A New Fluorescent Squaraine Probe for the Measurement of Membrane Polarity

Electronic Nature of Neutral and Charged Two-Photon Absorbing Squaraines for Fluorescence Bioimaging Application

The electronic properties of neutral 2,4-bis(4-bis(2-hydroxyethyl) amino-2-hydroxy-6-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)squaraine (1) and charged 2-((3-octadecylbenzothiazol-2(3H)-ylidene)methyl)-3-oxo-4-((3-(4-(pyridinium-1-yl)butyl)benzo-thiazol-3-ium-2-yl)methylene)cyclobut-1-enolate iodide (2) squaraine derivatives were analyzed based on comprehensive linear photophysical, photochemical, nonlinear optical studies (including two-photon absorption (2PA) and femtosecond transient absorption spectroscopy measurements), and quantum chemical calculations. The steady-state absorption, fluorescence, and excitation anisotropy spectra of these new squaraines revealed the values and mutual orientations of the main transition dipoles of 1 and 2 in solvents of different polarity, while their role in specific nonlinear optical properties was shown. The degenerate 2PA spectra of 1 and 2 exhibited similar shapes, with maximum cross sections of ∼300-400 GM, which were determined by the open aperture Z-scan method over a broad spectral range. The nature of the time-resolved excited-state absorption spectra of 1 and 2 was analyzed using a femtosecond transient absorption pump-probe technique and the characteristic relaxation times of 4-5 ps were revealed. Quantum chemical analyses of the electronic properties of 1 and 2 were performed using the ZINDO/S//DFT theory level, affording good agreement with experimental data. To demonstrate the potential of squaraines 1 and 2 as fluorescent probes for bioimaging, laser scanning fluorescence microscopy images of HeLa cells incubated with new squaraines were obtained.

Fluorescence monitoring of the effect of oxidized lipids on the process of protein fibrillization

The kinetics of lysozyme and insulin amyloid formation in the presence of the oxidized phospholipids (oxPLs) was investigated using Thioflavin T fluorescence assay. The kinetic parameters of fibrillization process (lag time and apparent rate constant) have been determined upon varying the following experimental parameters: the type of lipid assemblies (premicellar aggregates and lipid bilayer vesicles), pH, temperature and lipid-to-protein molar ratio. It was found that oxPLs premicellar aggregates induced the more pronounced increase of the maximum Thioflavin T fluorescence, which is proportional to the extent of fibril formation, compared to the vesicles composed of the oxidized and unoxidized lipids. In contrast, the oxPLs, used as dispersions or included into vesicles, inhibited fibril nucleation and elongation under near-physiological conditions in vitro compared to liposomes containing unoxidized lipids. The results obtained provide deeper insight into the molecular mechanisms of the oxidative stress-modulated conformational diseases, and could be employed for the anti-amyloid drug development.

Liposomes modified with cardiolipin can act as a platform to regulate the potential flux of NADP+-dependent isocitrate dehydrogenase

Cardiolipin (CL) is a phospholipid found in the outer mitochondrial membrane (OMM) and inner mitochondrial membrane (IMM) in animal cells. Isocitrate dehydrogenase (ICDH) is an important catalytic enzyme that is localized at the cytosol and mitochondria; the metabolic pathway catalyzed by ICDH differs between the OMM and IMM. To estimate the possible role of lipid membrane in the enzymatic activity of NADP⁺-dependent ICDH, CL-modified liposomes were prepared using CL/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol (Ch), and their characteristics were analyzed based on the fluorescent probe method. The relative enzymatic activity of ICDH decreased in the presence of CL/DPPC/Ch=(30/50/20) liposome, whereas activity increased in the presence of CL/DPPC/Ch=(5/75/20) liposome. NADP⁺ had the greatest substrate affinity and was dominant in the regulation of ICDH activity. Analysis of membrane properties indicated that membranes in CL-modified liposomes were dehydrated by ICDH binding. Using circular dichroism analysis, CL/DPPC/Ch=(30/50/20) liposome induced a conformational change in ICDH, indicating that CL-rich membrane domains could inhibit ICDH activity. These results suggest that lipid membranes, including CL molecules, could act as a platform to regulate ICDH-related metabolic pathways such as the tricarboxylic acid cycle and lipid synthesis.

