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Re: Alien bacteria and biosphere II

David Levine writes:
 > It's not my strong point, but didn't I read somewhere
 > once that while a lot of the proteins, etc. we live on,
 > while they can be either L- or R- versions (left handed
 > or right handed), are all mainly L-?  I think I read a
 > short story where someone went through a strange experiment
 > and came out with an R- based system, and no one knew it.
 > He was slowly starving to death because his system couldn't
 > use anything he ate.  Or am I barking up the wrong tree
 > here?

This is actually the case.  Certain chemical compounds can have two
isomers, that have the same formula and the same structure when
reflected across a plane of symmetry.  Nearly all isomeric compounds
used in biology on Earth are L-isomers.  Isomers are chemically
equivalent when reacting with corresponding isomers of the same
handedness, but usually L-isomers and R-isomers are not interchangeable
and an isomer of the wrong handedness is not metabolized.  On the other
hand, getting someone reflected in such a way that all their L-isomers
become R-isomers is highly unlikely :-).