Download Determination of Organic Compounds Formed in Simulated

ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
2001 © The Japan Society for Analytical Chemistry
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Determination of Organic Compounds Formed in Simulated
Interstellar Dust Environment
Yoshinori TAKANO,1 Kentaro USHIO,1 Hitomi MASUDA,1 Takeo KANEKO,1
Kensei KOBAYASHI,† 1 Jun-ichi TAKAHASHI,2 and Takeshi SAITO3
1† Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai,
Hodogaya-ku, Yokohama, 240-8501 Japan (E-mail: [email protected])
2 NTT Telecomunications Energy Laboratories, 3-1 Morinosato-wakamiya, Atsugi, 243-0198 Japan
Institute for Advanced Studies, 1-29-6 Shinjuku, Shinjuku-ku, Tokyo, 160-0022 Japan
3
Abiotic formation of amino acid precursors by irradiation of simulated interstellar dust (ISD) components were
quantitatively examined. Ultraviolet light and cosmic rays are believed to be two major energy sources for organic
formation in space. In order to investigate the formation of organic compound in ISDs, gas mixture including a C-source
(carbon monoxide) and a N-source (nitrogen or ammonia) was irradiated with UV light from a deuterium lamp, soft
X-rays from an electron synchrotron, high energy protons or electrons from accelerators, and γ-rays from 60Co. A wide
variety of amino acids were detected after acid hydrolysis in all the products but those by UV irradiation of carbon
monoxide, nitrogen and water. Total amount of glycine depended on the total deposited energy in the mixture of carbon
monoxide, ammonia and water, while it was independent from those energy sources. The present analytical results suggest
that the yield of amino acids in ISDs depend on their total deposited energy of UV and cosmic rays.
(Received on August 10, 2001; Accepted on September 13, 2001)
For the generation of life, fundamental building blocks
associated with life like amino acids should have been formed
in the primitive atmosphere and/or delivered by comets and
meteorites. As to the former, possible endogenous formation of
amino acids was first examined by Miller.1 He analyzed spark
discharge products in a gas mixture of CH4, NH3, H2 and H2O.
Nowadays, primitive earth atmosphere was not strongly
reduced components but only mildly reduced one.2 Kobayashi
et al.3, 4 reported the formation of bioorganic compounds in
simulated planetary atmospheres by high energy particles or
photons to show possible formation of bioorganic compounds
in the atmospheres of primitive earth.
The latter, extraterrestrial organic compounds, have been
discussed from the following points of view: (i) Source of
organic compounds for the first terrestrial biosphere,5 and (ii)
fossils of chemical evolution in prebiotic environment.6 In
addition, successful detection of enantiomeric excess in
meteoritic L-form amino acids7 implied that the origin of
biomolecular chirality came from exogenous influence. One
of the possible sources of biomolecular chirality proposed is
specific circularly polarized light.8
Greenberg et al.9 proposed a cyclic evolutionally model off
interstellar dusts: Organic compounds were formed and
transformed in interstellar dusts (ISDs) when they travels in
molecular clouds and diffuse clouds, then they were preserved
in comets when ISDs 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.6 Nitrogen (N2) may exist in ISD
environment, but it cannot be detected spectrographically.10
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.11 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.12 also showed that a variety
of organic compounds including glycine was found in the
product by UV irradiation of simulated ISD environment at 12
K.
Most of the previous works are, however, not quantitative so
that we cannot discuss energetics of abiotic formation of
organic compounds in ISD environments. Here we report the
formation of bioorgnic compounds by photon sources
quantitatively, and compare the roles of photons (UV, soft
X-rays and γ-rays) with high energy particles in chemical
evolution in interstellar space.
Experimental
Materials
Carbon monoxide, ammonia and nitrogen of ultra pure grade
used as starting materials for irradiation experiments were
purchased from Nihon Sanso Co. All reagents were of
analytical reagent grade. Deionized water was further purified
with a Millipore Milli-Q LaboSystemTM and a Millipore
Simpli Lab-UVTM (Japan Millipore Ltd., Tokyo, Japan) to
remove both inorganic ions and organic contaminants.
Instruments
An HPLC system for a amino acid analyzer was composed of
two high performance liquid chromatograph pumps (Shimadzu
LC-6A), a cation exchange column (Shimpak ISC-07/S1504, 4
mm i.d. ×150 mm), a post column derivatization system, and a
Shimadzu RF-535 fluoromeric detector. A GC/MS system of
a gas chromatograph combined with a mass spectrometer
(Finnigan-MAT GCQ) was also used with a Chrompack
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Chirasil-D-Val capillary column (0.25 mm i.d. ×25 m). All the
glass wares were heated in high temperature oven (Yamato
DR-22) at 500℃ in prior to use in order to eliminate any
possible contaminants.
