Hydrothermal systems provide a unique
habitat for the subsurface
biosphere, and possibly, for the origin of life. Amides are fundamental
to hydrothermal organic geochemistry and deep subsurface biology research,
in large part because of their involvement in metabolism, such as
in the forms of peptides and proteins, and also because of their participation
in the deep nitrogen cycle and their potential role in the origin
of life. Hydrothermal chemistry of amides is also of great interest
to astrobiology research because it may reveal potential formation
pathways of peptides and biomolecules in the space outside Earth.
Here, we describe a nonmineral-catalyzed synthetic pathway for amide
synthesis under hydrothermal conditions (250 °C and 40 bar, P
sat). We find that a suite of amides (12 examples)
are readily synthesized through a direct condensation between amines
and carboxylic acids, with an amide yield of up to 90% over a timescale
of hours. Time-series hydrothermal experiments were performed to obtain
apparent rate constants for amide synthesis. The observed hydrothermal
rate constants were significantly larger for certain amines (e.g.,
0.2 h–1 for benzylamine) than for others (e.g.,
0.05 h–1 for cyclohexylamine), which suggests a
strong substitution effect on amide formation. An amine acylation
mechanism is proposed, and also consistent with previous studies.
Furthermore, amide formation is found to be strongly inhibited in
high or low pH solutions (e.g., pH <2 or >12), which further
supports
that the condensation reaction should occur between the neutral amine
and acid. Our finding of a feasible and selective hydrothermal pathway
for amide bond formation may provide new insights into understanding
peptides and biomolecule synthesis in relevant hydrothermal environments.
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