Photocontrol of the β-Hairpin Polypeptide Structure through an Optimized Azobenzene-Based Amino Acid Analogue

A family of neurodegenerative diseases, including Huntington's disease (HD) and spinocerebellar ataxias, are associated with an abnormal polyglutamine (polyQ) expansion in mutant proteins that become prone to form amyloid-like aggregates. Prior studies have suggested a key role for β-hairpin formation as a driver of nucleation and aggregation, but direct experimental studies have been challenging. Toward such research, we set out to enable spatiotemporal control over β-hairpin formation by the introduction of a photosensitive β-turn mimic in the polypeptide backbone, consisting of a newly designed azobenzene derivative. The reported derivative overcomes the limitations of prior approaches associated with poor photochemical properties and imperfect structural compatibility with the desired β-turn structure. A new azobenzene-based β-turn mimic was designed, synthesized, and found to display improved photochemical properties, both prior and after incorporation into the backbone of a polyQ polypeptide. The two isomers of the azobenzene-polyQ peptide showed different aggregate structures of the polyQ peptide fibrils, as demonstrated by electron microscopy and solid-state NMR (ssNMR). Notably, only peptides in which the β-turn structure was stabilized (azobenzene in the cis configuration) closely reproduced the spectral fingerprints of toxic, β-hairpin-containing fibrils formed by mutant huntingtin protein fragments implicated in HD. These approaches and findings will enable better deciphering of the roles of β-hairpin structures in protein aggregation processes in HD and other amyloid-related neurodegenerative diseases.

Nonequilibrium molecular dynamics simulations of infrared laser-induced dissociation of a tetrameric Aβ42 β-barrel in a neuronal membrane model

Experimental studies have reported that the amyloid-β proteins can form pores in cell membranes, and this could be one possible source of toxicity in Alzheimer's disease. Dissociation of these pores could therefore be a potential therapeutic approach. It is known that high photon density free-electron laser experiments and laser-induced nonequilibrium molecular dynamics simulations (NEMD) can dissociate amyloid fibrils at specific frequencies in vitro. Our question is whether NEMD simulations can dissociate amyloid pores in a bilayer mimicking a neuronal membrane, and as an example, we select a tetrameric Aβ42 β-barrel. Our simulations shows that the resonance between the laser field and the amide I vibrational mode of the barrel destabilises all intramolecular and intermolecular hydrogen bonds of Aβ42 and converts the β-barrel to a random/coil disordered oligomer. Starting from this disordered oligomer, extensive standard MD simulations shows sampling of disordered Aβ42 states without any increase of β-sheet and reports that the orientational order of lipids is minimally disturbed. Interestingly, the frequency to be employed to dissociate this beta-barrel is specific to the amino acid sequence. Taken together with our previous simulation results, this study indicates that infrared laser irradiation can dissociate amyloid fibrils and oligomers in bulk solution and in a membrane environment without affecting the surrounding molecules, offering therefore a promising way to retard the progression of AD.

Infrared Laser-Induced Amyloid Fibril Dissociation: A Joint Experimental/Theoretical Study on the GNNQQNY Peptide

Neurodegenerative diseases are usually characterized by plaques made of well-ordered aggregates of distinct amyloid proteins. Dissociating these very stable amyloid plaques is a critical clinical issue. In this study, we present a joint mid-infrared free electron laser experiment/nonequilibrium molecular dynamics simulation to understand the dissociation process of a representative example GNNQQNY fibril. By tuning the laser frequency to the amide I band of the fibril, the resonance takes place and dissociation is occurred. With the calculated and observed wide-angle X-ray scattering profiles and secondary structures before and after laser irradiation being identical, we can propose a dissociation mechanism with high confidence from our simulations. We find that dissociation starts in the core of the fibrils by fragmenting the intermolecular hydrogen bonds and separating the peptides and then propagates to the fibril extremities leading to the formation of unstructured expanded oligomers. We suggest that this should be a generic mechanism of the laser-induced dissociation of amyloid fibrils.

Modulating the Phe–Phe dipeptide aggregation landscape via covalent attachment of an azobenzene photoswitch

A covalently modified trans-H-(l)AzoPhe–Phe–OH dipeptide shows reversible photo-switching between its native fibril and vesicle aggregate morphology.

Light and heat control over secondary structure and amyloid-like fiber formation in an overcrowded-alkene-modified Trp zipper

The external photocontrol over peptide folding, by the incorporation of molecular photoswitches into their structure, provides a powerful tool to study biological processes. However, it is limited so far to switches that exhibit only a rather limited geometrical change upon photoisomerization and that show thermal instability of the photoisomer. Here we describe the use of an overcrowded alkene photoswitch to control a model β-hairpin peptide. This photoresponsive unit undergoes a large conformational change and has two thermally stable isomers which has major influence on the secondary structure and the aggregation of the peptide, permitting the phototriggered formation of amyloid-like fibrils.

Reversible photocontrol of self-assembled peptide hydrogel viscoelasticity

Peptide hydrogels are promising biomaterials for applications ranging from drug delivery to tissue engineering.

Isomerization- and temperature-jump-induced dynamics of a photoswitchable β-hairpin

Conformational changes in proteins and peptides can be initiated by diverse processes. This raises the question how the variation of initiation mechanisms is connected to differences in folding or unfolding processes. In this work structural dynamics of a photoswitchable β-hairpin model peptide were initiated by two different mechanisms: temperature jump (T-jump) and isomerization of a backbone element. In both experiments the structural changes were followed by time-resolved IR spectroscopy in the nanosecond to microsecond range. When the photoisomerization of the azobenzene backbone switch initiated the folding reaction, pronounced absorption changes related to folding into the hairpin structure were found with a time constant of about 16 μs. In the T-jump experiment kinetics with the same time constant were observed. For both initiation processes the reaction dynamics revealed the same strong dependence of the reaction time on temperature. The highly similar transients in the microsecond range show that the peptide dynamics induced by T-jump and isomerization are both determined by the same mechanism and exclude a downhill-folding process. Furthermore, the combination of the two techniques allows a detailed model for folding and unfolding to be presented: The isomerization-induced folding process ends in a transition-state reaction scheme, in which a high energetic barrier of 48 kJ mol(-1) separates unfolded and folded structures.