Jerry writes:
What environment could possible cause homochiral proteins to form our of a racemic mixture of amino acids against the laws of chemistry?
I doubt that any could cause it to happen "against the laws of chemistry" but there certainly seem to be a few which might cause it to happen.
For instance the adsorption of one particular isomeric form of amino acids by minerals. In this case different faces show selectivity for different enantiomers.
Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality.Hazen RM, Filley TR, Goodfriend GA.
Proc Natl Acad Sci U S A. 2001 May 8;98(10):5487-90.
The emergence of biochemical homochirality was a key step in the origin of life, yet prebiotic mechanisms for chiral separation are not well constrained. Here we demonstrate a geochemically plausible scenario for chiral separation of amino acids by adsorption on mineral surfaces.
Crystals of the common rock-forming mineral calcite (CaCO3), when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of D- and L-enantiomers on pairs of mirror-related crystal-growth surfaces. This selective adsorption is greater on crystals with terraced surface textures, which indicates that D- and L-aspartic acid concentrate along step-like linear growth features.
Thus, selective adsorption of linear arrays of D- and L-amino acids on calcite, with subsequent condensation polymerization, represents a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.
I added the emphasis in bold to certain salient portions of the abstract.
Alternatively it may be that your assumption of equilibrium is not neccessarily warranted.
Recycling Frank: Spontaneous emergence of homochirality in noncatalytic systemsRaphal Plasson, Hugues Bersini and Auguste Commeyras
PNAS 2004 November 30, vol. 101, no. 48, pp.16733-16738
In this work, we introduce a prebiotically relevant protometabolic pattern corresponding to an engine of deracemization by using an external energy source.
The spontaneous formation of a nonracemic mixture of chiral compounds can be observed in out-of-equilibrium systems via a symmetry-breaking phenomenon. This observation is possible thanks to chirally selective autocatalytic reactions (Frank's model) [Frank, F. C. (1953) Biochim. Biophys. Acta 11, 459—463]. We show that the use of a Frank-like model in a recycled system composed of reversible chemical reactions, rather than the classical irreversible system, allows for the emergence of a synergetic autoinduction from simple reactions, without any autocatalytic or even catalytic reaction. This model is described as a theoretical framework, based on the stereoselective reactivity of preexisting chiral monomeric building blocks (polymerization, epimerization, and depolymerization) maintained out of equilibrium by a continuous energy income, via an activation reaction. It permits the self-conversion of all monomeric subunits into a single chiral configuration. Real prebiotic systems of amino acid derivatives can be described on this basis. They are shown to be able to spontaneously reach a stable nonracemic state in a few centuries. In such systems, the presence of epimerization reactions is no more destructive, but in contrast is the central driving force of the unstabilization of the racemic state.
This paper describes stereoselective reactions such as the formation of N-carboxyanhydrides of amino acids, as part of the activation reaction, which favour the subsequent formation of homochiral peptides.
TTFN,
WK
This message has been edited by Wounded King, 05-11-2005 11:23 AM