Liposome-assisted selective polycondensation of alpha-amino acids and peptides

The main question of this paper is whether and to what extend lipid bilayers can aid in the polycondensation of amino acids and peptides. This means in particular how such bilayers can favor the selection of certain sequences out of a large number of theoretical possible ones. In a first series of experiments we started from a library of Trp-containing dipeptides of the type Trp-X where X is an amino acid residue; and we could show that, when adding this mixture to the POPC liposomes containing a hydrophobic quinoline condensing agent (EEDQ), only the hydrophobic Trp-Trp dipeptide is selected out by the liposomes and transformed into a longer oligomer. Trp-oligomers up to 29 monomers long (water insoluble) could be obtained by using the matrix support of liposomes. Mixed POPC/DDAB liposomes (positive charge) were used to produce co-oligopeptides that contain Trp and Glu residues in the same sequence. Arg/Trp and His/Trp containing sequences were obtained in presence of negatively charged liposomes (mixed POPC/DOPA-liposomes). The polycondensation of racemic NCA-amino acids has been studied to clarify if homochiral sequences are produced preferentially in presence or absence of liposomes. LC-MS and isotope labeling of the L-amino acid, participating in the polymerization reaction achieved this on the level of a direct product analysis. So the individual stereoisomer distribution up to a polymerization degree of 10 (in the case of Trp) could be determined. The data for Trp and other amino acids (Leu, Ile) and amino acid mixtures (Trp/Leu, Trp/Ile, Leu/Ile and Trp/Leu/Ile) show that homochiral sequences are produced preferentially if compared with a random (Bernoulli) distribution.