β₂-Type Amyloidlike Fibrils of Poly-l-glutamic Acid Convert into Long, Highly Ordered Helices upon Dissolution in Dimethyl Sulfoxide

Abstract

Replacing water with dimethyl sulfoxide (DMSO) completely reshapes the free-energy landscapes of solvated proteins. In DMSO, a powerful hydrogen-bond (HB) acceptor, formation of HBs between backbone NH groups and solvent is favored over HBs involving protein's carbonyl groups. This entails a profound structural disruption of globular proteins and proteinaceous aggregates (e.g., amyloid fibrils) upon transfer to DMSO. Here, we investigate an unusual DMSO-induced conformational transition of β 2amyloid fibrils from poly-L-glutamic acid (PLGA). The infrared spectra of β 2 -PLGA dissolved in DMSO lack the typical features associated with disordered conformation that are observed when amyloid fibrils from other proteins are dispersed in DMSO. Instead, the frequency and unusual narrowness of the amide I band imply the presence of highly ordered helical structures, which is supported by complementary methods, including vibrational circular dichroism and Raman optical activity. We argue that the conformation most consistent with the spectroscopic data is that of a PLGA chain essentially lacking nonhelical segments such as bends that would provide DMSO acceptors with direct access to the backbone. A structural study of DMSO-dissolved β 2 -PLGA by synchrotron small-angle X-ray scattering reveals the presence of long uninterrupted helices lending direct support to this hypothesis. Our study highlights the dramatic effects that solvation may have on conformational transitions of large polypeptide assemblies.

Effects of Glycine on the Local Conformation and Internal Backbone Dynamics of Polypeptides

“…[θ] under the 1 B b band of pyrene ([θ] 1 B b ) as a function of pyrene content for (◯) Py-PGlu, (blue ◊) Py-PGly 15 Glu 85 , (blue △) Py-PGly 40 Glu 60 , and (blue □) Py-PGly 52 Glu 48 in DMSO. For comparative purposes, [θ] 1 B b of Py-P L Glu (orange ◯), which adopts a 3 10 -helical conformation in DMSO, , is also included.…”

“…For comparison purposes, [θ] 1 B b of a 3 10 -helical pyrene-labeled poly(L-glutamic acid) (Py-P L Glu) was also included (Figure S4F). 37,38 Figure 2 shows that Py-P L Glu exhibits a non-zero [θ] 1 B b , which increases with increasing x, indicative of a structured conformation. In contrast, the [θ] 1 B b values of the Py-PGlu and Py-PGlyGlu samples were significantly lower than those obtained for Py-P L Glu and scattered about zero for all pyrene contents, indicating that these samples exhibited negligible ellipticity.…”

Effect of Like Charges on the Conformation and Internal Dynamics of Polypeptides Probed by Pyrene Excimer Fluorescence

“…Consequently, if Py-PLGA or PLGNa adopted a secondary structure in DMSO, the N blob value corresponding to that structure was intermediate between that of a random coil and an α-helical conformation. As it turns out, these results agree with the conclusions drawn from a recent study on the structure of PLGA in DMSO . The study concluded that PLGA adopted an uninterrupted, rigid, rodlike structure, such as that of an α-helix or a similar helical structure in DMSO.…”

“…As it turns out, these results agree with the conclusions drawn from a recent study on the structure of PLGA in DMSO. 43 The study concluded that PLGA adopted an uninterrupted, rigid, rodlike structure, such as that of an α-helix or a similar helical structure in DMSO. Based on the experimental N blob values, the structure of Py-PLGA in DMSO had to be less dense than that of an α-helix with a backbone having fewer residues-perturn.…”

Reversible Freeze-Induced β-Sheet-to-Disorder Transition in Aggregated Homopolypeptide System

“…The self-association of polypeptide chains into insoluble aggregates (amorphous, or fibrillar: so-called amyloid fibrils) with a high β-sheet content is a common phenomenon extensively studied mainly due to its biomedical implications. , Networks of dense inter- and intramolecular interactions (primarily hydrogen bonds and van der Waals forces, but also salt bridges) suppress backbone fluctuations and disfavor structural transitions within β-aggregates, unless the transition itself involves fragmentation of larger polypeptide assemblies. For example, when amyloid fibrils are dissolved in dimethyl sulfoxide, simultaneous β-sheet → random coil (RC), or β-sheet → α-helix, transitions are observed. Apart from chemical denaturants, high hydrostatic pressure , and moderate cooling − (without freezing) have been shown to denature amyloid fibrils.…”