Irradiation with photons
Starting gas mixtures were filled in a Pyrex glass tube:
carbon monoxide for 350 Torr, ammonia or nitrogen for 350
Torr over liquid water which provide 20 Torr of water vapor at
room temperature. A 150 W deuterium lamp with a MgF2
window (Hamamatsu Photonics L1835) was used for UV (< 10
eV) irradiation (Fig. 1). Synchrotron radiation (SR) from the
ABL-5C beam line of the normal conducting accelerator ring at
NTT's SR facility was used for soft X-rays irradiation.
Schematic view of the SR experimental system for irradiation
with 1-2 keV X-rays was shown in preliminary report.13 The
same kind of gas mixture was irradiated with γ-rays (1.2 - 1.3
MeV) from a 60Co source in Research Center for Nuclear
Science and Technology, University of Tokyo.
Irradiation with high energy particles
In order to simulate actions of cosmic rays, the same kinds of
gas mixtures were irradiated with protons of 3.0 MeV
generated from a van de Graaff accelerator (Tokyo Institute of
Technology), or electrons of 400 MeV from the SF electron
synchrotron (Institute for Nuclear Study, University of Tokyo).
The total energy deposited in the gas mixture was given as the
product of number of the particles irradiated and an ionization
energy loss of single particle in the gas mixture.
Analysis of amino acids
After irradiation, an aliquot of the aqueous solution of the
irradiation products was hydrolyzed with 6 M HCl at 110℃ for
24 hours. Portions of hydrolyzed and unhydrolyzed fraction
were analyzed with the HPLC system where a post-colomn
derivatization with o-phthalaldehyde and N-acetyl-L-cystein in
boric acid buffer14 was applied.
In addition, the hydrolyzed products were subjected to
GC/MS analysis for identification and measurement of the D/L
ratio (optical isomers ratio) of amino acids. Derivatization for
GC/MS was performed as follows: Acetylchloride in
2-propanol was prepared by mildly dropping into iced
2-propanol. Prepared derivative reagent mixture was poured to
esterify amino acids at 110 ℃ . After gentle dryness,
trifluoroacetic anhydrite (TFAA) was added for acylation to
make N-(trifluoroacetyl) amino acid 2-propyl esters. Those
derivatives were extracted with dichloromethane three times.
The dichloromethane fractions were combined, and carefully
concentrated under nitrogen gas flow. In the GC/MS runs, the
oven temperature was raised from 35℃ to 140℃ at the rate of
3℃ min-1, and then raised to 190℃ at the rate of 10℃ min-1.
The analysis were ionized with 70 eV electrons (EI mode).
ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
Results and Discussion
Amino acids formation by irradiation of a mixture of CO, NH3
and H2O
A HPLC chromatogram of the product of UV irradiation of
the gas mixture of CO, NH3 and H2O (hereafter referred as
CAW) was shown in Fig. 2. A wide variety of proteinous
amino acids such as glycine, alanine, asparatic acid and
nonproteinous amino acids such as β-alanine, α- and
γ-aminobutyric acid were detected. The D/L ratio of alanine
was proved to be nearly 1.0 by GC/MS, as shown Fig. 3. Since
we used linearly polarized excitation sources, the result was
consistent with the fact that symmetric formation for optical
isomers were progressed. This fact, with the presence of
nonproteinous amino acids, indicated that the amino acids
found were not contaminated but indigenous to the product.
ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
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Amino acids formation by irradiation of a mixture of CO, N2
and H2O
Possible formation of amino acid precursors from a mixture
of CO, N2 and H2O (hereafter referred as CNW) was examined.
Table 2 summarized amino acid yields by irradiation with
photons or particles after acid hydrolysis. When the gas
mixture was irradiated with high energy particles (protons,
electrons) or high energy photons ( γ-rays, soft X-rays), a wide
variety of amino acids were detected. In the case of UV
irradiation, however, only trace amount of amino acids were
detected. Nitrogen triple bond is difficult to photo-dissociate
with UV photons (<10 eV), which would be the reason.
When the same gas mixture was irradiated with γ-rays or
high energy protons, hydrolysates of the products gave the
same kind of amino acids. Table 1 summarized amino acid
yields in both hydrolyzed and unhydrolyzed irradiation
products. In unhydrolyzed fractions, only trace of glycine was
detected. It showed that not free amino acids, but amino acid
precursors were formed during irradiation.
