Fig. 4.
Chemical structure of daPNA (Left) and RNA (Right) molecules. B, nucleic bases, attached qua carbonyl-spacers to the PNA respectively to the RNA backbone; R1–4, rest of the genetic material's molecular chain structure. The backbone monomer of the shown daPNA structure is 2,4-diaminobutanoic acid, which was identified in the Murchison meteorite. Beside 2,4-diaminobutanoic acid, ornithine had been suggested to be essential constituent of a further PNA backbone structure (24). Ornithine was identified in the Murchison meteorite tentatively.