HYDROPHOBIC CLUSTER FORMATION IS NECESSARY FOR DIBENZOFURAN-BASED AMINO-ACIDS TO FUNCTION AS BETA-SHEET NUCLEATORS

Three dibenzofuran-based amino acid residues, namely 4-(2-aminoethyl)-6-dibenzofuranpropanoic acid (1), 4-(aminomethyl)-6-dibenzofuranethanoic acid (2), and 4-amino-6-dibenzofuranmethanoic acid (3), were prepared in order to compare their physical properties so as to further understand residue 1's ability to nucleate antiparallel beta-sheet formation within small peptides in aqueous solution. FT-IR, variable temperature NMR, and an X-ray crystallography study reveal that amide analogs of 1 and 2 can adopt intramolecularly hydrogen bonded conformations in nonpolar solvents, whereas amides composed of residue 3 cannot. Spectroscopic studies reveal that linear heptapeptides containing 1 are capable of adopting a dynamic antiparallel beta-sheet structure in aqueous solution, whereas residues 2 and 3 are incapable of nucleating a beta-sheet structure in an otherwise identical peptide sequence. The efficacy of residue 1 as a beta-sheet nucleator appears to result from a 15-membered ring intramolecularly hydrogen-bonded hydrophobic cluster conformation which serves as a partial beta-sheet template enabling neighboring residues to be added to the growing sheet with a favorable equilibrium constant. The hydrophobic cluster conformation in these heptapeptides is stabilized by the interactions between the dibenzofuran skeleton in 1 and the hydrophobic side chains of the alpha-amino acid residues flanking 1. In order for residue 1 to nucleate a beta-sheet structure, it must be flanked with hydrophobic alpha-amino acid residues. Therefore, it is the tripeptide sequence-hydrophobic residue-1-hydrophobic residue-which mediates beta-sheet nucleation. Residues 2 and 3 are not capable of promoting hydrophobic cluster formation, which apparently precludes these residues from being effective beta-sheet nucleators, even though residue 2 facilitates intramolecular hydrogen bonding capable of supporting an antiparallel beta-sheet structure. Spectroscopic methods for characterizing the hydrophobic cluster and the resulting beta-sheet structure are described within.