Cover picture: The µ helix of poly-L-glutamine. (Top) Atom species are denoted by a standard coloring: C, green; N, blue; O, red; H, white. (Bottom) Main-chain atoms are colored red, and side chains green. (Left) Longitudinal view from the N-terminus of a 73-residue chain; the terminal H atom is removed. (Right) Lateral view; the N-terminus is to the left. This helix has a cylindrical pore, the diameter of which is 3.7 Å. A conformation-energy calculation predicts that poly-L-glutamine, as a rare exception, can form a stable µ helix. This stability is attributable to the presence of six pieces of the amide string motif. One of the amide strings is indicated by superimposed chemical symbols in the bottom right figure. The µ helix has been proposed as a candidate molecular structure of the cation channel formed by long-chain poly-L-glutamine. See the article by Monoi et al. on p. 2892.