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Last Updated: 03/10/05

Vesicle Forming Compounds Made by the UV Photolysis of Interstellar Ice Analogs

The infrared spectra of dense molecular clouds have taught us that these huge clouds are composed of microscopic grains with a mantle of ice composed primarily of H₂O, and also including simple carbon compounds such as CO, CO₂, CH₃OH, and NH₃. Moreover, we know that the ices in these interstellar enviornments are often processed by ionizing radiation. In the Astrochemistry Laboratory we carry out experiments in the laboratory where we reproduce the effects of this radiation processing on interstellar ice grains. These ices begin with only a few simple species but larger compounds are produced, with molecules of almost 300 amu made from compounds of only one carbon atom each! Careful control and isotopic labeling experiments demonstrate that these are not contaminants. Some of these new molecules have properties that are of prebiotic interest. For example, some of these compounds form membrane-like structures that may have been important for the early evolution of Life.

One of the more interesting results of this work is shown below. When our organic residues are placed in water, one or more of the compounds present spontaneously form membranes that produce 'vesicles' of the sort shown below.

The top image shows what the vesicles look like in visibly light. Interestingly, the membranes in these structures fluoresce, that is, they absorb UV radiation and re-emit the energy at visible wavelengths. The lower image shows the same field as the upper image, the difference being that in the lower image, the sample is being illuminated by UV light and the vesicles are 'glowing.'

What is really happening here on a molecular level? Well, at the correct concentration and pH, certain compounds spontaneously self-assemble into spherical bi-layer structures known as vesicles. Mixtures of molecules from the Murchison meteorite and from our ice simulations have this capacity. We suspect long chain linear carboxylic acids are the active molecules in these cases, but little is known about the prebio-physics of the vesicles formed by these prebiotic molecules.

The compounds that form these membrane-like vesicle structures called amphiphiles are detergent-like in their structure. They must have a polar, hydrophilic, head group and a hydrophobic tail of the right length. The head groups point outward, facing the water that is both on the inside and outside of the membrane and the tails face inwards towards each other.

Amphiphiles are present in the Murchison meteorite (Deamer 1992) and are produced in our interstellar ice simulation experiments (Dworkin et al 2001). Whether produced in the primordial soup, or brought from space by meteorites (or most likely a mixture of the two), such self-assembling molecules probably played a key role in the emergence of life because of the unique advantages they can provide. For example, enclosed vesicular structures are capable of concentrating molecules, overcoming dilution and facilitating interactions that develop or maintain macromolecules. Moreover, since the aromatic components of the prebiotic organic inventory include non-polar pigments, they have the potential to capture light energy and screen the interior from damaging UV light when partitioned into the hydrophobic phase of a bi-layer. This emergent set of biophysical properties can only arise from amphiphilic molecules that have the ability to self-assemble into more complex vesicular structures.

For more information on these residues and the membranes they produce, see the press release about this article. Additional information on our work in the area of astrobiology and the origin of life can be found here.

This figure originally appeared in Dworkin, J. P., Deamer, D. W., Sandford, S. A., & Allamandola, L. J. (2001). Self-Assembling Amphiphilic Molecules: Synthesis in Simulated Interstellar/Precometary Ices. Proc. Nat. Acad. Sci. 98, 815-819.

We were also co-authors on a more recent article about the earlierst membranes by David Deamer of UC Santa Cruz: Deamer, D. W., Dworkin, J. P., Sandford, S. A., Bernstein, M. P., Allamandola, L. J. (2003). The First Cell Membranes Astrobiology 2, 371-381. Both of these, and the articles below can be dowloaded at our publications page.

Past articles about the formation of organic molecules in space include: Allamandola, L. J., Bernstein, M. P., & Sandford, S. A. (1997). Photochemical evolution of interstellar/precometary organic material. In Astronomical and Biochemical Origins and the Search for Life in the Universe, C.B. Cosmovici, S. Bowyer, & D. Werthimer (eds.), Proc. 5th International Conf. on Bioastronomy, IAU Coll. #161, Capri, 1-5 July 1996, (Editrice Compositori: Bologna), pp. 23-47.

Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Chang, S., & Scharberg, M. A. (1995). Organic Compounds Produced by Photolysis of Realistic Interstellar and Cometary Ice Analogs Containing Methanol. Astrophys. J. 454, 327-344.

Allamandola, L. J., Sandford, S. A., & Valero, G. (1988). Photochemical and thermal evolution of interstellar/pre-cometary ice analogs. Icarus 76, 225-252.