L-amino acids catalyze the formation of an excess of D-glyceraldehyde, and thus of other D sugars, under credible prebiotic conditions

Abstract

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...

A Chemical Engineering Perspective on the Origins of Life

“…Predominantly L-handed amino-oxazolines-precursors to RNA-are formed from the reaction of amino-oxazole with glyceraldehyde at a 1% excess of the L-enantiomer in the presence of chiral amino acids [136]. After the emergence of L-RNA, L-amino acids could then have determined the handedness of the D-sugars we see in life today [137].…”

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“…It is not the arabino-6 initially formed by this addition reaction that ends up as the pyrimidine ribonucleotides; if it were, then there would be no way to produce enantiomerically pure ribonucleotides unless a prebiotic route to enantiopure glyceraldehyde 5 existed. Routes to non-racemic glyceraldehyde can be envisaged [9], but obtaining this three-carbon sugar enantiopure looks very difficult. A means of enantioenrichment is, therefore, needed at some point in the synthetic route.…”

Artificial enzymes, cancer chemotherapy, conjugation and nanoelectronics, and prebiotic chemistry

“…As we described [103], we found that L amino acids that we examined and that were likely ones on prebiotic earth did in fact catalyze the formation of an excess of D glyceraldehyde under credible prebiotic conditions. We performed simple aldol condensations of glycolaldehyde and formaldehyde in water solution with the L amino acids, including L -methyl valine.…”