Methylammonium methylcarbamate thermal formation in interstellar ice analogs: a glycine salt precursor in protostellar environments

Abstract

Context. Analyses of dust cometary grains collected by the Stardust spacecraft have shown the presence of amines and amino acids molecules, and among them glycine (NH 2 CH 2 COOH). We show how the glycine molecule could be produced in the protostellar environments before its introduction into comets. Aims. We study the evolution of the interstellar ice analogues affected by both thermal heating and vacuum ultraviolet (VUV) photons, in addition to the nature of the formed molecules and the confrontation of our experimental results with astronomical observations. Methods. Infrared spectroscopy and mass spectrometry are used to monitor the evolution of the H 2 O:CO 2 :CH 3 NH 2 and CO 2 :CH 3 NH 2 ice mixtures during both warming processes and VUV photolysis. Results. We first show how carbon dioxide (CO 2 ) and methylamine (CH 3 NH 2 ) thermally react in water-dominated ice to form methylammonium methylcarbamate [CH 3 NH + 3 ][CH 3 NHCOO − ] noted C. We then determine the reaction rate and activation energy. We show that C thermal formation can occurs in the 50-70 K temperature range of a protostellar environment. Secondly, we report that a VUV photolysis of a pure C sample produces a glycine salt, methylammonium glycinate [CH 3 NH + 3 ][NH 2 CH 2 COO − ] noted G. We propose a scenario explaining how C and subsequently G can be synthesized in interstellar ices and precometary grains. Conclusions. [CH 3 NH + 3 ][CH 3 NHCOO − ] could be readily formed and would act as a glycine salt precursor in protostellar environments dominated by thermal and UV processing. We propose a new pathway leading to a glycine salt, which is consistent with the detection of glycine and methylamine within the returned samples of comet 81P/Wild 2 from the Stardust mission.

Chiral molecule formation in interstellar ice analogs: alpha-aminoethanol NH2CH(CH3)OH

“…It is probably insufficiently stable to be seen, either because alpha-aminoethanol is not stable in the gas phase or because it undergoes collisionally induced unimolecular dissociation in our experimental conditions. This is consistent with the mass spectrum of aminomethanol reported by Bossa et al (2009a). The molecular ion of aminomethanol (NH 2 CH 2 OH) at m/z 47 was 15 times lower in intensity than the base peak at m/z 46 corresponding to the loss of one hydrogen atom in the alpha position.…”

“…For the k 5 and k 6 we used the value given by . The desorption rate of aminethanol (k 7 ) was estimated from the measured desorption energy of aminomethanol (Bossa et al 2009a), since they both desorb at the same temperature. The initial abundance for NH 3 and CH 3 CHO are 10% and 1% with respect to H 2 O respectively.…”

“…The second type of thermal reaction is addition reactions involving aminolysis or hydrolysis reactions. For example, it has been shown that amine RNH 2 and CO 2 can react in a water ice environment to produce carbamates in relevant astrophysical conditions and timescales (Bossa et al 2008(Bossa et al , 2009a. The simplest aminoalcohol, aminomethanol NH 2 CH 2 OH, has been recently thermally produced in interstellar ice analogs at temperatures lower than 100 K from a reaction between NH 3 and H 2 CO (Bossa et al 2009b).…”

Adsorption of Methylamine at the Surface of Ice. A Grand Canonical Monte Carlo Simulation Study

“…The <N> values were then determined in consecutive 10 8 steps long equilibrium simulations. Furthermore, at five selected  values (see Table 1), 2500 sample configurations per system, separated by 2×10 5 Monte Carlo steps, were saved for detailed structural, orientational and energetic analyses in an additional 5×10 8 Monte Carlo steps long run. Finally, all properties calculated were averaged over the sample configurations collected as well as over the both sides of simulated ice slab.…”

“…6 Methylamine is detected in dense clouds 9 and understood to be present on icy grains at concentrations of less than 1% relative to water. However, this is too low for its detections by astronomical infrared observations 8 and no structural information is available about ice-adsorbed methylamine molecules.…”