Technical Comments

Comment on “Ribose and related sugars from ultraviolet irradiation of interstellar ice analogs”

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Science  13 Jan 2017:
Vol. 355, Issue 6321, pp. 141
DOI: 10.1126/science.aag2990

Abstract

Meinert et al. (Reports, 8 April 2016, p. 208) reported the formation of prebiotic molecules, including ribose, in an interstellar ice analog experiment. We show that if their experimental procedure is accurately described, much or most of their products may have been formed during their analysis process, not in the parent ice.

Meinert et al. (1) recently reported the use of two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-TOFMS) to characterize derivatized two-to-five carbon atoms aldoses, sugar alcohols, and sugar acids, and three-to-five carbon atoms ketoses, all extracted after ultraviolet irradiation of a simulant of interstellar ices containing water (H2O), methanol (CH3OH), and ammonia (NH3). They proposed that these were formed by aldol reactions after initial formation of carbon monoxide (CO), carbon dioxide (CO2), HCO, and formaldehyde (H2CO).

Unfortunately, their experimental procedure raises some questions. For example, they report that the residue ices were extracted with water. Furthermore, formaldehyde was most likely included in the residue, because de Marcellus et al. (2) have shown that mixtures containing H2O, CH3OH, and NH3 in different proportions, when ultraviolet-irradiated, produce formaldehyde, glycolaldehyde, and glyceraldehydes. This literature makes it appropriate to question what fractions of the products detected by Meinert et al. were formed in the ices and which fractions were formed during workup and derivatization. Thus, aldol reactions also could also have occurred in the aqueous extracts during derivatization, if (as they report) water extracts (150 μL) were treated with N, O-bis (trimethylsilyl) trifluoroacetamide (BSTFA 10 μL), and pyridine (25 μL) at 80°C for 2 hours. For comparison, product mixtures similar to those that they report are known to arise in aqueous solution of 1.86 M of formaldehyde and 0.2 mol/L of various organic bases—such as pyridine, collidine, and picoline—at 60°C (3), suggesting that aldol reactions in organic solvents cause the synthesis of carbohydrates such as aldohexose, ketohexose, aldopentose, and ketopentose.

To answer this question, I mixed glycolaldehyde (0.5 mmol, 1 equivalent, 30 mg), formaldehyde solution (1200 μL, ~16.0 mmol, about 32 equivalents), and pyridine (200 μL). A solution of glycolaldehyde (30 mg), formaldehyde (1200 μL), and water (200 μL) served as a negative blank. Each was heated 80°C for 2 hours. As expected, the solution with pyridine turned light brown; the blank did not. Supporting the conclusion that this color change indicates successive aldol reactions are various reports in the literature (3, 4) of analogous aldol reactions occurring in pyridine solution.

Regardless of which products may have arisen during workup, it is unlikely that they were formed following the scheme shown in figure 3 in (1), which adopts unconventional structural formulas (R-HO is unusual; R-OH is the typical form). Multiple studies [(5) and references therein] have examined the formose reaction since (6) and have ruled out much of that mechanism. The interconversion of figures S3 and S4 in (1) requires a hydride shift that is unknown at low temperatures and likely not driven by ultraviolet light.

I agree with the need to understand the relative contributions of endogenously (on Earth) and exogenously formed carbohydrates. This understanding would be best furthered by clearly distinguishing products produced by ultraviolet irradiation from those produced during workup.

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