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We report the 0.75 A crystal structure of a racemic mixture of the 12-residue designed peptide "Alpha-1" (Acetyl-ELLKKLLEELKG), the L-enantiomer of which is described in the accompanying paper. Equivalent solutions of the centrosymmetric bilayers were determined by two direct phasing programs in space groups P1 and P1bar. The unit cell contains two L-alpha-helices and two D-alpha-helices. The columnar-sheet bilayer motif seen in L-Alpha-1 is maintained in the D,L-Alpha-1 structure except that each sheet of head-to-tail helices is composed of one enantiomer and is related to its neighboring sheets by inversion symmetry. Comparison to the L-Alpha-1 structure provides further insight into peptide design. The high resolution and small asymmetric unit allowed building an intricate model (R = 13.1%, Rfree = 14.5%) that incorporates much of the discrete disorder of peptide and solvent. Ethanolamine and 2-methyl-2,4-pentanediol (MPD) molecules bind near helix termini. Rigid body analysis identifies sites of restricted displacements and torsions. Side-chain discrete disorder propagates into the backbone of one helix but not the other. Although no side chain in Alpha-1 is rigid, the environments in the crystal restrict some of them to no or only one active torsion.

A 12-residue peptide designed to form an alpha-helix and self-associate into an antiparallel 4-alpha-helical bundle yields a 0.9 A crystal structure revealing unanticipated features. The structure was determined by direct phasing with the "Shake-and-Bake" program, and contains four crystallographically distinct 12-mer peptide molecules plus solvent for a total of 479 atoms. The crystal is formed from nearly ideal alpha-helices hydrogen bonded head-to-tail into columns, which in turn pack side-by-side into sheets spanning the width of the crystal. Within each sheet, the alpha-helices run antiparallel and are closely spaced (9-10 A center-to-center). The sheets are more loosely packed against each other (13-14 A between helix centers). Each sheet is amphiphilic: apolar leucine side chains project from one face, charged lysine and glutamate side chains from the other face. The sheets are stacked with two polar faces opposing and two apolar faces opposing. The result is a periodic biomaterial composed of packed protein bilayers, with alternating polar and apolar interfaces. All of the 30 water molecules in the unit cell lie in the polar interface or between the stacked termini of helices. A section through the sheet reveals that the helices packed at the apolar interface resemble the four-alpha-helical bundle of the design, but the helices overhang parts of the adjacent bundles, and the helix crossing angles are less steep than intended (7-11 degrees rather than 18 degrees).