Peptide nucleic acids rather than RNA may have been the first genetic molecule

Abstract

Numerous problems exist with the current thinking of RNA as the first genetic material. No plausible prebiotic processes have yet been demonstrated to produce the nucleosides or nucleotides or for efficient two-way nonenzymatic replication. Peptide nucleic acid (PNA) is a promising precursor to RNA, consisting of N-(2-aminoethyl)glycine (AEG) and the adenine, uracil, guanine, and cytosine-N-acetic acids. However, PNA has not yet been demonstrated to be prebiotic. We show here that AEG is produced directly in electric discharge reactions from CH4, N2, NH3, and H2O. Electric discharges also produce ethylenediamine, as do NH4CN polymerizations. AEG is produced from the robust Strecker synthesis with ethylenediamine. The NH4CN polymerization in the presence of glycine leads to the adenine and guanine-N 9 -acetic acids, and the cytosine and uracil-N 1 -acetic acids are produced in high yield from the reaction of cyanoacetaldehyde with hydantoic acid, rather than urea. Preliminary experiments suggest that AEG may polymerize rapidly at 100°C to give the polypeptide backbone of PNA. The ease of synthesis of the components of PNA and possibility of polymerization of AEG reinforce the possibility that PNA may have been the first genetic material.prebiotic synthesis ͉ first genetic material ͉ pre-RNA world ͉ RNA world ͉ chemical evolution T he discovery of the catalytic activity of RNA (1, 2) brought the concept of an RNA world (3-7) into wide acceptance. However, the instability of ribose and other sugars (8), the great difficulty of prebiotic synthesis of the glycosidic bonds of the necessary nucleotides (9, 10), and the inability to achieve twoway non-enzymatic template polymerizations (11, 12) have raised serious questions about whether RNA could have been the first genetic material (13), although there are dissenting opinions (6,14). A pre-RNA world in which the backbone of the first genetic material would have been different from the ribose phosphate seems more likely, but the nature of this backbone is unknown. One proposal offers peptide nucleic acids (PNA) as a possible precursor to RNA (15) because PNA binds DNA and forms double and triple helical structures that are related to the Watson-Crick helix (16-18).The backbone of PNA is a polymer of N-(2-aminoethyl)glycine (AEG), which is sometimes referred to as ethylenediamine monoacetic acid. It is interesting to note that Westheimer listed AEG as one of a number of possible backbones to replace ribose phosphate (19). This was four years before the invention of peptide nucleic acids. The bases are attached to the backbone by a base-substituted acetyl unit, as shown in Fig. 1. The simplicity of the components of PNA suggests that prebiotic syntheses might be feasible. We therefore examined a number of prebiotic syntheses, including electric discharges and NH 4 CN polymerizations for ethylenediamine (ED) and AEG, as well as the adenine and guanine-N 9 -acetic acids and the cytosine and uracil-N 1 -acetic acids. We show here that the components of PNA are synthesized u...

Evidence for reactive reduced phosphorus species in the early Archean ocean

“…or peptides (31) in early replicating molecules. In contrast to calcium phosphate minerals, schreibersite is more reactive and, hence, may have provided soluble P species like phosphite necessary for the formation of organophosphates during the origin of life (9,13).…”

The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification.

“…The RNA world is a purely speculative phylogenetic hypothesis ; we cannot use such an uncorroborated hypothesis, plus a logically untenable assumption, to root the tree objectively ! We do not know if an RNA world ever existed (Cavalier-Smith, 2001) ; some chemists think it likely that RNA replaced an earlier polynucleotide (XNA) shortly after that had invented protein synthesis and protein catalysis provided the first ribonucleotides (Orgel, 1998 ;Nelson et al, 2000). The sequence XNA world, XNA-protein world, RNAprotein world, DNA-RNA-protein world is as plausible as the RNA world hypothesis at present.…”

Systems chemistry of α-amino acids and peptides

“…Experimental support has been provided through the identification of pathways for the synthesis of diamino acid precursors of PNAs [35] and their identification in interstellar ice analogues [36]. However, a pathway leading to PNA monomers in prebiotic environments must involve the nucleic base moiety in addition to the amino acid backbone, the availability of a favoured process leading to the whole structure remains to be demonstrated.…”