Many similar experiments followed, by Miller and others, using
both electrical discharge and ultraviolet light. The compositions
of the gas mixtures were varied, and included H2S,
CO, and CO2. Almost any starting
mixture containing compounds of both nitrogen and carbon led to
amino acids, as long as free oxygen was absent.
It seems that the spontaneous formation of amino acids by lightning
and ultraviolet radiation would have been virtually inevitable on
Earth in a reducing atmosphere, but impossible in an oxidizing atmosphere.
The first products in these experiments usually were hydrogen cyanide
(HCN), cyanogen (NC-CN), cyanoacetylene (H-C=C-C=N), formaldehyde
(HCHO), acetaldehyde (CH3CHO), and
These products then reacted to form various nitriles (R-CN), which
subsequently hydrolyzed in aqueous solution:
The results were mixtures of formic, acetic, propionic, lactic,
succinic, and other organic acids; glycine, alanine, aspartic and
glutamic acids, and other biological and nonbiological amino acids;
urea, methylurea, and various other small molecules.
None of these artificially synthesized molecules showed optical
activity; they all were equal mixtures of D and L isomers. As has
been mentioned previously, the optical activity that biological
molecules exhibit today is a result of choices by enzymes in living