On the origin of terrestrial homochirality for nucleosides and amino acids

Abstract

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...

The Origin of Biological Homochirality

“…Enantioenrichment in solution is thus dictated by thermodynamics for chiral compounds that happen to form relatively insoluble racemic compounds. This concept was first recognized by Morowitz (1969) 40 yr ago and was more recently elaborated by Blackmond's work probing eutectic composition for a variety of amino acids and other chiral compounds , and by Breslow (Breslow and Levine 2006) who recently also reported high eutectic ee values for several nucelosides of prebiotic importance (Breslow and Cheng 2009). A recent theoretical treatment based on a two-dimensional lattice model successfully predicts the ternary phase behavior of amino acids based on the interactions that stabilize the racemic crystal, providing molecular level insight into the observed enantiomer partitioning (Lombardo et al 2009).…”

The origin of biological homochirality

“…This concept was first recognized by Morowitz [23] 40 years ago and was more recently elaborated by Klussmann et al [22,24] and by Breslow & Levine [25]. Also Breslow & Cheng recently reported high eutectic ee values for several nucelosides of prebiotic importance [26]. A recent theoretical treatment based on a two-dimensional lattice model successfully predicts the ternary phase behaviour of amino acids based on the interactions that stabilize the racemic crystal, providing molecular-level insight into the observed enantiomer partitioning [27].…”

Reply to Bada: The relevance of meteoritic alpha-methyl amino acids to prebiotic homochirality

“…In his letter Bada (1) is complimentary with respect to the work we actually performed and reported in a recent issue of PNAS (2). However, he is skeptical about the relevance of the alpha-methyl amino acids found by others in the Murchison meteorite, species with a few percent of an excess of the L enantiomers.…”