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

Abstract

Poly(glutamic acid) at low pH self-assembles after incubation at higher temperature into fibrils composed of antiparallel sheets that are stacked in a β2-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 are 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 β-branched aliphatic residues, Val and Ile, that could potentially pack with Glu side chains, these oligopeptides do show evidence of β2-stacking. By contrast, for Leu, with longer side chains, only β1-stacking is seen while with even larger Phe side chains, either β-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 for formation of long, multistrand, often twisted structures. Substitution of Glu with residues having selected side chains yields a variety of morphologies, leading to both β1- and β2-structures, that overall suggests two different packing modes for the hydrophobic side chains depending on size and type.

Transition dipole coupling modeling of optical activity enhancements in macromolecular protein systems

“…types 1 to 4 ) and local chromophore structure ( F or T ). This is consistent with previous VCD fibril measurements revealing a remarkable variability of spectral shapes with respect to studied systems . However, the theoretical g ‐factors seem to be quite small compared to maximal values observed experimentally (~10 −2 ‐10 −3 , e.g., Figure ).…”

β‐sheet assembly in amyloidogenic glutamic acid nanostructures: Insights from X‐ray scattering and infrared nanospectroscopy

“…Dotted lines are guides for the eyes indicating the positions of main peaks appearing in the spectra. The mode at 1600 cm −1 , associated to β 2 ‐sheets, is highlighted in bold…”

“…In recent years, Glu ‐ rich peptides have gained more attention mainly due to the ability of carboxylic side chains to form certain types of H‐bond networks . Particularly, it has been demonstrated that Glu‐enriched sequences stack into antiparallel sheets, forming β 2 ‐type structures held together by bifurcated H bonds between amide carbonyls and carboxylic acid side chains in neighboring sheets . These structural insights have been derived mainly from Fourier transform infrared (FTIR) absorption and circular and vibrational circular dichroism (CD and VCD) .…”

Mini review: Instrumentation for vibrational circular dichroism spectroscopy, still a role for dispersive instruments

“…One check is to rotate the sample about the optical axis to test for orientation sensitivity. In particular, enhanced VCD of fibrillar samples have been shown to be diagnostically useful, although the origin of the enhancement is still not fully clear . While it is desirable to express VCD in molar units, eg, ε and Δε, for analyses by theoretical modeling, IR‐based biomolecular experiments tend to be inaccurate for both concentration and path lengths, making these values unreliable.…”