A new pathway to aspartic acid from urea and maleic acid affected by ultraviolet light.

The photochemistry of a mixture of ureaand maleic acid, which are thought to have been widelypresent on the primitive Earth, was studied in order toexamine a possibility of the formation of amino acids. When an aqueous solution of urea and maleic acid wasirradiated with an ultraviolet light of wavelength 172 nm,urea was revealed to be rather resistant to photochemicaldecomposition. In contrast, maleic acid was completelydecomposed within 4 h, reflecting the reactivity of a C-Cdouble bond in the molecule. In the reaction mixture, 2-isoureidosuccinic acid was detected. The acid wasconsidered to be formed by addition of an isoureido radicalwhich had been produced from urea by the action of ahydroxyl radical, to a C-C double bond of maleic acid. Theisoureido group of the product was revealed to undergothermal rearrangement to afford 2-ureidosuccinic acid (N-carbamoylaspartic acid). The result suggested a novelpathway leading to the formation of aspartic acid from non-amino acid precursors, possibly effected by UV-light on theprimitive Earth. The formation of ureidocarboxylic acidsis of another significance, since they are capable ofundergoing thermal polymerization, resulting in formationof polyamino acids.