Crystal Structures of Chain-Folded Antiparallel β-Sheet Assemblies from Sequence-Designed Periodic Polypeptides

The crystal structures and textures of a family of sequence-designed periodic polypeptides were investigated and analyzed using X-ray diffraction, vibrational spectroscopy, and cross-polarization magic angle spinning 13C nuclear magnetic resonance. The repetitive amino acid sequences are described by −[(AG)xEG]−, with integer x from 3 to 6. These macromolecules were prepared via bacterial expression of artificial genes and are monodisperse. Crystalline samples were obtained, and the interpretation of the X-ray diffraction results was aided by the generation of computer-simulated X-ray diffraction patterns. This allowed direct comparisons to be made with the observed texture-oriented X-ray diffraction photographs. All diffraction and spectroscopic evidence supports an antiparallel (ap) β-sheet structure, and all structures index on orthorhombic sublattices similar to those reported for Bombyx mori silk fibroin and poly(l-alanylglycine). The unit cell parameters for poly(AG)3EG, for example, are a = 0.948 nm (hydrogen-bond direction), b = 1.060 nm (ap β-sheet stacking direction), and c = 0.695 nm (chain direction). Selective line broadening is observed for wide-angle diffraction signals with l ≠ 0 (for the 211 in particular) and gives an estimated crystal size of <4 nm in the chain direction. This, coupled with the appearance of a low-angle particle interference peak at 3.6 nm, indicates a crystal size over an order of magnitude less than the chain length and suggests an adjacent reentry chain-folded lamellar structure incorporating the ap β-sheet architecture. A structure with polar ap β-sheets and γ-turns, stacking with the hydrophobic methyl groups of the alanyl residues in contact, is selected by X-ray structure refinement to give the best match with the experimental data. The pattern of crystallization behavior of the poly(AG)xEG family is consistent with the folding periodicity being in-phase with the amino acid sequence so that the glutamic acid residues are confined to the lamellar surfaces.