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-dibenzofuranethano ic acid (2), and 4-amino-6-dibenzofuranmethanoic acid (3), were prepar ed in order to compare their physical properties so as to further unde rstand residue 1's ability to nucleate antiparallel beta-sheet formati on within small peptides in aqueous solution. FT-IR, variable temperat ure 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. Spect roscopic studies reveal that linear heptapeptides containing 1 are cap able of adopting a dynamic antiparallel beta-sheet structure in aqueou s solution, whereas residues 2 and 3 are incapable of nucleating a bet a-sheet structure in an otherwise identical peptide sequence. The effi cacy of residue 1 as a beta-sheet nucleator appears to result from a 1 5-membered ring intramolecularly hydrogen-bonded hydrophobic cluster c onformation which serves as a partial beta-sheet template enabling nei ghboring residues to be added to the growing sheet with a favorable eq uilibrium constant. The hydrophobic cluster conformation in these hept apeptides is stabilized by the interactions between the dibenzofuran s keleton in 1 and the hydrophobic side chains of the alpha-amino acid r esidues flanking 1. In order for residue 1 to nucleate a beta-sheet st ructure, it must be flanked with hydrophobic alpha-amino acid residues . Therefore, it is the tripeptide sequence-hydrophobic residue-1-hydro phobic residue-which mediates beta-sheet nucleation. Residues 2 and 3 are not capable of promoting hydrophobic cluster formation, which appa rently precludes these residues from being effective beta-sheet nuclea tors, even though residue 2 facilitates intramolecular hydrogen bondin g capable of supporting an antiparallel beta-sheet structure. Spectros copic methods for characterizing the hydrophobic cluster and the resul ting beta-sheet structure are described within.