Binuclear ruthenium(II) complexes for amyloid fibrils recognition

doi:10.1016/j.chemphys.2014.10.015

Chemical Physics

Volume 445, 5 December 2014, Pages 1-4

https://doi.org/10.1016/j.chemphys.2014.10.015 Get rights and content

Highlights

•
Interactions of binuclear ruthenium(II) complexes with amyloid fibrils.
•
Dimer ruthenium(II) compounds are sensitive amyloid fibrils biomarkers.
•
Recognition of amyloid-chromophore adducts by two-photon excited emission.

Abstract

Metal–organic compounds represent a unique class of biomarkers with promising photophysical properties useful for imaging. Here interactions of insulin fibrils with two binuclear complexes [μ-(11,11′-bidppz)(phen)₄Ru₂]⁴⁺ (1) and [μ-C4(cpdppz)(phen)₄Ru₂]⁴⁺ (2) are studied by linear dichroism (LD) and fluorescence. These ruthenium(II) compounds could provide a new generation of amyloid binding chromophores with long lived lifetimes, good luminescence quantum yields for the bound molecules and photo-stability useful in multiphoton luminescence imaging.

Graphical abstract

Introduction

Fibrilization of peptides leads to formation of amyloid fibril structures [1]. The process is promoted by certain mutations that affect protein folding which may result in erratic structures, such as self-assembled isolable aggregates believed to lead to amyloidosis and in consequence to serious diseases [2], [3], [4]. One of the most common detection method for amyloids in vivo and in vitro is staining with organic dyes such as Thioflavine T and Congo Red [5]. However, those chromophores are far from being ideal, and in respect of progress in diagnostic technology, new chromophores, more effective, specific and with good photophysical properties are in demand for fibrils recognition. Transition metal complexes constitute an interesting class of probes exhibiting versatile properties that, due to the rigid coordination framework with possibilities to vary the three-dimensional structure could be tuned for specific tasks [6], [7]. Ruthenium(II) complexes, in particular those comprising the dipyrido[3,2-a:2′,3′-c]phenazine (dppz) ligand and derivatives thereof, have been intensively studied due to their environmental sensitive luminescence which make them versatile spectroscopic probes for biomolecules including amyloid fibrils [8]. Strongly hydrophobic environment leads to the efficient fluorescence, whereas polarity leads to complete quenching (so called the light-switch effect [9]). Ruthenium complexes structure can be also extended to dimers. Monomeric [Ru(phen)₂dppz]²⁺ moieties are connected either by a single bond between the dppz moieties that is resulting in dimeric complex (1) which is relatively rigid or by connecting with aliphatic chain that makes it flexible (2) [structures: see insets in Fig. 1].

Intramolecular structure is particularly important for potential applications of these compounds for non-bleaching multiphoton imaging [10] which is using low energy photons outside the absorption edges that allows performing non-invasive in vivo studies in the near-infrared (NIR) without damaging biological materials. Moreover this detection approach has better spatial resolution and long penetration depth [11]. Thus interactions between amyloids and two binuclear ruthenium(II) complexes of different rigidity were investigated. Amyloid fibrils formed by insulin can be oriented in a flow linear dichroism (LD) [12] what opens the possibility to understand the mechanism of interactions between chromophores and the fibrils. Results from LD and fluorescence are analyzed in terms of binding geometry and accessibility for multiphoton detection.

Section snippets

Materials and methods

Samples were prepared as follows: a native insulin protein from bovine albumin was purchased from Sigma Aldrich, dissolved in pH = 2 (0.01 M HCl) water buffer and adjusted to final concentration of 5 mg/ml. The solution was filtered through 0.2 μm filter and further heated to 65 °C for 24 h. After fibrilization the samples were centrifuged at 3000 rpm for 5 min in order to remove globular particulates. Ruthenium(II) complexes were kindly provided by Per Lincoln at Chalmers University of Technology.

Results and discussion

LD is defined as the difference in absorbance between light that is linearly polarized parallel and light that is linearly polarized perpendicular to the macroscopic axis of orientation [13]. It studied case LD showed that both complexes are interacting with insulin fibrils and transition dipole moments are oriented parallel to the long molecular axis with absorption maxima at 425 nm for (1) and 375 nm for (2) respectively [Fig. 1]. These results are consistent with computational simulations

Conclusions

In conclusion it is the first report showing that dimer ruthenium(II) complexes can be successfully used for recognition of amyloid protein fibrils what was confirmed by linear dichroism and fluorescence experiments. Binding geometry of the complex determines the fluorescence efficiency and in consequence accessibility for nonlinear optical detection methodology.

