Artificial peptides containing Ca,a- disubstituted amino acids: synthesis, conformational studies, and application as �-strand mimics

Abstract

Short peptides containing Cα,α-dipropylglycine (Dpg) at alternating sequence positions were synthesized and examined for conformational behavior. Peptide assembly was performed using Fmoc-solid-phase chemistry where the coupling with PyAOP could be significantly enhanced at elevated temperature. Circular dichroism (CD) and NMR conformational studies revealed that incorporation of Dpg residues induced folded structures into peptides. It was observed that Dpg residues adopted helical conformation in a helix-promoting sequence. The resulting helical structure was comprised of consecutive β-turns whose structure was stabilized by salt bridge in aqueous solution. In this study, the preparation of sterically and polyfunctional Cα,α-disubstituted amino acids via alkylation of ethyl nitroacetate and transformation into derivatives ready for incorporation into peptides are described. Treatment of ethyl nitroacetate with N,N-diisopropylethylamine in the presence of a catalytic amount of tetraalkylammonium salt, followed by the addition of an activated alkyl halide or Michael acceptor, gave the doubly C-alkylated product in good to excellent yields. Selective nitro reduction with Zn in acetic or hydrogen over Raney Ni gave the corresponding amino ester that, upon saponification, can be protected with the fluorenylmethyloxycarbonyl (Fmoc) group. The synthesis of a sterically demanding Cα,α-dibenzylglycine (Dbzg), and an orthogonally protected, tetrafunctional Cα,α-disubstituted analogue of aspartic acid Bcmg is described. The preparation of amyloid fibril blocker peptides based on amyloid peptide hydrophobic core Aβ16-20 is described. These blocker peptides containing sterically hindered ααAA are β-strand mimics and are likely to interact with the amyloid hydrophobic core based on “like likes like” residue relationships. Amino acid symmetrical anhydride method was employed for the peptide synthesis. It was observed that Fmoc amino acid symmetrical anhydrides were efficient and readily available reagents for acylation of the N-terminus of highly hindered ααAAs. Comparison of a variety of coupling protocols showed that the symmetrical anhydride method always provided the superior results. Amyloid fibril inhibitor AMY-1 was synthesized and examined for its biological activities. It was observed that AMY-1 could significantly reduce the aggregation of amyloidogenic Aβ10-35 at different ratio at either room temperature and 37 ºC.