🚀 go-pugleaf

In article <1362@tdat.teradata.COM> swf@tdat.teradata.com (Stanley Friesen)
writes:
>In article <1992Oct30.195102.28879@cs.rochester.edu> dietz@cs.rochester.edu
>(Paul Dietz) writes:
>|Prebiotic Miller-Urey type
>|experiments have not, to my knowledge, ever produced nucleotides, nor
>|have they produced anything but racemic mixtures of mostly
>|biologically irrelevant molecules.
>
>I am fairly sure I heard about the production of nucleotides, and even short
>poly-nucleotides, in Meller-Urey type experiments.  True, they were still
>racemic.

     I'm not sure I understand the significance of this.  What's wrong
with racemic mixtures?  It would seem to me that a preferred chirality
would have arisen at a relatively late stage of biotic (not prebiotic)
development.  The only reason that we have any preferences that way is
that we rely on enzymes to speed things up.  It's only after enzymes get
sufficiently complicated that chirality would start to matter.  At that
point, there would be selective pressures to make most of the
biochemistry "fall into line" with whatever the first few enzymes
required.
     On more fundamental grounds, the free energy difference between two
chiralities of the same molecule is very small.  The kinetic properties
of the two (in the absence of enzymes) are also nearly identical.
Prebiotic synthesis can therefore not have generated significantly
different amounts of (for instance) R- and L-amino acids.  Whatever
differences in the distribution of the two we see now must have arisen
as a consequence of biological evolution and not as a prelude to it.  To
put it yet another way, it's not much of a criticism of Miller-Urey
experiments to say that they produce only racemic mixtures.  (Paul's
other points are harder to dismiss.  Our understanding of prebiotic
evolution is, if you'll pardon the pun, primitive.)

				Marc R. Roussel
                                mroussel@alchemy.chem.utoronto.ca