Conflict of interest

There is no conflict of interest.

Acknowledgments

This work was supported by Swedish Research Council (VR) grant. P.H. thanks Per Lincoln for the gift of ruthenium(II) compounds.

References (19)

J. Wigenius et al.
Biochem. Biophys. Res. Commun.
(2011)
J. Olesiak-Banska et al.
Chem. Phys.
(2012)
P. Hanczyc et al.
Nat. Photonics
(2013)
E. Dawkins et al.
J. Neurochem.
(2014)
D.J. Irwin et al.
Nat. Rev. Neurol.
(2013)
A. Jacquin et al.
J. Neurol.
(2014)
J. Duboisset et al.
J. Phys. Chem. B
(2013)
P. Hanczyc et al.
J. Phys. Chem. B
(2013)
V. Sathisha et al.
Talanta
(2014)

There are more references available in the full text version of this article.

Cited by (11)

A FRET pair for quantitative and superresolution imaging of amyloid fibril formation
2022, Sensors and Actuators B: Chemical
Citation Excerpt :
For this reason, recent studies have provided a wide range of fluorescent dyes shown to be capable of detecting different types of amyloid aggregates. A few examples of recent developments of luminescent dyes to stain amyloid aggregates and fibrils, even in cells and tissues, involve curcumine derivatives [14], BODIPY-based dyes [15], molecular rotors [16], oxazine fluorophores [17], benzimidazole derivatives [18], conjugated polythiophenes [19,20], thiazolium dyes [21,22] and ruthenium complexes [23,24], among many others. Some of these examples have been recently revised in references [8,25].
The presence of neuritic plaques and amyloid fibrils arising from the misfolding of certain proteins is the principal molecular indicator of neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Methodologies for studying the early stages of amyloid aggregation are rapidly arising to provide a better understanding of the mechanism of fibrillization and cytotoxicity and to identify potential targets for diagnosis and therapy. The method presented here involves the simultaneous use of two different fluorophores, a quinolimide derivative and Nile Blue A. These are capable of interacting with and reporting on the formation of preamyloid aggregates and fibrils of apoferritin through fluorescence resonance energy transfer (FRET), which occurs between them, thus maximizing the contrast in detection and quantitative information of such amyloid species by using multidimensional fluorescence lifetime imaging microscopy (FLIM).
Luminescent imaging of insulin amyloid aggregation using a sensitive ruthenium-based probe in the red region
2021, Journal of Inorganic Biochemistry
Citation Excerpt :
Due to these limitations, research is directed towards the investigation of new luminescent probes that present more specific spectroscopic changes for protein aggregates. Ruthenium complexes, in special, have been used to study a series of different amyloid proteins such as amyloid beta, insulin, hiAPP (islet amyloid polypeptide) and PrP106–126 (prion protein) [17–20]. They have shown a variety of applications according to their structures, from promising luminescent probes to anti-amyloid properties.
A sensitive and selective strategy to identify insulin fibrils remains a challenge for researchers in amyloid protein research. Thus, it is critical to detect, in vitro, the species generated during amyloid aggregation, particularly the fibrillar species. Here we demonstrate that the luminescent complex cis-[Ru(phen)₂(3,4Apy)₂]²⁺ (RuApy; phen = 1,10-phenanthroline; 3,4Apy = 3,4-diaminopyridine) is a rapid, low-cost alternative to in vitro detection of fibrillar insulin, using conventional optical techniques. The RuApy complex displays emission intensity enhancement at 655 nm when associated with insulin, which enables imaging of the conformational changes of the protein's self-aggregation. The complex shows high sensitivity to fibrillar insulin with a limit of detection of 0.85 μM and binding affinity of 12.40 ± 1.84 μM which is comparable to those of Thioflavin T and Congo red, with the advantage of minimizing background fluorescence, absorption of light by biomolecules, and light scattering from physiologic salts in the medium.
Non-conventional photoactive transition metal complexes that mediated sensing and inhibition of amyloidogenic aggregates
2021, Coordination Chemistry Reviews
Citation Excerpt :
Molecular docking study showed that binding of 3 with negatively charged residues produced hydrophobic protection of 3 to enhance its luminescence. In 2014, Hanczyc reported that two binuclear ruthenium(II) complexes, [µ-(11,11′-(bidppz)(phen)4Ru2]4+ (4) and [µ-C4-(cpdppz)(phen)4Ru2]4+ (5) are promising candidates for sensing the amyloid fibrils using linear dichroism (LD) and fluorescence spectral techniques [77]. Complexes 4 and 5 were weakly luminescent at ~ 625 nm.
