Investigation of active crystal morphogenesis peptide sequences from peptide libraries by crystallization on peptide functionalized beads

In this study, split-and-mix peptide libraries from one to four amino acids bound to functionalized beads were used to identify active morphogenesis peptides for the systems CaCO₃ and dl-alanine. Density gradient ultracentrifugation was used to remove all beads without crystals as well as crystals homogeneously nucleated in solution. From the remaining fractions, beads were selected, which account for differences in crystal morphologies from the default crystal morphology on a micrometer scale and their amino acid sequence was analyzed. Our results show that multiple and different peptide sequences are found to be active in the morphogenesis of CaCO₃ and dl-alanine. It was not possible to find a correlation, which connects active single amino acids with the sequences of di-, tri- and tetrapeptides. However, peptide charge was found to be important for morphogenesis. Peptides active in CaCO₃ morphogenesis were enriched in basic amino acids while those active for dl-alanine morphogenesis contained more acidic amino acids. This can be explained by charge-charge interactions of the crystallizing species with the countercharged peptide moieties. Tests for chiral separation for the dl-alanine system showed that with the applied oligopeptide libraries, no enantioselective crystallization was achieved to a significant extent. The presented combinatorial crystallization assay provides an easy tool for crystallization control, which can be used for the straightforward selection of interesting species under a light microscope. However, suitable staining techniques to identify individual beads with crystals, which are for example a pure enantiomer still need to be developed.