Universe Biased Toward L-Amino Acids and D-Sugars

Is the Universe Biased (Toward L-Amino Acids and D-Sugars)  

Another interesting question concerns the origin of chirality ( handedness ) on Earth Why are the alpha-amino acids that compose proteins on Earth all L (not D) in absolute configuration (see the figure on page 379) while glucose and related saccharides are all D (not L) Why are not the amino acids D and the saccharides L A protein molecule com- 

Initial analysis of the Murchison meteorite that crashed in Australia in 1969 appeared at the time to answer two important questions. Careful study of die common amino acids found on the meteorite indicated that they were racemic (equal L and D). This established their extraterrestrial origin since the amino acids on Earth are L. In addition, it supported the intuitive notion that in space, in the absence of life, there is no intrinsic bias toward L- or D-amino acids. The findings from the Murchison meteorite lent support to the hypothesis of seeding the Earth with amino acids. 

In order to evolve a homochiral world, three general steps have been recognized  

The first step remains the biggest mystery. It may have been a statistical fluke on Earth but this is now a less-than-satisfactory explanation given a seeming bias in the universe. Another possibihty may be circularly polarized light from stars which could, in principle, favor formation of one enantiomer over the other. Physicists discovered 50 years ago that there is asymmetry in the weak nuclear force possibly giving some intrinsic bias in matter favoring one enantiomer over the other. At present these remain speculations. 

---