KOICHIRO MATSUNO

Abstract. Oligopeptides synthesized from glycine and alanine in the flow reactor simulating hydrothermal circulation of seawater through hot vents in the ocean on the primitive earth demonstrated that the activity of dissecting the peptide bond of glycylalanine coincided with the buildup of those oligomers of alanylalanine, alanylgylcine, alanylalanylalanine, alanylalanylalanylalanine. Hydrothermal environments can provide oligopeptides with both the capacities of integration and disintegration. Such a coexistence of both the capacities comes to enhance the specificity of oligomers to be synthesized in hydrothermal environments.

Hydrothermal vents on the sea floor of the primitive earth have been proposed as likely locales among others for prebiotic synthesis (Corliss et al., 1979). What makes hydrothermal environments significant prebiotically is constant circulation of seawater from hot vents to surrounding cold seawater and back again repeatedly. In particular, the interface between hot and cold seawater in hydrothermal environments can serve as a selective sieve for saving chemical products synthesized in hot vents if the products are immediately transferred into the cold region before they are disintegrated while remaining in the hot region. The selective enhancement of the synthesis could become multiplicative if the products can revisit hot vents repeatedly.

We already attempted an experimental model simulating geological conditions for hydrothermal circulation of seawater through hot vents (Matsuno, 1997; Imai et al., 1999), and observed an exponential growth of oligopeptides with the elapse of time at least initially when the circulating reaction solution was initially glycine dissolved in water (Imai et al., 1999). One specific implication of the significance of the interface chemistry in the simulated hydrothermal environment was that the yields of triglycine synthesized from gylcine were more than 1000 times greater than the yields expected from equilibrium reactions at the high temperature available from the simulated hot vents (Ross, 2000).

A principal feature of the flow reactor we constructed for simulating hydrothermal circulation of reaction solution was that a high-temperature, high-pressure fluid is injected into a low-temperature chamber maintaining the same high pressure while the whole fluid is circulated in a closed manner in the system with a fixed turnover rate. The high-temperature, high-pressure fluid was prepared in a pressurized and heated section of the closed circuit of the fluid, whereas the low-temperature chamber was maintained by immersing it in an external cooling apparatus. The high-temperature, high-pressure fluid jet from a nozzle into the low-temperature chamber was intended to simulate a submarine hydrothermal vent. At the same time, the fluid flowing out of the low-temperature chamber was first depressurized to normal atmospheric pressure to sample a very small fluid volume for the purpose of measurement at a given time interval and then followed by conversion back to the high-temperature, high-pressure fluid.

We first prepared 500 ml solution of 100mM glycine without and with 10mM diketopiperazine (DKP) at pH 2.5 adjusted by HCl at room temperature. The addition of DKP was intended to see how intermediary reaction products would contribute to the on-going synthetic reactions. The pressure of the high-temperature, high-pressure chamber with a volume 15ml was set at 24.0 MPa which is only slightly above the pressure of the critical point of water (22.1MPa). The flow rate was about 10 ml /min. This gave the cycle time 34 seconds of reactants rounding the closed flow pathway in stirred conditions. The temperature of the high-temperature chamber was set at 250^oC. Figure 1 demonstrates the time courses of the yields of oligoglycines without (a) and with (b) DKP. There is observed a significant enhancement of the synthesis of the longer oligoglycines due to the addition of DKP as the intermediary reaction products.

In order to see how the intermediary reaction products would contribute to further elongation of oligopeptides thus synthesized, we tried 500 ml solution of 40mM L-alanine and 10mM glycine at pH 2.5 adjusted by HCl at room temperature, while other conditions were the similar to the previous one. Figure 2 demonstrates the time courses of the yields of oligopeptides identified. We have identified at least six different oligpeptides; Ala-Gly, Gly-Ala, Ala-Ala, Gly-Ala-Ala, Ala-Ala-Ala, Ala-Ala-Ala-Ala, where Ala for L-alanine and Gly for glycine. We then observed that the rises of Ala-Gly, Ala-Ala-Ala, Gly-Ala-Ala, and Ala-Ala-Ala-Ala coincided with the decrease of Gly-Ala. It is thus suggested that the activity of dissecting the Gly-Ala bond could enhance the production of the four oligopeptides. The driving factor for selective elongation of oligopeptides supplemented by the activity of dissecting the intermediary products is the interface chemistry between the hot and cold regions through constant circulation of the reaction solution from the hot to the cold repeatedly.

Activity of dissecting intermediary oligopeptides for further elongation, that is similar to a protease activity, can in turn become highly selective in the oligopeptide elongation when it is further supplemented by an exponential growth of the products. Constant circulation of seawater through hot vents into surrounding cold seawater can install such a multiplicative amplification of the synthesized products if the interface chemistry capable of rapidly transferring the synthesized products into the cold water is certainly operative. As a matter of fact, cycle time for those reaction products to revisit any one of the hot vents in the primitive ocean could certainly have remained finite, even though it may have been over tens of thousands of years. In particular, those oligopeptides carrying proto-protease activities may have been synthesized even prebiotically in hydrothermal environments on the primitive earth or on the similar planets elsewhere.

Williams, D., Bainbridge, A., Crane, K. & van Andel, T. H. Submarine thermal springs on the Galapagos

rift. Science 203, 1073-1083 (1979).

Imai, E., Honda, H., Hatori, K., Brack, A. & Matsuno, K. Elongation of oligopeptides in a simulated

hydrothermal system. Science 283, 831-833.

Matsuno, K. A design principle of a flow reactor simulating prebiotic evolution. Viva Origino 25, 191-204

(1997).

Ogata, Y., Imai, E., Honda, H., Hatori, K. & Matsuno, K. Hydrothermal circulation of seawater through hot

vents and contribution of interface chemistry to prebiotic synthesis. Origins Life Evol. Biosphere (2000), in

press.

Ross, D. Private communication (2000).