Marianna Dakanali

Dyes with Segmental Mobility: Molecular Rotors

Molecular rotors are fluorescent molecules that are characterized by the ability to form twisted states through the rotation of one segment of the structure with respect to the rest of the molecule. Intramolecular rotation changes the ground- state and excited-state energies, and molecular rotors deexcite from the twisted state either without photon emission or with a different wavelength than from the LE state. Intramolecular rotation is strongly dependent on the solvent. Solvent polarity, hydrogen bond formation, isomerization, excimer formation, and steric hindrance are predominant forms of solvent-fluorophore interaction. Of highest importance is sterichindrance,becauseitlinksthesolventsmicroviscositytotheformationrateof TICT states, which, in turn, determines the spectral emission. For this reason, molecular rotors have found a wide range of applications as fluorescent sensors of microviscosity and solvent free volume. Application examples include bulk viscos- ity measurement, probing dynamics of polymer formation, protein sensing and probing of protein aggregation, and microviscosity probing in living cells.

Rational Design of Amyloid Binding Agents Based on the Molecular Rotor Motif

Alzheimer’s disease (AD) is characterized by a progressive loss of cognitive function and constitutes the most common and fatal neurodegenerative disorder.[1] Genetic and clinical evidence supports the hypothesis that accumulation of amyloid deposits in the brain plays an important role in the pathology of the disease. This event is associated with perturbations of biological functions in the surrounding tissue leading to neuronal cell death, thus contributing to the disease process. The deposits are comprised primarily of amyloid (Aβ) peptides, a 39–43 amino acid sequence that self aggregates into a fibrillar β-pleated sheet motif. While the exact three-dimensional structure of the aggregated Aβ peptides is not known, a model structure that sustains the property of aggregation has been proposed.[2] This creates opportunities for in vivo imaging of amyloid deposits that can not only help evaluate the time course and evolution of the disease, but can also allow the timely monitoring of therapeutic treatments.[3]

Aminonaphthalene 2-Cyanoacrylate (ANCA) Probes Fluorescently Discriminate between Amyloid-β and Prion Plaques in Brain

A major challenge for diagnosing and monitoring the progression of amyloid-based diseases is the capability to distinguish between amyloid deposits that are associated with related, but distinctly different, diseases. Here, we demonstrate that aminonaphthalenyl 2-cyanoacrylate-based probes can fluorescently discriminate between different types of amyloid deposits in brain. The discriminating capability of these molecular rotors is due to the stabilization of the ground versus excited states of these probes as a function of the polarity of their microenvironment (i.e., within the binding pocket on the amyloid). This property makes it possible, for the first time, to estimate the inherent static relative permittivity (ε(0)) of the binding pocket of each amyloid within tissue. The capability to selectively follow the deposition of specific amyloids in tissue may provide important information for therapeutic development that is not readily accessible from currently available technology.

A Short Biomimetic Approach to the Fully Functionalized Bicyclic Framework of Type A Acylphloroglucinols

A biomimetic approach toward type A polyprenylated acylphloroglucinols (PPAPs) is described. The method is based on a C-alkylation-cation cyclization reaction sequence, leading to a convenient buildup of molecular complexity, employing the simple and readily available deoxycohumulone and an appropriately functionalized hydroxy halide. Thus, a versatile construction of the fully functionalized bicyclic framework of type A PPAPs (5) was achieved.

Intrinsic and extrinsic temperature-dependency of viscosity-sensitive fluorescent molecular rotors.

Molecular rotors are a group of environment-sensitive fluorescent probes whose quantum yield depends on the ability to form twisted intramolecular charge-transfer (TICT) states. TICT formation is dominantly governed by the solvent’s microviscosity, but polarity and the ability of the solvent to form hydrogen bonds play an additional role. The relationship between quantum yield ϕF and viscosity η is widely accepted as a power-law,

Self-calibrating viscosity probes: Design and subcellular localization

\phi_F = C \cdot \eta ^x

Synthesis and evaluation of self-calibrating ratiometric viscosity sensors

. In this study, we isolated the direct influence of the temperature on the TICT formation rate by examining several molecular rotors in protic and aprotic solvents over a range of temperatures. Each solvent’s viscosity was determined as a function of temperature and used in the above power-law to determine how the proportionality constant C varies with temperature. We found that the power-law relationship fully explains the variations of the measured steady-state intensity by temperature-induced variations of the solvent viscosity, and C can be assumed to be temperature-independent. The exponent x, however, was found to be significantly higher in aprotic solvents than in protic solvents. We conclude that the ability of the solvent to form hydrogen bonds has a major influence on the relationship between viscosity and quantum yield. To use molecular rotors for the quantitative determination of viscosity or microviscosity, the exponent x needs to be determined for each dye-solvent combination.

Enantioselective synthesis of the ABC ring motif of norzoanthamine based on asymmetric Robinson annulation reactions.

We describe the design, synthesis and fluorescence profiles of new self-calibrating viscosity dyes in which a coumarin (reference fluorophore) has been covalently linked with a molecular rotor (viscosity sensor). Characterization of their fluorescence properties was made with separate excitation of the units and through resonance energy transfer from the reference to the sensor dye. We have modified the linker and the substitution of the rotor in order to change the hydrophilicity of these probes thereby altering their subcellular localization. For instance, hydrophilic dye 12 shows a homogeneous distribution inside the cell and represents a suitable probe for viscosity measurements in the cytoplasm.

Subcellular Localization and Activity of Gambogic Acid

We describe the design, synthesis and fluorescent profile of a family of self-calibrating dyes that provide ratiometric measurements of fluid viscosity. The design is based on covalently linking a primary fluorophore (reference) that displays a viscosity-independent fluorescence emission with a secondary fluorophore (sensor) that exhibits a viscosity-sensitive fluorescence emission. Characterization of fluorescent properties was made with separate excitation of the units and through Resonance Energy Transfer from the reference to the sensor dye. The chemical structures of both fluorophores and the linker length have been evaluated in order to optimize the overall brightness and sensitivity of the viscosity measurements. We also present an application of such ratiometric dyes for the detection of membrane viscosity changes in a liposome model.

A-ring oxygenation modulates the chemistry and bioactivity of caged Garcinia xanthones

An enantioselective strategy for the synthesis of tetracyclic motif 5, representing the northern fragment of norzoanthamine, is presented. Key to the strategy is the use of two asymmetric Robinson annulation reactions that produce the tricyclic ABC ring system with excellent stereoselectivity. Further functionalization at the periphery of the C ring produces compound 5 containing six contiguous stereocenters of the natural product.