Why only L amino acids?

Andras Aszodi a-aszodi at nimr.mrc.ac.uk
Wed Jul 20 06:58:11 EST 1994


In article <307l06$ncp at zikzak.apana.org.au>, shnub at zikzak.apana.org.au (Paul Baulch) asks:

[in reply to a suggestion that biomolecules may owe their chirality to
interactions with chiral mineral (surfaces)]:

|>altabios at bham.ac.uk (John E. Fox) writes: [...]

|>>There are some theories that minerals acted as early templates, could these
|>>have some prefered orientation?


|> If so, the minerals themselves surely would have had stereochemistry.
|> Do any minerals with stereochemical lattices exist?


Firstly, a pedantic remark: I think you use "stereochemistry" where the expression
"chirality" or "asymmetry" would be more appropriate. If this is indeed the case
and your question is "Do any minerals with chiral lattices exist?", then
the answer is a definitive YES. One spectacular example is quartz; asymmetric,
left- and right-handed crystals (which are even macroscopically asymmetric in addition
to their optical activity which indicates microscopic asymmetry, in this
case the asymmetry of the crystal lattice) are often found.

So I think biomolecules could have interacted with chiral mineral surfaces.
Unfortunately this is not the solution since an imbalance in chirality is
required anyway: to explain the predominance of one optical isomer over
its mirror image in a primordial biomolecule requires - according to this
hypothesis - the presence of an asymmetric mineral that acted as a chiral
template/catalyst/whatever. Now: what made this mineral asymmetric?...

A very general rule in stereochemistry is that asymmetric precursors produce
either asymmetric products or a 50%-50% mixture of enantiomers (chiral isomers),
known as a "raceme mixture". There is no known way of doing chiral synthesis
starting from asymmetric reactants WITHOUT ANY CHIRAL interaction. (Cf. the
sad story published in Angew. Chem. recently where the 50:50 enantiomer ratio was claimed
to have been shifted by magnetic fields; unfortunately it turned out to be a swindle,
the samples were doctored with chiral reagents.) The problem is very
important for the chemical and drugs industries since fairly often only
a given enantiomer of a drug is active biologically: in other words, half
of the product (the wrong mirror image) must be separated from the
desired enantiomer and either thrown away or "re-racemised", re-extracted, etc.

My guess is that it would be rather difficult to explain the predominance
of one enantiomer over the other among biomolecules using stereochemical
arguments only; we need chirality to produce chirality and either in the test-tube
or in the primordial ocean the "rule of racemes" applies. We need a GLOBAL chiral
and non-chemical effect: how about the Earth's rotation? ;-)  

    ,
Andras



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