Synthesis and CD spectra of fluoro- and hydroxy-substituted β-peptides

β-Amino acids 1-3 with OH and F substituents in the α-position have been prepared (Scheme) from the natural (S)-α-amino acids alanine, valine, and leucine, and incorporated into β-hexa- and β-heptapeptides 4-12. The peptide syntheses were performed according to a conventional solution strategy (Boc/Bn protection) with fragment coupling. The new β-peptides with (series a) and without (series b) terminal protection were isolated in HPLC-pure form and characterized by NMR spectroscopy and MALDI mass spectrometry. The chemical properties as well as the patterns of the CD spectra (Figs. 3-5) depend upon constitution (OH, F, F₂ substitution) and configuration (l or u) of the amino acid residues, upon the total number of OH and F substituents in the peptide chain, and upon the solvent used (H₂O, MeOH, CF₃CH₂OH, (CF₃)₂CHOH). No reliable clues regarding the structures can be obtained from these CD spectra. Only a full NMR analysis will be able to answer the questions: a) with which known secondary structures (Figs. 1 and 2) of β-peptides are the OH and F derivatives compatible? b) Are new secondary structures enforced by the polar and/or H-bonding backbone substituents? Furthermore, the β-peptides described here will enable us to study changes in chemical, enzymatic, and metabolic stability, and in physiological properties caused by the heteroatoms.