4:40 PM - 4:55 PM
[MIS07-05] Peptide Formation from amino acid precursors under simulated submarine hydrothermal environments
Keywords:Submarine hydrothermal systems, Chemical Evolution, Amino acid precursors, Peptides, Flash and quench
Experimental: We chose the following compounds or materials for hydrothermal experiments: (i) Free glycine (Gly); (ii) aminoacetonitrile (AAN), a Strecker-type precursor; (iii) hydantoin (Hyd), a glycine precursor found in carbonaceous chondrites; (iv) CAW; complex organic molecules formed by proton irradiation of a mixture of CO, NH3 and H2O . CAW gave various amino acids after acid hydrolysis, where glycine was predominant. The CAW stock solution used in this study yielded ca.10 mM glycine after acid hydrolysis.
In order to simulate submarine hydrothermal reactions, we used a flow reactor, composed of a HPLC pump, an infrared gold image furnace, a cold bath and a backpressure regulator . Injected aqueous samples could be heated up to 400°C in the furnace, and then quenched in the ice bath (0 °C). Carrier used was purified water (neutral) or 1 mM HCl (acidic). In the present study, 0.25 mL of each sample was processed in the furnace at room temperature (reference) or 200-400°C for 2 min, and then quenched. Amino acids in the effluent were determined by cation exchange HPLC (Shimadzu LC-20 amino acid analyzer with a fluorescence detector) before or after acid-hydrolysis. Peptides were determined by ion-pair HPLC (Shimadzu LC-20 HPLC system with a UV detector).
Results and Discussion: Stability of each material at 300°C was examined by using 4 mM of Gly, AAN or Hyd, or 10-times diluted CAW. Hyd, AAN and CAW gave some glycine in unhydrolized products. Recovery was calculated by glycine amount after hydrolysis against the reference. Hyd and CAW were more stable than Gly and AAN.
Peptide formation experiments were performed by using 40 mM each of Gly, AAN or Hyd, or the CAW stock solution. Diketopiperazine (DKP; a cyclic dimer of Gly) and diglycine (Gly2) were detected in the unhydrolized samples after Gly or Hyd was heat-processed. Hyd gave much more Gly2 and DKP than Gly. The Best yield of Gly2 was 1.3 %, which was observed when Hyd was processed at 200 or 250°C under neutral conditions. CAW gave so many peaks in ion-pair chromatograms, so that we could not identify peptides there.
In order to examine possible catalytic abilities of CAW, 10-times diluted CAW was added to 40 mM Gly or 40 mM Hyd and then heat-processed. Yields of Gly2 were increased when CAW was added to Gly or Hyd and heated at 200°C under acidic conditions.
It was shown that amino acid precursors such as Hyd and CAW was more stable than free amino acids, and that amino acid precursors like Hyd could yield more peptides than free amino acids. It was also found that CAW (complex amino acid precursors) could facilitate peptide formation. It is known that CAW has amide bonds in its structure, which could be a reason why CAW could work as a condensing agent.
In summary, amino acid precursors were more plausible starting materials than free amino acids in prebiotic reactions in primordial terrestrial environments. Kurihara et al.  showed that organic aggregates could be formed from water-soluble organics in hydrothermal environments. It is of interest to examine whether CAW could give such aggregates after processed in the flow reactor.
This work was partly supported by JSPS KAKENHI Grant Number JP17H02991.
 H. Yanagawa and K. Kobayashi, Orig. Life Evol. Biosph., 22, 147-159 (1995).
 E. Imai et al., Science, 283, 831-833 (1999).
 Y. Takano et al., Appl. Phys. Lett., 84, 1410-1412 (2004).
 Md. N. Islam et al., Bull. Chem. Soc. Jpn., 76, 1172-1178 (2003).
 Y. Kurihara et al., Chem. Lett., 41, 441-443 (2012).