How Polypeptides Form

Polypeptides and proteins are composed of amino acids. Their two most common structural motives are a-helix and P-sheet. The a-helix shows helical chirality with 3.6 residues per turn, a special case of axial chirality. As we have seen in Chap. 2, in proteins, the L-configuration of the monomer amino acids dictates the R-helical chirality of the helix.6 This helicity arises from energetic constraints in the bonding of L-amino acids. A chain consisting of D-amino acids would produce spirals with an M-helical twist. This assumption was confirmed by chiroptical data of the P-amyloid peptide and its chemically synthesized all-D enantiomer. The circular dichroism spectra showed equal optical rotation in value but opposite optical rotation in sign (Cribbs et al. 1997), suggesting that the folded forms of the all-D and all-L peptides are mirror images when viewed in three dimensions (see also Petsko 1992; Milton et al. 1992). So, how did life's polypeptides and proteins form, and do they contain any structural motive?

The desired information on the formation of polypeptides and proteins can be gained by their synthesis under controlled conditions in the laboratory. The growth of the a-helical and P-sheet motifs under variation of the amino acid monomers' chirality is of particular importance. These studies have been systematically performed in Andre Brack's research team at the Centre de Biophysique Moleculaire in Orleans, France.

Here, a slight enantiomeric excess in a mixture of activated amino acid enan-tiomers was shown to be amplified by a stereoselective polymerization forming

6 The structural protein collagen is an exception to the rule: each of the three amino acid chains of collagen is twisted into an M-helix with only three amino acids per turn (Borchers et al. 2004).

a-helical structures. Brack and Spach demonstrated that a right-handed a-helical peptidic seed composed of L-amino acids incorporates L-amino acid monomers 18 times faster than D-amino acids (Brack and Spach 1971). Similarly, a P-sheet conformation of synthetic polypeptides enriched in a dominant enantiomer was found to be more stable than a random coil of racemic amino acids (Brack and Spach 1980). Such macro-molecular routes of peptide growth towards a-helices or P-sheets show considerable potential in the amplification of enantiomeric enrichments in the re-actants; they were theoretically modelled through the Majority Rule caused by the strong cooperativity effect claimed by Green and Selinger (1998) and Green et al. (1999). More recently, an enantiomeric enhancement through self-assemblies of amphiphilic activated amino acid analogues on the surface of water has been reported (Zepik et al. 2002). But do these items inevitably point towards the biotic or selection theories?

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