New Analysis Finds No Evidence for Tryptophan in Interstellar Cloud

Last year, a researcher claimed the first detection of an amino acid, tryptophan, in an interstellar cloud. New research that leverages laboratory experiments and data from JWST refutes this finding, suggesting that the hunt for the first amino acid in the interstellar medium isn’t over yet.

A Hunt for Amino Acids

Researchers have long sought evidence for amino acids, the building blocks of all life on Earth, in the dusty clouds of the interstellar medium. Several amino acids have turned up in meteorites and the gaseous envelopes of protostars, but claimed detections of these critical molecules in cold, star-forming clouds have been refuted. Now, another potential detection awaits confirmation.

diagram of a tryptophan molecule

The structure of a tryptophan molecule. [Dhariwal et al. 2024]

The amino acid in question is tryptophan, which is perhaps best known for being unfairly blamed for post-Thanksgiving sleepiness due to its presence in turkey meat. Research published in 2023 found evidence for tryptophan in the gas suffusing the star cluster IC 348, based on a comparison of an infrared spectrum from the Spitzer Space Telescope to a laboratory spectrum of the molecule. How does this finding hold up to further tests?

comparison of results from new and previous laboratory spectra of tryptophan

Comparison of the Spitzer spectrum used in the claimed detection of tryptophan (orange) to the new laboratory spectrum (blue) and the previously used laboratory spectrum (red). The panels at the bottom show individual spectral features. Click to enlarge. [Dhariwal et al. 2024]

A Three-Pronged Investigation

Aditya Dhariwal (University of British Columbia) and collaborators used several methods to evaluate the claimed detection, beginning with a laboratory spectrum of the molecule. It’s no easy task to simulate the conditions of the interstellar medium in a lab: the density of interstellar gas can be far lower than the best lab vacuum, and temperatures in cold clouds hover just above absolute zero. This makes it challenging to generate spectra that match what would be seen from molecules in interstellar space.

The previous study used a lab spectrum of tryptophan in the form of a solid pellet. Dhariwal’s team pointed out that unlike in the interstellar medium, tryptophan in a solid state likely takes the form of a zwitterion: a molecule that has no electric charge overall but is composed of parts that carry electric charge.

The team used a new method to prepare a sample of tryptophan in non-zwitterionic form and compared the collected infrared spectrum to the Spitzer observations. Crucially, the spectral lines seen by Spitzer and assigned to tryptophan have no matches in the new spectrum.

Paths Forward

JWST spectrum of IC 348

Spectrum of IC 348 from JWST (top) and individual spectral lines from that spectrum (bottom). The blue dotted lines mark the locations of the reported tryptophan lines. Click to enlarge. [Dhariwal et al. 2024]

Next, the team analyzed new infrared spectra of IC 348 from JWST. The JWST spectra clearly contained spectral lines from hydrogen and other elements in the cloud, but none of the lines attributed to tryptophan were present.

Finally, Dhariwal’s team revisited the Spitzer observations used in the previous analysis. Their examination of the spectrum led them to suspect that the purported emission lines of tryptophan were instrumental artifacts rather than true spectral lines.

This study demonstrates the fascinating back and forth of science at work. Although the latest investigation finds no compelling evidence for tryptophan in the interstellar medium, the authors identified paths forward. Laboratory spectra of tryptophan gas at radio wavelengths allow for searches of the sky at longer wavelengths, and though challenging to collect, spectra of tryptophan gas at infrared wavelengths could open yet another window. The search goes on!


“On the Origin of Infrared Bands Attributed to Tryptophan in Spitzer Observations of IC 348,” Aditya Dhariwal et al 2024 ApJL 968 L9. doi:10.3847/2041-8213/ad4d9a