Nano-pulsed discharge plasma-induced abiotic oligopeptide formation from diketopiperazine.

Conventional oligopeptide synthesis techniques involve environmentally harmful procedures and materials. In addition, the efficient accumulation of oligopeptides under Hadean Earth environments regarding the origin of life remains still unclear. In these processes, the formation of diketopiperazine is a big issue due to the strong inhibition for further elongation beyond dipeptides. Hydrothermal media enables environmentally friendly oligopeptide synthesis. However, hydrothermal oligopeptide synthesis produces large amounts of diketopiperazine (DKP), due to its thermodynamic stability. DKP inhibits dipeptide elongation and also constitutes an inhibitory pathway in conventional oligopeptide synthesis. Here, we show an efficient pathway for oligopeptide formation using a specially designed experimental setup to run both thermal and non-thermal discharge plasma, generated by nano-pulsed electric discharge with 16-23 kV voltage and 300-430 A current within ca. 500 ns. DKP (14%) was converted to dipeptides and higher oligopeptides in an aqueous solution containing alanine-DKP at pH 4.5, after 20 min of 50 pps thermal plasma irradiation. This is the first study to report efficient oligopeptide synthesis in aqueous medium using nano-pulsed plasma (with thermal plasma being more efficient than non-thermal plasma) via DKP ring-opening. This unexpected finding is implicative to evaluate the pathway how the oligopeptides could have accumulated in the primitive Earth with high-energy plasma sources such as thunder as well as to facilitate the green synthesis of oligopeptides.