Hydrophobic Peptide Channels and Encapsulated Water Wires

摘要

Abstract: Peptide nanotubes with filled and empty pores and close-packed structures are formed in closelynrelated pentapeptides. Enantiomorphic sequences, Boc-nDPro-Aib-Xxx-Aib-Val-OMe (Xxx Leu, 1; Val, 2;nAla, 3; Phe, 4) and Boc-Pro-Aib-nDXxx-Aib-nDVal-OMe (nDXxx DLeu, 5;nDVal, 6;nDAla, 7;nDPhe, 8), yieldnmolecular structures with a very similar backbone conformation but varied packing patterns in crystals.nPeptides 1, 2, 5, and 6 show tubular structures with the molecules self-assembling along the crystallographicnsix-fold axis (c-axis) and revealing a honeycomb arrangement laterally (ab plane). Two forms of entrappednwater wires have been characterized in 2: 2a with dO O 2.6 Å and 2b with dO O 3.5 Å. The latter isnobserved in 6 (6a) also. A polymorphic form of 6 (6b), grown from a solution of methanol water, wasnobserved to crystallize in a monoclinic system as a close-packed structure. Single-file water wirenarrangements encapsulated inside hydrophobic channels formed by peptide nanotubes could be establishednby modeling the published structures in the cases of a cyclic peptide and a dipeptide. In all the entrappednwater wires, each water molecule is involved in a hydrogen bond with a previous and succeeding waternmolecule. The O H group of the water not involved in any hydrogen bond does not seem to be involvednin an energetically significant interaction with the nanotube interior, a general feature of the one-dimensionalnwater wires encapsulated in hydrophobic environements.Water wires in hydrophobic channels are contrastednwith the single-file arrangements in amphipathic channels formed by aquaporins.