Proton Irradiation Forms Dipeptides in Interstellar Ice Analogues

Peptide-bond formation is usually treated as a chemical step that needs either liquid water chemistry, later planetary processing, or a more elaborate precursor route. This paper tests a narrower question with direct astrobiological consequences: whether solid glycine on cold grains can be pushed into peptide chemistry by energetic particles alone. The authors irradiate glycine, partially deuterated D3-glycine, and fully deuterated D5-glycine at 20 K, then track both water formation and peptide-linked products with in situ spectroscopy and ex situ mass spectrometry.

Figure 1 lays out the proposed setting and reaction: two glycine molecules on an interstellar grain form glycylglycine plus water under proton bombardment, with the peptide bond as the chemical product of interest.

Core Contribution

The main contribution is not the claim that radiation makes complex organics; that has substantial precedent in ice-analogue experiments. The new point is the combination of isotope labeling, water-product tracking, amide-band assignment, and mass-spectral identification aimed specifically at a non-aqueous glycine-to-dipeptide pathway. In the authors' interpretation, water is not a solvent here. It is a coproduct of condensation chemistry:

The deuterated analogue gives the corresponding D2O pathway, which lets the experiment separate peptide-forming condensation from other radiation-driven sources of H2O and D2O.

Technical Approach

The experiments use two irradiation regimes. The in situ AQLA setup exposes glycine isotopologues on ZnSe windows at 20 K to 10 keV protons, increasing the fluence up to $6\times10^{15}$ H+ cm$^{-2}$ while recording FTIR spectra after irradiation steps. Afterward, temperature-programmed desorption heats the sample at 10 K/min while QMS tracks m/z = 18, 19, and 20 for H2O, HDO, and D2O. A second chamber uses 1 MeV protons at a fluence of $1.24\times10^{15}$ H+ cm$^{-2}$ to simulate higher-energy cosmic-ray processing and leaves a residue for IR analysis.

The isotope design is the strength of the experiment. D5-glycine should mainly reveal deuterated-water chemistry; D3-glycine retains hydrogen at the alpha carbon and can show competing H2O and D2O channels. Figure 4 shows that D2O in D3-glycine grows quickly at lower fluence and then plateaus, while H2O rises later; D5-glycine forms D2O but not H2O under the reported conditions. That pattern supports the authors' reading that more than one reaction route contributes to water formation.

Results and Analysis

The FTIR data first show radiation products expected from glycine degradation and recombination. After 10 keV irradiation, CO2 appears at 2341 cm^-1, OCN^- near 2165 cm^-1, and CO near 2140 cm^-1. D2O signatures grow in the 2800–2300 cm^-1 region for D5- and D3-glycine, while H2O appears mainly in D3-glycine around the 3750–2750 cm^-1 range. QMS then confirms desorption of water isotopologues between roughly 140 and 230 K: m/z = 18 for H2O, 19 for HDO, and 20 for D2O.

The peptide evidence comes from two complementary readouts. The 1 MeV residue contains fitted IR bands assigned to amide 1, 2, and 3 features: 1665 cm^-1 for carbonyl stretching, 1573 cm^-1 for NH2 scissoring, and 1325 cm^-1 for a combined CN/NH vibration. The authors are careful that these bands alone do not prove peptide bonds, because overlapping carbonyl-containing products can contribute in the same region. The stronger evidence is their concurrence with water formation and mass spectrometry.

Figure 6 is the key product-level result. ESI-MS detects remaining glycine at m/z = 76.04 for nondeuterated glycine and D3-glycine, and m/z = 78.05 for D5-glycine. It also detects protonated glycylglycine at m/z = 133.06 in nondeuterated and D3 samples, and deuterated glycylglycine at m/z = 137.09 in the D5 sample. The spectra include additional products extending to m/z = 300, including a candidate N-formylglycinamide signal at m/z = 103.05 and a deuterated counterpart at m/z = 106.07.

The evidence supports the paper's laboratory claim: proton processing of solid glycine can produce detectable dipeptide-linked material under cold, non-aqueous conditions. The broader astrophysical claim is more conditional. The experiments model selected radiation environments, but they do not measure actual interstellar abundances, survival during transport, or delivery efficiency to planetary surfaces. The result is best read as a plausible solid-state pathway that should be folded into prebiotic chemistry models, not as a measurement of how much peptide material space actually supplies.