses are discussed, with one involving the partial polymerization of amino acids and the partial hydrolysis of polypeptides suggested as potentially viable prebiotically. Aspects of the turbulent, chirality‐destructive primeval environment are described, with the conclusion that all of the above mechanisms for theterrestrialprebiotic origin of chirality would be non‐viable, and that an alternative extraterrestrial source for the accumulation of chiral molecules on primitive Earth must have been operative. A scenario for this is outlined, in which we postulate that asymmetric photolysis of the organic mantles on interstellar grains in molecular clouds by circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae produces chiral molecules in the grain mantles. These grains with their chiral mantles are ultimately accumulated by Earth as the planet sweeps through the molecular clouds, or by impact delivery to Earth during the prebiotic era after incorporation of the grains into comets or asteroids. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51234

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

One of the most intriguing and crucial problems of the prebiotic evolution and the origin of life is the explanation of the origin of biohomochirality. A scheme of conversions originated by formaldehyde (FA) as hypothetical primer of biohomochirality is proposed. The merit of FA as executor of this function is based ‐inter alia ‐ on the distinguished role of FA as one of the earliest and simplest molecules in both warm, terrestrial and cold, extraterrestrial scenarios of the origin of life. The confirmation of the role of FA as primer of biohomochirality would support the option of an RNA world as an alternative to the protein world. The suggested hypothesis puts forward for the first time a concrete sequence of chemical reactions which can lead to biohomochirality. The spontaneous breaking of the mirror symmetry is secured by the application of the well‐known Frank scheme (combination of autocatalysis and ‘‘annihilation’’ of L and D enantiomers) to the series of interactions of FA ‘‘trimers’’ (i.e. C3H6O3compounds) of (aaa), (apa) and (app) types, where the monomeric groups (a) means ‘‘achirons’’ (a=CHn, n≥2 and C=M, M=C,O) and (p) mean ‘‘prochirons’’ (p=HC*OM, M=H,C). ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51247

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

PNA is a pseudopeptide DNA mimic in which the natural nucleobases have been retained, but the backbone consists ofN‐(2‐aminoethyl)glycine units to which the nucleobases are attached via methylene carbonyl linkers. The finding that PNA forms Warson‐Crick‐like helices with complementary DNA, RNA or PNA combined with the fact PNA is held together by amide bonds has made PNA of interest as a model for a primordial genetic material. Furthermore, the PNA backbone is achiral, while preferred chirality can be induced in PNA‐PNA double helices by attached chiral ligands, thereby providing a new way of ‘‘chiral amplification.’’ Finally, it has been demonstrated that PNA‐template directed synthesis of RNA and PNA is feasible. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51248

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We see chiral asymmetry in nature at all levels: from elementary particles to living beings. This naturally makes us wonder if these asymmetries are interrelated. Is it possible that the particular asymmetry we see in life’s chemistry is a consequence of the chiral asymmetry (parity violation) at the level of electroweak interactions? Here we present a theory that relates the strength of a chiral asymmetry and random chiral fluctuations to the probability that molecules with a particular handedness will dominate in a symmetry breaking transition. This theory tells us that, under reasonable prebiotic conditions, the molecular chiral asymmetry could be determined by chiral asymmetries as small as those due to weak neutral currents. ©1994 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51249

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Parity nonconserving nuclear‐spin‐dependent weak interaction of an electron with the nucleus induces difference between hyperfine structure constants in optical isomers. For heavy radicals withZ∼80 the effect may exceed 10 Hz, so that its observation is not so far away from the possibilities of the modern experimental technics. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51250

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Circular dichroism of optically active isotropic media of chiral molecules Im(n+−n−)/Im(n++n−) falls off as &ohgr;−2at frequenciesRy≪&ohgr;≪Z2Ryand as &ohgr;−3at &ohgr;≫Z2Ry, whereZis a typical nuclear charge of atoms in the chiral group. The contribution of the spin of electron to the circular dichroism appears in the second order in spin‐orbit perturbation only. The polarization of photoelectrons in the absorption of unpolarized light is connected with the chirality of molecule and constitutesZ2&agr;3from the degree of geometrical asymmetry. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51251

出版商:AIP    

年代:1996

数据来源: AIP