Amino Acid Structures

(information from: http://web99.arc.nasa.gov/~astrochm/aastruct.html)
Here is an amino acid. The name comes from the fact that this molecule has an amino group (in red) and an acid group (in blue). Anything that has both is an amino acid. In between the "amino" and the "acid" groups are some number of carbon atoms (C) bearing hydrogen atoms (H). The "R" in the structure means that there can also be attached some other random groups, but no matter what that R is, you've still got an amino acid. An interesting aspect of most amino acids is that they exist in left and right handed forms, and this is called chirality. Life on Earth is made of left handed amino acids, and this may be a result of how these molecules formed in space.
 
 

Amino Acid Chirality

(information from: http://web99.arc.nasa.gov/~astrochm/aachiral.html)
chirality with hands from http://www.nai.arc.nasa.gov/ A "chiral" molecule is one that is not superimposable with its mirror image. Like left and right hands that have a thumb, fingers in the same order, but are mirror images and not the same, chiral molecules have the same things attached in the same order, but are mirror images and not the same.

Although most amino acids can exist in both left and right handed forms, Life on Earth is made of left handed amino acids, almost exlusively. No one knows why this is the case. However, Drs. John Cronin and Sandra Pizzarello have shown that some of the amino acids that fall to earth from space are more left than right. Thus, the fact that we are made of L amino acids may be because of amino acids from space.

Why do amino acids in space favor L? No one really knows, but it is known that radiation can also exist in left and right handed forms. So, there is a theory called the Bonner hypothesis, that proposes that left handed radiation in space (from a rotating neutron star for example) could lead to left handed amino acids in space, which would explain the left handed amino acids in meteorites. This is still speculative but our paper makes it much more plausible. In fact, this observations was one of the main reasons why we persued this research. Although there were theories about how the amino acids could form in space in the ice, no one had shown that it was viable to make amino acids this way, until now. 


Further reading: (web)

Here is a cool web site that has wonderful 3d images of amino acids.

A nice Scientific American article about meteorites and handedness (chirality) of molecules.

Nice Cambridge Astronomy article about how handedness of molecules could arise in space from polarized radiation.
There are other theories.

Further reading: (Primary articles in the peer reviewed literature in alphabetical order)

Cronin, J. R., & Pizzarello, S. Enantiomeric excesses in meteoritic amino acids. Science 275, 951-955 (1997).

Rubenstein, E. Bonner, W. A., Brown, G. S., & Bailey, J. Polarized stellar light. Science 283, 1415 (1999).

Rubenstein, E. Bonner, W. A., Noyes, H. P., & Brown, G. S. Supernovae and Life. Nature 306, 118 (1983).

Bonner, W. A., & Bean, B. D. asymmetric photolysis with elliptically polarized light. Orig. Life Evol. Biosphere 30, 513-517 (2000).