title: Hydrophobic peptide channels and encapsulated water wires.
abstract: Peptide nanotubes with filled and empty pores and close-packed structures are formed in closely related pentapeptides. Enantiomorphic sequences, Boc-(D)Pro-Aib-Xxx-Aib-Val-OMe (Xxx = Leu, 1; Val, 2; Ala, 3; Phe, 4) and Boc-Pro-Aib-(D)Xxx-Aib-(D)Val-OMe ((D)Xxx = (D)Leu, 5; (D)Val, 6; (D)Ala, 7; (D)Phe, 8), yield molecular structures with a very similar backbone conformation but varied packing patterns in crystals. Peptides 1, 2, 5, and 6 show tubular structures with the molecules self-assembling along the crystallographic six-fold axis (c-axis) and revealing a honeycomb arrangement laterally (ab plane). Two forms of entrapped water wires have been characterized in 2: 2a with d(O...O) = 2.6 A and 2b with d(O...O) = 3.5 A. The latter is observed in 6 (6a) also. A polymorphic form of 6 (6b), grown from a solution of methanol-water, was observed to crystallize in a monoclinic system as a close-packed structure. Single-file water wire arrangements encapsulated inside hydrophobic channels formed by peptide nanotubes could be established by modeling the published structures in the cases of a cyclic peptide and a dipeptide. In all the entrapped water wires, each water molecule is involved in a hydrogen bond with a previous and succeeding water molecule. The O-H group of the water not involved in any hydrogen bond does not seem to be involved in an energetically significant interaction with the nanotube interior, a general feature of the one-dimensional water wires encapsulated in hydrophobic environments. Water wires in hydrophobic channels are contrasted with the single-file arrangements in amphipathic channels formed by aquaporins.
authors.affiliation: Department of Physics, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Upadhyayula S
authors.initials: US
authors.lastname: Raghavender
authors.lastname: Kantharaju
authors.forename: Subrayashastry
authors.initials: S
authors.lastname: Aravinda
authors.forename: Narayanaswamy
authors.initials: N
authors.lastname: Shamala
authors.forename: Padmanabhan
authors.initials: P
authors.lastname: Balaram
mesh_terms: D018360:Crystallography, X-Ray; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D008968:Molecular Conformation; D043942:Nanotubes / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D010455:Peptides; D014867:Water