Quite a puzzler in astrochemistry and astrobiology is where exactly prebiotic amino acids form. We know they can form in interstellar space. We’ve found them inside chondritic meteorites. All the ingredients exist in the interstellar medium. There have even been reports of glycine detected in interstellar space. On the other hand, those reports are still unverified. But are we really looking for the right thing…?
Glycine, being the simplest amino acid, has always been a good candidate molecule to search for in the interstellar medium (ISM). While over 70 amino acids were found in the Murchison meteorite, including 8 out of the 20 used by known living organisms, even glycine proves elusive in astronomical spectra. The most logical assumptions are that either amino acids form inside meteorites, or those that form in interstellar space exist in some form which makes their spectra difficult to recognise.
As it happens, there’s good reason to believe that the latter could be true. Over the sort of timescales involved in astrochemistry, molecules have ample time to relax to their most stable form. Due to bombardment with energetic photons, combined with effects like quantum tunneling, that means they can even isomerise; their atoms reshuffle to a more stable state.
There are actually 17 possible isomers of glycine. Using DFT calculations, Lattelais et al found that methyl-formyl-amine here, is the most stable of them. Glycine itself is only the third most stable. Any glycine present in the ISM will eventually isomerise into this stuff instead. That could be an explanation as to why most searches for glycine haven’t been particularly fruitful.
The thing is though, that glycine (in a similar way to certain other molecules) will readily accept a stray proton, going from a glycine molecule to a hydroglycine cation (which is technically a type of ammonium cation). You might think this was a recurring theme in astrochemistry. In fact, you’d be right. It seems any molecule with free π electrons can readily exist in a protonated form.
Interestingly enough, when the same calculations are run with an extra proton in the fray, the hydroglycine cation is the most stable molecule of the set. It’s definitely notable that virtually all of the cations detected in the ISM are protonated species. Indeed, ion molecule reactions drive a lot of interstellar chemistry. In other words, it’s highly likely that glycine may exist in space in this form.
So this seems like quite a good place to start future astrochemical searches for glycine. The other troubles are that glycine has a complicated and extremely weak spectrum at radio frequencies; a fact that can make things difficult to detect. The group have actually calculated some infrared frequencies too. I won’t copy out the full details, but the strongest peaks in the spectrum would be at 2.7µm, 2.8µm, 3.2µm, 5.5µm, 6.6µm, 6.8µm, 6.9µm, 8.3µm and 14.8µm. Actually, those are in the right ballpark for a few astronomical spectral lines. Unfortunately, DFT isn’t the most accurate prediction, but it’s good enough to give you the right idea.
Regardless of whether or not amino acids are present in space, calculations like this will do a lot to make sure we know what we’re looking for. If you can’t find a molecule, calculate if it should exist. If it isn’t the most stable, try the protonated form.
I wonder if the same method could work for other molecules being searched for in space, like ribose or furan… Perhaps similar techniques could put us one (small) step closer in the search for alien life. If not alien life itself, then perhaps at least the prebiotic chemistry from whence it came…
M. Lattelais, Y. Ellinger, B. Zanda (2007). Theoretical study of prebiotic precursors-2: about glycine, its N-carboxyanhydride and their protonated ions International Journal of Astrobiology, 6 (01) DOI: 10.1017/S1473550406003521