title: Effect of local sequence inversions on the crystalline antiparallel beta-sheet lamellar structures of periodic polypeptides: implications for chain-folding.
abstract: The crystal structure and texture of the monodisperse periodic polypeptide [(AG)3EG(GA)3EG]10 (poly(+/-AG)3EG: A=alanine, G=glycine, E=glutamic acid) were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and electron microscopy. Structure determination was aided by comparison with the recently described structure for the related periodic polypeptide [(AG)3EG]36 by Krejchi et al. (Macromolecules 1997;30:5012). Texture-oriented samples of poly(+/-AG)3EG were obtained by crystallization of the polymer from aqueous formic acid solution. The evidence supports an antiparallel (ap) beta-sheet protein structure and the X-ray diffraction signals index on an orthorhombic unit cell with parameters: a=0.950 nm (hydrogen-bond direction), b=1.052 nm (apbeta-sheet stacking direction), c=6.95 nm (chain direction). The absence of the (010) diffraction signal, a prominent signal in the poly(AG)3EG diffraction pattern, implies that the apbeta-sheets are 'apolar', i.e. both surfaces are equally populated with alanyl methyl groups. Selective line broadening of wide-angle diffraction signals with l not equal to 0 gives an estimated crystal size of approximately/= 4 nm in the chain direction. This observation, coupled with the appearance of low-angle particle interference peaks, indicates a crystal thickness considerably less than the chain length and suggests an adjacent-re-entry chain-folded lamellar structure incorporating the apbeta-sheet architecture. The polypeptide folds through gamma-turns, in-phase with the pseudo-octapeptide repeat; the glutamic acid residues occur on the lamellar surfaces. These results and those from the crystalline lamellae of poly(AG)3EG suggest that beta-turns are not compatible with these repetitively stacked apbeta-sheet structures. This implies that intersheet interactions of alanyl methyl groups and glycyl alpha-protons are not sufficiently strong to dictate the folding geometry in these structures.
authors.affiliation: Polymer Science and Engineering Department, University of Massachusetts, Amherst 01003, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Parkhe
authors.forename: S J
authors.initials: SJ
authors.lastname: Cooper
authors.forename: E D
authors.initials: ED
authors.lastname: Atkins
authors.forename: M J
authors.initials: MJ
authors.lastname: Fournier
authors.forename: T L
authors.initials: TL
authors.lastname: Mason
authors.forename: D A
authors.initials: DA
authors.lastname: Tirrell
mesh_terms: D003460:Crystallization; D008854:Microscopy, Electron; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D020449:Repetitive Sequences, Amino Acid; D014961:X-Ray Diffraction
chemical_list: D010455:Peptides