Self-assembling cyclic β3-peptide nanotubes as artificial transmembrane ion channels

A new class of self-assembling transmembrane ion channels based on cyclic beta(3)-peptides is described. Cyclic peptide subunits were designed to adopt flat, ring shaped conformations and stack through extensive backbone-backbone hydrogen bonding to form tubular channel structures. Candidate channel-forming peptides cyclo[(-beta(3)-HTrp)(4-)] 1, cyclo[(-beta(3)-HTrp-beta-HLeu)(2-)] 2, and cyclo[(-beta(3)-HLeu)(4-)] 3 were examined in liposome-based proton transport assays and single channel conductance experiments. Compounds 1 and 2 exhibited remarkable ion transport activities with single-channel K+ conductance of 56 pS for peptide 1, while compound 3 was inactive, possibly due to its poor solubility. Additionally, the putative structure of transmembrane channels formed by peptides 1 and 2 was supported by FT-IR spectroscopy of membrane-peptide preparations. The present system not only complements that of channel-forming cyclic D,L-alpha-peptides previously reported from this laboratory but also is expected to exhibit novel properties arising from the unnatural beta(3)-peptide backbone.