How does glycine in ice evolve under irradiation?

Philippe Parent and Carine Laffon
CNRS, UMR 7614, Laboratoire de Chimie Physique, Matière et Rayonnement, 11 rue Pierre et Marie Curie, 75231 PARIS CEDEX 05-France.

To understand how life begun on Earth, it is essential to know what organic compounds were available, and how they interacted with the planetary environment. This prebiotic organic chemistry might have begun in the icy grains of the interstellar clouds, where key molecules are synthesized and incorporated into nascent solar systems. These amounts of organic material were then delivered to the Earth by accretion of icy bodies, providing organic compounds that could have been directly incorporated into early forms of life. Among these organic materials, amino acids were essential components for life to begin. Recently, it has been shown that several amino acids are synthesized by UV irradiation of interstellar ice analogues containing H₂O, CH₂OH, NH₃, CO and CO₂ [1, 2], demonstrating that the spontaneous generation of amino acids in interstellar medium is possible. Once created, the survival and transfer of the amino acids from space to planets were necessary for the appearance of life, and, especially, their resistance to the spatial radiation is a key issue [3, 4].

Fig.1: O1s NEXAFS of glycine (top) and glycine in ice (bottom) before (blue) and after (red) irradiation. Glycine is shown in inset.

Figure 1 shows the O1s NEXAFS spectra of pure glycine (top panel) and 10% of glycine diluted in H2O ice (bottom panel), before (blue curve) and after (red curve) X-ray irradiation at 20 K, recorded at SOLEIL on the TEMPO beamline. In pure glycine, the strong peak at 532 eV corresponds to the O1s excitation to the π* orbital of the acidic function of glycine. After the irradiation, several byproducts can be identified at the O K-edge: O2, NO, CO and CO2. The evolution of the peaks intensities as a function of the dose is reported in the figure 2, for pure glycine (open circles) and glycine in ice (filled squares). Most importantly, we see that glycine is destroyed at the same rate in ice than in the in the pure compound (grey curves), showing that ice plays no role in the primary photodecomposition of glycine. However, clear differences are seen in the yield of CO or NO, evidencing that some secondary photochemical routes involve the ice matrix, through, for instance, the OH radical. N K-edge data have been also recorded (not shown). They display a wide kind of nitrogen-bearing byproducts including the -C≡N, the -C=N-, the N-C=O chemical functions, the NH radical and the NO and N2 molecules. Most of these species have never been reported so far by use of laboratory techniques, and NEXAFS brings here an inestimable method to unravel the dissociation routes of such complex molecules plunged in a reactive medium as water ice.   Fig.2 : Evolution of the intensities of the O1s NEXAFS features of glycine and of the byproducts O2, CO2, NO and CO with the irradiation dose (open circles , pure glycine, filled squares, glycine in ice.

[1] M. P. Bernstein, J. Dworkin, S. Sandforf, G. W. Cooper et al. Nature 416 (2002) 401.
[2] G. M. Munos Caro, U. J. Meierhenrich, W. A. Schutte et al. Nature 416 (2002) 403.
[3] P. Ehrenfreund, M. Bernstein, J. Dworkin, et al. Astrophys. J., 550 (2001) L95.
[4] P. Ehrenfreund et al., Rep. Prog. Phys. 65 (2002) 1427–1487.