Fig. 4 shows the relationship between yields of aliphatic
amino acids (YUV) and their carbon numbers ( Cn; Glycine: 2,
Alanine: 3, α-aminobutyric acid: 4). The larger the carbon
number of amino acid, the smaller the yield of the amino acid is.
It has been reported that this is a common tendency of
abiotically-formed amino acids, such as those found in
carbonaceous chondrites.15 The semilogarithmic linear relations
found between them are:
( R = 0.99 )
ln YUV = - 1.65 Cn + 7.85
ln YProton = - 1.82 Cn + 11.33 ( R = 0.99 )
These linear relations postulate a synthetic reaction in which
larger molecules are formed from smaller ones in the
homologues by the addition of one element species in the
period of eventual hydrolysis.
Energetics of the formation of amino acid precursors in space
Figs. 5 and 6 shows the relationship between total energy
deposited to the gas mixtures and yield of glycine (after
hydrolysis). In the case of the CAW mixture, glycine yield
was in proportion to the total energy deposit, but was
independent from the types of energy. From the CNW mixture,
protons, electrons and g-rays gave glycine in the same energy
yield, but UV did not give detectable amount of glycine. Soft
X-rays yielded glycine, but the energy yield is lower that of
particles or γ-rays.
Table 3 summarizes enegy yields of glycine by irradiation of
gas mixtures: G-values stand for total numbers of molecules
per 100 eV energy deposit.16 G-value of glycine from the CAW
mixture by UV was 0.022, which was larger at least by four
orders of magnitude than the CNW mixture. It is suggested that
ammonia is necessary for photochemical formation of
bioorganic compounds in space.
Galactic cosmic rays produce secondary γ-rays, soft X-rays
and UV, as well as charged particles, in the process of losing
their kinetic energy when they interact with materials. The
present results suggest that not only galactic cosmic rays but
also secondary photons and particles can produce amino acid
precursors and other N-containing organic compounds in
interstellar space where ammonia is available as a N- source.
Extreme UV (EUV) light of 1 - 100 nm of wavelength seems
to be available in space. It is suggested that G-value of glycine
from the CAW by EUV is between that by UV and that by soft
X-rays, i.e., that would range from 10-3 to 10-6 from the CNW
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ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
University of Tokyo, for their 60Co management and
experimental advice. They also thank Dr. K. Kawasaki, Tokyo
Institute of Technology, and the staff of the SF synchrotron,
INS, University of Tokyo, for their kind assistance in the
operation of the accelerators.
This present work was supported in part by Grant-in-Aid for
Exploratory Research from Japan Society for the Promotion of
Science (No. 13874066).
mixture. Thus EUV might make larger contribution to the
formation of bioorganics than the longer wavelength UV. It is
difficult, however, to obtain such short wavelength UV on the
ground due to the lack of appropriate window materials. In
order to verify abiotic formation of bioorganic compounds in
space, the exposed facility of Japan Experimental Module
(JEM) is promising for the EUV irradiation experiments since
solar shorter wavelength UV is available there.17
Reference
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Summary and Future Prospects
When the ISDs type gas mixture (the CAW mixture) was
irradiated with photons or high energy particles, wide variety of
amino acids were detected in the hydrolysates of the products,
and the energy yield by UV are of the same level as other
higher energy sources. In the case of the CNW mixture, UV
did not give amino acids. The latter fact suggested that cosmic
rays are more important energy sources for abiotic formation of
bioorganic compounds in primitive Earth atmosphere than solar
UV.3 On the other hand, not only cosmic rays but also UV is
important to produce bioorganic compounds in interstellar
space where ammonia exists.
Here gaseous mixtures are used to evaluate the formation of
amino acid precursors in space. In interstellar space, most
materials are frozen onto the surface of dusts. It is of interest
to compare energy yields in solid and gaseous phase.11
D/L ratio of amino acid having chiral carbon was nearly 1.0,
indicating those amino acids were abiotically synthesized,
however, proteinous amino acids associated with life are chiral,
that is, one-handedness of L-form. Enantiomeric excess in
meteoritic L-form amino acids7 may imply the origin of
biomolecular chirality came from exogenous influence such as
neutron star radiation specific circularly polarized light to
induce asymmetric synthesis. Experiments using circularly
polarized light by synchrotron radiation are also required for
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The authors would like to thank Dr. T.Hosokawa for machine
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