Crystalline Aggregates of the Repetitive Polypeptide {(AlaGly)3GluGly(GlyAla)3GluGly}10: Structure and Dynamics Probed by 13C Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

A repetitive polymer comprising 10 repeats of the oligopeptide (AlaGly)₃GluGly(GlyAla)₃ GluGly has been synthesized via bacterial expression of an artificial gene. This polymer was designed to assemble into lamellar crystals of predictable thickness, with the oligo(AlaGly) and oligo(GlyAla) portions forming the crystal stems and the polar, bulky Glu residues lying in reverse turns between adjacent, antiparallel β-strands. The solid-state structure and dynamics of this material, before and after crystallization, have been probed with a range of magic angle spinning ¹³C NMR experiments. Chemical shifts of backbone carbons indicate that the oligo(AlaGly) elements adopt β-conformations in all samples. Changes in both the chemical shift and cross-polarization behavior of the alanine side chains upon crystallization provide evidence for location of alanine within the crystalline β-sheet regions. In contrast, although crystallization induces changes in the chemical shift of C_α of the glutamic acid residue, no change occurs in the dynamic behavior of the Glu side chain carbons. The latter observation suggests that this side chain is excluded from the crystalline region and supports the assignment of Glu to turn positions at the lamellar surface.