Centrosymmetric bilayers in the 0.75 angstrom resolution structure of a designed alpha-helical peptide, D,L-Alpha-1

We report the 0.75 Angstrom crystal structure of a racemic mixture of the 1 2-residue designed peptide "Alpha-1" (Acetyl-ELLKKLLEELKG), the L-enantiome r of which is described in the accompanying paper. Equivalent solutions of the centrosymmetric bilayers were determined by two direct phasing programs in space groups pi and P (1) over bar. The unit cell contains two L-alpha- helices and two D-alpha-helices. The columnar-sheet bilayer motif seen in L -Alpha-l is maintained in the D,L-Alpha-l 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-l stru cture provides further insight into peptide design. The high resolution and small asymmetric unit allowed building an intricate model (R = 13.1%, R-fr ee = 14.5%) that incorporates much of the discrete disorder of peptide and solvent. Ethanolamine and 2-methyl-2,4-pentanediol (MPD) molecules bind nea r helix termini. Rigid body analysis identifies sites of restricted displac ements and torsions. Side-chain discrete disorder propagates into the backb one of one helix but not the other. Although no side chain in Alpha-1 is ri gid, the environments in the crystal restrict some of them to no or only on e active torsion.