Giant macrolactams based on β-sheet peptides

Abstract

This paper reports the use of natural amino acids, the tripeptide β-strand mimic Hao, and the β-turn mimic δ-linked ornithine to generate water-soluble 54-, 78-, and 102-membered-ring macrolactams. These giant macrocycles were efficiently prepared by synthesis of the corresponding protected linear peptides, followed by solution-phase cyclization and deprotection. The protected linear peptide precursors were synthesized on 2-chlorotrityl chloride resin by conventional Fmoc-based solid-phase peptide synthesis. Macrocyclization was typically performed using HCTU and N,N-diisopropylethylamine in DMF at ca. 0.5 mM concentration. The macrocycles were isolated in 13-45% overall yield after HPLC purification and lyophilization. 1D, 2D TOCSY, and 2D ROESY (1)H NMR studies of the 54- and 78-membered-ring macrolactams establish that these compounds fold to form β-sheet structures in aqueous solutions.

FIGURE 1

Analytical HPLC traces in the syntheses of peptides 3a, 4a, and 6. HPLC data were recorded using an Agilent Zorbax SB-C18 column (50 mm × 4.6 mm) with a gradient of 5–100% CH₃CN in H₂O with 0.1% TFA and a flow of 1.0 mL/min over 20 minutes.

FIGURE 2

Selected expansion of the ROESY spectrum of peptide 3a (2.0 mM in D₂O at 500 MHz and 298 K with a 300-ms spin lock time).

FIGURE 3

Key interstrand H_α–H_α, NH–NH, and H_α–NH NOEs of peptide 3a observed in H₂O/D₂O (9:1) at 2.0 mM and 298 K.

Figure 4

Diffusion coefficients of peptide 3a in D₂O at 298 K.

FIGURE 5

Selected expansion of the ROESY spectrum of peptide 4b (1.0 mM in D₂O at 500 MHz and 288 K with a 200-ms spin lock time).

FIGURE 6

Key NOEs of peptide 5 observed in CD₃OD at 1.0 mM and 298 K.

FIGURE 7

Δδ^δOrn values of peptides 3 in D₂O at 298 K.

SCHEME 1

Synthesis of Peptide 3a