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tokens: agrees well with the assumption. In addition to the reactions presented above, the following unimolecular hydrogen molecule elimination (UHME) process may occur during the photolysis of methanol at low temperatures (Gerakines et al. 1996 ; Hama et al. 2009 ): The reaction products of UHME process may be incorporated into AAs. If these reactions occur in a statistical manner, the frequencies of reactions ( 4 b ) and ( 5 a ) are twice as large as those of their counterparts (reactions ( 4 a ) and ( 5 b ), respectively) because the number of C–H bonds in CH 2 DOH is larger than that of the C–D bond, which may affect the D/H ratio of the reaction products. The production of C–H bonds is also possible from other pathways that do not use methanol as a reactant. Briggs et al. ( 1992 ) reported the formation of various COMs such as glycolic acid (HOCH 2 COOH), glycerol (HOCH 2 CH(OH)CH 2 OH), and Gly after the photolysis of solid mixtures (CO:H 2 O:NH 3 = 5:5:1) at 10 K. Although their gas composition is different from that in our study, similar products can be produced in the present experiment, which has the potential to decrease the deuteration level of Gly. Although the formation of other AAs was not reported by Briggs et al., others reported the formation of various AAs upon the hydrolysis of the organic residues even in the absence of methanol (Muñoz Caro et al. 2002 ), which should have the potential to drastically decrease their deuteration levels. 4.2. Effect of Hydrolysis on Deuteration Levels of Amino Acids We demonstrated that the acid hydrolysis of the organic residues generally results in significant increases in AA abundances, which is consistent with that demonstrated in previous studies (Nuevo et al. 2008 ; Evans et al. 2012 ). On the other hand, there has been no report on the variation in the deuteration levels of AAs after the acid hydrolysis of the organic residues. The present result suggests that the five AAs can be divided into two groups depending on whether the deuteration level decreases upon hydrolysis: those whose D/H is significantly decreased ( α -Ala and β -Ala; referred to as the Ala group) and those whose D/H did not change (Gly, Sar, and Ser; referred to as the Gly group). This indicates that the two groups have different formation mechanisms upon hydrolysis. We propose one possible scenario to explain the observed D/H ratio: we assume that these five AAs are produced by reactions of small radicals during warming of the irradiated ice and that most AAs formed are incorporated into macromolecules via relatively weak bonds such as peptide bonds. Upon hydrolysis, these weak bonds are broken and the unit AAs are released. If this is the only pathway for the formation of AAs, the deuteration levels of the AAs would not vary between the free and bound samples, as is the case for Gly-group AAs. On the other hand, for Ala-group AAs, if there are other formation pathways that do not use deuterated radicals and/or molecules