title: Apolar peptide models for conformational heterogeneity, hydration, and packing of polypeptide helices: crystal structure of hepta- and octapeptides containing alpha-aminoisobutyric acid.
abstract: The crystal structures of two helical peptides Boc-Val-Ala-Leu-Aib-Val-ala-Leu-OMe (VALU-7) and Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Aib-OMe (VALU-8) have been determined to a resolution of 1.0 and 0.9 A, respectively. Both the seven and eight residue peptides crystallize with two conformers per asymmetric unit. The VALU-8 conformers are completely helical and differ only at the C-terminus by a sign reversal of the phi, psi angles of the last residue. One of the VALU-7 conformers occurs as a normal alpha-helix, whereas in the other, the N(7)--O(3) alpha-type hydrogen bond is ruptured by the entry of a water molecule (W) into the helix, which in turn makes hydrogen bonds N(7)...W = 2.97 A and W...O(3) = 2.77 A. The other side of the water molecule is surrounded by a hydrophobic pocket. These two conformers give a static representation of a step in a possible helix unwinding or folding process. In the VALU-8 crystal the helices aggregate in a parallel mode, whereas the aggregation is anti-parallel in the VALU-7 crystal. The crystal parameters are VALU-7, P2(1), a = 10.203 (3) A, b = 19.744 (6) A, c = 22.561 (6) A, beta = 96.76 degrees, Z = 4, C38H69N7O10.0.5H2O, R = 6.65% for 3674 reflections observed greater than 3 sigma (F); and VALU-8, P2(1), a = 10.593 (4) A, b = 27.57 (6) A, c = 17.745 (5) A, beta = 95.76 (3) degrees, Z = 4, C42H76N8O11.0.25 CH3OH, R = 6.63% for 4701 reflections observed greater than 3 sigma (F).
authors.affiliation: Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C. 20375-5000.
authors.forename: I L
authors.initials: IL
authors.lastname: Karle
authors.forename: J L
authors.initials: JL
authors.lastname: Flippen-Anderson
authors.forename: K
authors.initials: K
authors.lastname: Uma
authors.forename: P
authors.initials: P
authors.lastname: Balaram
mesh_terms: D000595:Amino Acid Sequence; D000621:Aminoisobutyric Acids; D006860:Hydrogen Bonding; D008954:Models, Biological; D008969:Molecular Sequence Data; D009842:Oligopeptides; D010455:Peptides; D011487:Protein Conformation; D014867:Water; D014961:X-Ray Diffraction
chemical_list: D000621:Aminoisobutyric Acids; D009842:Oligopeptides; D010455:Peptides; D014867:Water; C063870:tert-butyloxycarbonyl-valyl-alanyl-leucyl-2-aminoisobutyryl-valyl-alanyl-leucyl methyl ester; C063872:t-butyloxycarbonyl-valyl-alanyl-leucyl-2-aminoisobutyryl-valyl-alanyl-leucyl-2-aminoisobutyryl methyl ester