986
Chemistry Letters 2002
Formation of Amino Acids from Possible Interstellar Media by ꢀ-rays and UV Irradiation
Yoshinori Takano,y Hitomi Masuda,y Takeo Kaneko,y and Kensei Kobayashiꢀy;yy
yDepartment of Chemistry and Biotechnology, Yokohama National University, Hodogaya-ku, Yokohama 240-8501
yyInstitute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara 229-8510
(Received June 21, 2002; CL-020518)
Formation of amino acids from mixtures of methanol,
A starting mixture of methanol and ammonia aqueous
solution was prepared; the molar ratio of methanol, ammonia
and water was 1 : 1 : 2:8. For the UV irradiation, the mixture was
sealed in a quartz vessel. The three phases of the mixture was
achieved as follows: The solid condition was made in liquid
nitrogen bath to keep at 77 K. The liquid condition was performed
at ambient temperature. Starting gas mixtures were filled in a
Pyrex glass tube: Methanol for 350 Torr, ammonia for 350 Torr
over liquid water which provide 20 Torr of water vapor at 353 K
with heating coil wrapped. A 150 W deuterium lamp with a MgF2
window (Hamamatsu Photonics L1835) was used for UV
(<15 eV) irradiation. Since irradiation window was attached
with quartz window, above 160 nm wavelength was obtained.
The energy deposit was 3:1 ꢃ 1020 eV each.
ammonia and water in various phases by ꢀ-rays and UV
irradiation were quantitatively verified. A wide variety of amino
acids were detected in every hydrosate of the product. Each G-
value (the number of formed molecules per deposit energy of
100 eV) was around 10ꢁ2 order, which was independent from the
phase of the starting materials and kind of the energy.
Wide variety of organic compounds have been discovered in
extraterrestrial environments, which are discussed from the
content of the emergence of life on the primitive Earth.
Extraterrestrial organic compounds, have been discussed from
the following points of view: (i) Source of organic compounds for
the first terrestrial biosphere,1 and (ii) fossils of chemical
evolution in prebiotic environment.2 Greenberg et al.3 proposed
a cyclic evolutionally model of interstellar dusts: Organic
compounds were formed and transformed in interstellar dusts
(ISDs) when they travels in molecular clouds and diffuse clouds,
thentheywere preservedincomets whenISDs grown as comets in
the proto-solar system. Thus it seems that the first step of the
abiotic formation of organic compounds takes place in ISDs in
molecular clouds. Representative carbon sources for abiotic
formation of organics are carbon monoxide, formaldehyde and
methanol, and a major nitrogen source is ammonia.2;4 Nitrogen
(N2) may exist in ISD environment, but it cannot be detected
spectrometrically.5 Those ices are irradiated with UV from
neighboring stars and galactic cosmic rays. There have many
studies to simulate possible chemical reactions in ISDs.
Kasamatsu et al.6 showed that amino acid precursors (molecules
which provide amino acids after hydrolysis) were formed when an
icy mixture of carbon monoxide, ammonia and water were
irradiated with high energy protons. Briggs et al.7 showed that a
variety of organic compounds including glycine was found in the
product by UV irradiation of simulated ISD environment at 12 K.
Recent simulation experiments8;9 also reported the abiotic
formation of other types of amino acids in ice mantles at 12 K.
Most of the previous works are, however, not quantitatively
examined abiotic formation of organic compounds. Here we
compared the G-value rate (the number of formed molecule/
100 eV)10 of amino acids with ꢀ-rays and UV photons from
various phases; solid phase at 77 K, liquid phase at 293 K and
gaseous phase at 353 K.
The same kind of the gas mixture in three phases sealed in
Pyrex tubes were irradiated with ꢀ-rays (1.2–1.3 MeV) from a
60Co source in Research Center for Nuclear Science and
Technology, University of Tokyo. The energy deposit was
2:3 ꢃ 1019 eV each. After irradiation, an aliquot of the irradiation
products was hydrolyzed with 6 M HCl at 110 ꢂC for 24 h. Amino
acids in the hydrolyzed and unhydrolyzed fraction were analyzed
with an ion-exchanged HPLC system where a post-column
derivatization with o-phthalaldehyde and N-acetyl-L-cystein was
applied.11 The HPLC system used was composed of two high
performance liquid chromatograph pumps (Shimadzu LC-6A), a
cation
exchange
column
(Shimpak
ISC-07/S1504,
4 mm i.d. ꢃ 150 mm), a post column derivatization system, and
a Shimadzu RF-535 fluoromeric detector.
An ion-exchanged chromatogram of the product of UV
irradiation of the gas mixture of methanol, ammonia and water
was shown in Figure 1. A wide variety of proteinous amino acids
such as glycine, alanine, aspartic acid and non-proteinous amino
acids such as ꢁ-alanine, ꢂ- and ꢀ-aminobutyric acid were
detected. With the presence of non-proteinous amino acids,
indicated that the amino acids found were not contaminated but
indigenous to the product. In the unhydrolyzed fraction, only
small amount of glycine was detected. It showed that not free
amino acids, but amino acid precursors were formed during
irradiation. When the same gas mixture was irradiated with ꢀ-
rays, hydrolysate of the products gave similar kinds of amino
acids. In the both products, the semi logarithmic linear relations
between carbon number of amino acid and amount of product are
observed.12
Hereafter we will use the G-value (the number of formed
molecules per 100 eV) of glycine in the hydrolyzed products
when the yields of amino acids are discussed, because (i) glycine
is the most abundant amino acid in the hydrolyzed products, and
(ii) it was proved that glycine was formed in proportion to total
energy deposite.13;14 The G-values of glycine in the irradiation
experiments were shown in Figure 2. In both experiments of ꢀ-
rays and UV irradiations, G-values were around 10ꢁ2 in all of the
Methanol and ammonia used were of ultra pure grade as
starting materials for irradiation experiments. Deionized water
was further purified with a Millipore Milli-Q LaboSystemTM and
a Millipore Simpli Lab-UV (Japan Millipore Ltd., Tokyo, Japan)
to remove both inorganic ions and organic contaminants. All the
glass wares were heated in high temperature oven (Yamato DR-
22) at 500 ꢂC in prior to use in order to eliminate any possible
contaminants.
Copyright Ó 2002 The Chemical Society of Japan
Takano, Yoshinori
