Photostability of amino acids to Lyman α radiation: Glycine
Graphical abstract
Glycine Lyman α photostability and its degradation were followed by 252Cf-PDMS. Positive and negative desorption ions emitted are analyzed, their yields time dependence is determined.
Highlights
► Lyman-α UV Lamp generates chemical reactions in glycine. ► 252Cf fission fragments impact on target sample and induce desorption. ► Ion desorption yield measurement gives the abundance of intact molecules in surface. ► Dependence of glycine abundance on irradiation dose is exponential.
Introduction
Pre-biotic molecules arising from space, including aldehydes, ethers, nitriles, quinones, and amino acids, may have played an important role in the origin and evolution of life [1], [2], [3]. Among these compounds, the amino acids are involved in a crucial way because they are the basic components of proteins, fundamental constituents of all living organisms.
The vacuum ultraviolet (VUV) photostability of amino acids is of considerable interest in view of the possible delivery of these molecules, and even more complex molecular species, from space to the primitive Earth, and the role that they could have played in the origin and development of life on our planet [4], [5], [6]. Amino acids are found in both meteorites and micrometeorites [7], [8] and much effort is currently being undertaken to observe them in the interstellar medium (ISM) [9].
Amino acids have been identified in several carbonaceous meteorites in concentrations of up to 3 parts per million versus carbon [10]. Whereas life on Earth is based on l-amino acids (left-handed), those measured in meteorites are 50% in d-form (right-handed) or racemic (mixed handedness). Small excesses of left-handed extraterrestrial amino acids, such as isovaline, have been reported in the Murchison and Murray meteorites [7]. The fact that meteoritic amino acids are deuterium-enriched [11], [12], [13] may imply an interstellar heritage [14]. Formation of amino acids in the interstellar medium (ISM) may be possible via specific gas-phase reactions in dark clouds [15]. At present, the detection of glycine (H2NCH2COOH; the simplest amino acid) in the interstellar gas remains controversial [9], [16]; an upper limit of 10−10 (per H2) has recently been estimated in the low-mass protostar IRAS 16293-2422 [17].
To assess the availability of amino acids for pre-biotic chemistry, it is important to investigate the stability of such compounds in extraterrestrial environments. Recent studies focused on the thermal properties of amino acids and their stability upon impact [18], [19], other work studies the preservation of amino acids enantiomeric excess with the stability under gamma radiation in comets and meteorites [20]. Another work tested the photostability of four amino acids in Ar, N2, and H2O ices to explore whether they could withstand radiation in space in order to investigate how the size and structure of amino acids would influence their destruction by UV photons [14].
In this paper we report experimental results on photostability of glycine. This molecule is proteinaceous and has been found in meteoritic materials [21]. The sample was exposed to a spectral region around 1216 Å (10.2 eV) UV radiation and its degradation were followed by Plasma Desorption Mass Spectrometry (252Cf-PDMS). This is a new methodology for studying photostability, in which the emission of positive and negative ions are analyzed instead of the optical absorption by neutrals, offering possibilities for further understanding of the polymerization induced photolysis of amino acids and/or phase transition induced by UV radiation, as well as the prompt ion emission of the formed chemical species.
Section snippets
Experimental methods and results
The 252Cf-PDMS experimental set-up was constructed at the Van de Graaff Laboratory, in the Physical Department of Pontifícia Universidade Católica do Rio de Janeiro – Brazil, and described elsewhere [22], [23]. This technique has been largely employed to study several physical chemistry phenomena, some of them of astrophysical interest such as the electronic sputtering of astrophysical ices [24], [25], [26], [27], [28]. The experimental set-up, including the H2 UV lamp combined with the
The methodology based on the electronic sputtering
Whenever a very fast and highly charged ion impinges an insulator, it mainly interacts with the target electrons and triggers several phenomena; in particular, chemical reactions are induced and target material is ejected through a process called electronic sputtering [33]. Swelling or craters may occur and their sizes (characterized by L) lay over the nanometer scale. Since the wavelength of the Lyman α line is 121.6 nm, the thickness of the layer under photolysis is at least one or two orders
Summary
In this work we analyzed the photostability of the simplest of amino acids, glycine exposed to 1216 Å (10.2 eV) UV radiation and its degradation were followed by Plasma Desorption Mass Spectrometry (252Cf-PDMS). This is a new methodology for studying photostability, reason why positive and negative secondary ions were analyzed for testing coherence of results. Our analysis reveals that: (a) the interaction of FF 252Cf with glycine molecule produces a negative ion desorption yield slightly more
Acknowledgements
This work was partially supported by Brazilian agencies CNPq and FAPERJ. One of us (AMF-R) would like to acknowledge to Prof. F.N. Rodrigues for his helpful discussion and suggestions about cross section and temperature, and another of us (EFS) would like to acknowledge to Dr. S. Pilling for the discussion about the determination of the total photon flux of the H2-UV lamp.
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Permanent address: Natural Science Department of the Institute of Biosciences, Federal University of the State of Rio de Janeiro (UNIRIO), Av. Pasteur, 458 Bloco III 512D CEP 22290-240 Urca - Rio de Janeiro/RJ, Brazil. Tel.: +55 2122445527; Fax: +55 2122756059.
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