Role of Side Chains in β-Sheet Self-Assembly into Peptide Fibrils. IR and VCD Spectroscopic Studies of Glutamic Acid-Containing Peptides

Poly(glutamic acid) at low pH self-assembles after incubation at higher temperature into fibrils composed of. antiparallel sheets that are stacked in a beta(2)-type structure whose amide carbonyls have bifurcated H-bonds involving the side. chains from the next sheet. Oligomers of Glu can also form such structures, and isotope labeling has, provided insight into their out-of-register antiparallel structure [Biomacromolecules 2013, 14, 3880-3891]. In this paper we report IR and VCD spectra and transmission electron micrograph (TEM) images for a series of alternately sequenced oligomers, Lys-(Aaa-Glu)(5)-Lys-NH2, where Aaa was varied over a variety of polar, aliphatic; or aromatic residues. Their spectral and, TEM data show that these oligopeptides self-assemble into different structures, both local and morphological, that ate dependent on both the nature of the Aaa side chains and growth conditions employed. Such alternate peptides substituted with small or polar residues, Ala and Thr, do not yield fibrils; but with beta-branched aliphatic residues, Val and Ile, that could potentially pack with Glu side Chains, these oligopeptides do show evidence of beta(2)-stacking. By contrast, for Leu, with longer side chains, Only beta(1)-stacking is seen while with even larger Phe side Chains, either beta-form can be detected separately, depending on preparation conditions. These structures-are dependent on high temperature incubation after reducing the pH and in some cases after sonication of initial fibril forms and reincubation. Some of these fibrillar peptides, but not all, show enhanced VCD) which can offer evidence far formation of long, multistrand, often twisted structures. Substitution, of Glu with residues having selected side chains yields a variety of morphologies, leading to both beta(1)- and beta(2)-structures; that overall suggests two different packing modes for the hydrophobic side chains depending on size and type.