Alzheimer’s disease (AD), a devastating neurodegenerative disease, is associated with the abnormal accumulation and aggregation of β-amyloid proteins (Aβ) along with the deposition of high levels of Cu, Fe and Zn ions in the brain, causing neuronal cell deaths to lead the cognitive disabilities and even death. As there is a direct relationship between AD and Aβ aggregation, an intense research activity has been made to develop drug materials that serve as probes and inhibitors for controlling the pathways of Aβ peptide aggregation. However, their relatively instability in aqueous medium, tedious sample treatment, multistep syntheses, or low detection ability limit their potential applications. Therefore, the development of photoactive metal complexes for the selective detection and inhibition effects of Aβ aggregation is a thrust area in biomedical research. In this review, the use of non-conventional photoactive metal complexes including Ru(II), Re(I), Ir(III) and Pt(II) has the potential advantages of probes for monitoring and inhibiting the fibrillation as well as the toxicity of Aβ over conventional dyes such as Thioflavin T (ThT). The geometry, multiple electronic/spin states and redox nature of metal centres have made them tunable properties. Upon binding to the Aβ peptide aggregates, they exhibit promising potential as anti-AD agents due to their fascinating photophysical properties include red emissions, large Stokes shifts, and long lifetimes, which differentiate the competitive binding of other short-lived fluorescent molecules via photoluminescence, and time-resolved measurements. In addition, metal complexes display their remarkable selectivity and superiority over ThT. Competition study between photoactive metal complexes and ThT on fibrillation process show their effective binding of metal complex with Aβ₄₂ fibrils by hindering the ThT binding to give higher binding constants than that of ThT. Computational studies predicted a hydrophobic domain between amino acid binding sites and the functional group of photoactive metal complexes via different noncovalent interactions. Thus, attractive characteristics of photoactive metal complexes could influence remarkable evolutions in new dimensions, which in turn address current challenges in the clinical use of the detection and inhibition of Aβ fibrils.
Surface patterns of insulin fibrils revealed by time-resolved spectroscopy measurements of fluorescent probes
2018, Journal of Luminescence
Citation Excerpt :
The emitting species can be detected and distinguished using wavelength tunable time-resolved fluorescence spectroscopy [20] and their energy distribution revealed by transient absorption [21]. Among many small molecules that bind electrostatically to fibrils [22–26], ThT is the most common fluorophore which has few-fold increase of quantum yield in presence of amyloid fibrils [27]. But to resolve different modes of probe binding, it requires time-resolved setup operating in ultrafast domain that can measure the ultrafast relaxation from the twisted internal charge transfer (TICT) state [20].
Amyloid fibrils are a hallmark of neurodegeneration. The structural diversity of amyloids necessitates sensitive methods and probes that can be reliably used to characterize them. Here, we study insulin fibrils and its polymorphs seeded with LVEALYL peptide in context of probing the surface patterns using Thioflavin T (ThT) and polythiophene derivative – Poly[2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS) polymer. We investigated the dynamics and lifetimes of these two probes using time-resolved absorption and fluorescence spectroscopy. The photoluminescence emission lifetimes of the probes showed different relaxation times in the presence of structurally different amyloid fibrils. However, only PTEBS revealed sensitivity to the surface patterns of the fibrils that was explained by uneven charge distribution and exposure of different amino acids to the fibril surface where probe is interacting with the fibril. We find that PTEBS binding to distinct amyloid surfaces causes perturbation of the main chain of the polymer creating new conditions for energy distribution in the excited states that favors formation of various emitting species. The results indicate that PTEBS and ThT are comparable in terms of recognition of fibril polymorphs whereas polymer probe provides additional dimension to study fibrils surface patterns that can be revealed using time-resolved spectroscopy.
Probing Amyloid Nanostructures Using Photoluminescent Metal Complexes
2021, European Journal of Inorganic Chemistry
Interrogating Amyloid Aggregates using Fluorescent Probes
2019, Chemical Reviews

View all citing articles on Scopus

View full text

Binuclear ruthenium(II) complexes for amyloid fibrils recognition

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

Conflict of interest

Acknowledgments

Biochem. Biophys. Res. Commun.

Chem. Phys.

Nat. Photonics

J. Neurochem.

Nat. Rev. Neurol.

J. Neurol.

J. Phys. Chem. B

J. Phys. Chem. B

Talanta