In honor of Stanley Miller

Stanley Miller is famous for his groundbreaking experiment in the 50's, where he recapitulated the atmospheric conditions of the young earth, added some water and some UV light and created amino acid, thereby showing that the components of life can be created abiotically. He was also an advocate of the peptidic origin of life (as opposed to the RNA). He passed away this week. He was one of the scientists that truly inspired me when I was very young, and the inspiration for Amino-Rx.

Stereo is Homer

Over lunch we were discussing why life had chosen left-handed amino-acids versus right-handed ones. Was there any advantage for that initial choice? Rob suggested that comets actually carry more left handed amino-acids so they may have been more abundant on earth. Turns out he was right: "Amino acids found in meteorites from space, which must have formed abiotically, also show significantly more of the left-handed variety, perhaps from circularly polarized UV light in the early solar system (Engel and Macko 1997; Cronin and Pizzarello 1999)"
Other reasons that may have contributed for that preference, is that left-handed AA may be required for catalytic production of other left-handed organic molecules (sugar, lipids). Although this is kind of a chicken and egg thing. More surprisingly, some bacteria actually use right handed amino-acids: "From the ratios of right- to left-handed amino acids in seawater, McCarthy and his colleagues conclude that a substantial fraction of the dissolved organic matter comes from bacteria. This challenges the traditional view that algae produce most of the ocean's soluble biological material. Bada notes that bacteria coat themselves with right-handed amino acids because the unusual structures provide a tough exterior that resists other organisms. This is what helps bacteria evade digestive enzymes in human stomachs, he says."
Interestingly, a new hypothesis suggests that while reactions may produce equal amounts of L and D, the L AA dissolve better in water..."Donna Blackmond at Imperial College London and colleagues dissolved a mixture of solid L and D versions of the amino acid serine in water. They found that a small difference in the initial proportion of one version gets amplified in the resulting solution. So a 100:1 mixture of L- and D-serine produces a solution made up almost entirely of L-serine, but so does a 100:99 mixture (Nature, vol 441, p 621)."
So should we expect life on Mars to also be left-handed if it exists?