Charge transport across metal-molecule interfaces has an important role in organic electronics. Typically, chemical link groups such as thiols or amines are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal-carbon coupling has been shown through C60, benzene and π-stacked benzene, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe(3))-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique. Gold atoms at the electrode displace the SnMe(3) linkers, leading to the formation of direct Au-C bonded single-molecule junctions with a conductance that is ∼100 times larger than analogous alkanes with most other terminations. The conductance of these Au-C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au-C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal-organic contacts.

Understanding electron transport across π-π-stacked systems will help to answer fundamental questions about biochemical redox processes and benefit the design of new materials and molecular devices. Herein we employed the STM break-junction technique to measure the single-molecule conductance of multiple π-π-stacked aromatic rings. We studied electron transport through up to four stacked benzene rings held together in an eclipsed fashion via a paracyclophane scaffold. We found that the strained hydrocarbons studied herein couple directly to gold electrodes during the measurements; hence, we did not require any heteroatom binding groups as electrical contacts. Density functional theory-based calculations suggest that the gold atoms of the electrodes bind to two neighboring carbon atoms of the outermost cyclophane benzene rings in η(2) fashion. Our measurements show an exponential decay of the conductance with an increasing number of stacked benzene rings, indicating a nonresonant tunneling mechanism. Furthermore, STM tip-substrate displacement data provide additional evidence that the electrodes bind to the outermost benzene rings of the π-π-stacked molecular wires.

Before life could start on earth, it was important that the amino acid building blocks be present in a predominant handedness called the L configuration and that the ribose of RNA be predominantly in the D configuration. Because ordinary chemical processes would produce them in equal L and D amounts, it has long been a puzzle how the needed selectivities could have arisen. Carbonaceous chondrites such as the Murchison meteorite, which landed in Australia in 1969, brought some unusual amino acids with a methyl group replacing their ␣ hydrogen. They cannot racemize and have a small but real excess of those with the L configuration. We have shown that they can partake in a synthesis of normal L amino acids under credible prebiotic conditions. We and others showed that small preferences can be amplified into solutions with very high dominance of the L amino acids because of the higher solubility of the pure L form than of the more stable DL racemic compound crystal. Here, we show that such solubility-based amplification of small excesses of three D nucleosides, uridine, adenosine, and cytidine, can also occur to form solutions with very high D dominance under credible prebiotic conditions. Guanosine crystallizes as a conglomerate and does not amplify in this way. However, under prebiotic conditions it could have been formed from homochiral D ribose from the hydrolysis of amplified adenosine or cytidine. chiral amplification ͉ Murchison meteorite ͉ transamination ͉ water solubilities E ver since the discovery of chirality there has been speculation about why and how our protein amino acids have the L configuration and the ribose and deoxyribose in nucleic acids have the D configuration (1, 2). It is generally understood that such homochirality was needed in prebiotic times so proteins could have their well defined structures, impossible with a random mixture of the L and D amino acid enantiomers (but possible if all of the amino acids were D, as may have happened in other parts of the universe). Similarly, ribose must have been in its homochiral D form so the RNA of the prebiotic RNA world and the DNA and RNA of our world could have well defined structures such as those in their helices, impossible with a random mixture of D and L ribose. Because chemical reactions to synthesize these amino acids and ribose would normally form a DL equal mixture in the absence of some chiral influence, scientists have proposed various ideas about how the compounds could have been formed in the prebiotic world with the complete homochirality we see in proteins and nucleic acids (1). However, as Rikken and Raupach (3) commented, ''Clearly the question of the origin of the homochirality of life is far from answered.'' An important piece of experimental evidence on this question arrived on earth in 1969: the Murchison meteorite that landed in Australia carried a number of ␣-methyl amino acids that all show small excesses of the L forms (Fig. 1), named L with methyl groups replacing the ␣ protons of L amino acids (4, 5). They have the hig...

Previous work by us, and others, has shown that the formation of amino acids on prebiotic earth with the geometric arrangement called the L configuration can be understood. Some meteorites of the carbonaceous chondritic type deliver unusual amino acids, with alpha-methyl groups, which have an excess of the L isomers. We previously showed that in decarboxylative transamination reactions under credible prebiotic conditions they produce normal amino acids that also have a preference for the L isomer, as is found in our proteins. We, and others, showed that as little as a 1% excess of the L isomers could be amplified up to a 95/5 ratio of L over D on simple evaporation of a solution, so life could start with such a solution in which the dominant L isomers would be selectively chosen. We now find that the geometry of sugars referred to D, as in D-ribose or D-glucose, is not an independent mystery. D-glyceraldehyde, the simplest sugar with a D center, is the basic unit on which other sugars are built. We find that the synthesis of glyceraldehyde by reaction of formaldehyde with glycolaldehyde is catalyzed under prebiotic conditions to D/L ratios greater than 1, to as much as 60/40, by a representative group of L-amino acids (with the exception of L-proline). The D/L glyceraldehyde ratio in water solution is amplified to 92/8 using simple selective solubilities of the D and the DL forms. This D center would then be carried into the prebiotic syntheses of larger sugars.aldol reaction | formaldehyde | formose reaction | meteorites F or life to start, it was necessary that the amino acids that are building blocks for proteins, and the sugars that play many biological roles and are part of both RNA and DNA, have a single handedness, called chirality. The amino acids could have been all in the handedness referred to as L, analogous to the left hand, or they could have been all D, analogous to the right hand, but on our planet the L geometry was preferred. After many years of speculation about why this is so, evidence has now accumulated that the L handedness of amino acids was derived from a special group of amino acids, with an extra methyl group that prevented loss of their small excess of the L form by a process that can scramble the handedness of ordinary amino acids on prolonged heating. These special L-amino acids with the extra methyl group were isolated from a meteorite that fell in Murchison Australia in 1969 (1, 2). We showed that they could generate normal amino acids (without the methyl group) under credible prebiotic conditions, and those also had an excess of the L isomer, along with a smaller amount of the D amino acids (3).Once there was a small excess of the L-amino acid, we showed that it could be amplified in solution, because the DL 1∶1 mixture was less soluble in water (4). By evaporating a water solution with only a 1% excess of L-phenylalanine, for instance, we obtained a solution with a 95/5 L/D ratio; equal amounts of the D and L components precipitated from the solution as a less soluble crystal in...

We show how the amino acids needed on prebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids-that have been identified in the Murchison meteorite-with alpha-keto acids under credible prebiotic conditions. When they are simply heated together they perform a process of decarboxylative transamination but with almost no chiral transfer, and that in the wrong direction, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reaction intermediates is formed, and now there is the desired L to L transformation, producing small enantioexcesses of the normal L-amino acids. We also show how these can be amplified, not by making more of the L form but by increasing its concentration in water solution. The process can start with a miniscule excess and in one step generate water solutions with L/D ratios in the over 90% region. Kinetic processes can exceed the results from equilibria. We have also examined such amplifications with ribonucleosides, and have shown that initial modest excesses of the D-nucleosides can be amplified to afford water solutions with D to L ratios in the high 90's. We have shown that the homochiral compound has two effects on the solubility of the racemate. On one hand it decreases the solubility of the racemate by its role in the solubility product, as a theoretical equation predicts. On the other hand, it increases the solubility of the racemate by changing the nature of the solvent, acting as a cosolvent with the water. This explains why the amplification, while large, is not as large as the simple theoretical equation predicts. Thus when credible examples are produced where small enantioexcesses of D-ribose are created under credible prebiotic conditions, the prerequisites for the RNA world will have been exemplified.

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