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Phosphatidylcholine liposomes were modified with cardiolipin and characterized.

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DPPC liposomes did not affect the activity of ICDH.

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ICDH activity was enhanced with liposomes at 5 mol% cardiolipin.

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ICDH activity was lowered with liposomes at 30 mol% cardiolipin.

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Liposomes with high content of cardiolipin led to conformational changes of ICDH.

Controlled aggregation and enhanced two-photon absorption of a water-soluble squaraine dye with a poly(acrylic acid) template

Controlling the aggregation behavior of organic dyes is important for understanding and exploring supramolecular assembly utilizing the specific characteristics of aggregation. Regulating J-aggregation by electrostatic interactions between anionic polyelectrolytes and cationic dyes has gained growing interest. Here, we report the formation of J-aggregates of a water-soluble cationic squaraine dye, 4-(pyridinium-1-yl)butylbenzothiazolium squaraine (SQ), using poly(acrylic acid) sodium salt (PAA-Na) as a template. Electrostatic interactions between the PAA-Na polyelectrolyte and the cationic SQ dye enhanced J-aggregation; the absorbance of the resulting J-band with the polyelectrolyte template was much sharper than the absorbance of the J-aggregate formed using a high concentration of NaCl. Significantly, removal of the polyelectrolyte PPA-Na template by the introduction of calcium ions, which can form stronger ionic binding with carboxylate groups, dissociated J-aggregates, freeing the SQ molecules back to unaggregated or lower aggregate forms. To demonstrate the reversibility of the J-aggregate formation cycle, an in situ experiment was conducted that showed 60% reversibility of the second cycle. In addition, an enhancement by ca. 23 times per repeat unit of the two-photon absorption (2PA) cross section was observed at 920 nm for the polyelectrolyte template-SQ J-aggregate compared to unaggregated or lower aggregate SQ. These results suggest a prominent role of polyelectrolyte templated SQ J-aggregation in the enhancement of 2PA efficiency and provide a means of modulating supramolecular assembly.

A class of fluorescent heterocyclic dyes revisited: photophysics, structure, and solvent effects

The photophysical behavior of a series of 2-methylthio-5-(Z-carbonyl)thieno-[3,4-e]-3,4-dihydro-1,2,3-triazine-4-ones was investigated by absorption and emission spectroscopy in a range of solvents representing a systematic variation in polarity, polarizability, as well as hydrogen bond donating and accepting capabilities. In all the investigated compounds, the heterocyclic 6-membered ring of the chromophore is involved in interactions between electron donor and acceptor substituents on the thiophene ring. Throughout the series, the charge-donating methylthio group is kept constant, while the acceptor (Z-carbonyl) is varied between amide, ester, and ketone functionalities. The fluorescent first electronically excited state is primarily of intramolecular HOMO-LUMO charge transfer character. All members of the series exhibit solvent-dependent photophysics, although the magnitude of the dependence varies with the nature of the acceptor group. In addition to the solvent-sensitive photophysics, the investigated class of compounds shows high thermal and chemical stability. Among this class of heterocyclic dyes, the amide-substituted compound is superior with respect to high quantum yield and lifetime, and also shows the largest change in emission lifetimes and fluorescence quantum yields upon solvent variation (about 5-fold).

Examining Protein-Lipid Interactions in Model Systems with a New Squarylium Fluorescent Dye

The applicability of newly synthesized squarylium dye Sq to probing the changes in physical characteristics of lipid bilayer on the formation of protein-lipid complexes has been evaluated. Lipid vesicles composed of zwitterionic phospholipid phosphatidylcholine (PC) and its mixtures with positively charged detergent cetyltrimethylammonium bromide (CTAB), anionic phospholipid cardiolipin (CL), and cholesterol (Chol) were employed as lipid component of model membrane systems while protein constituent was represented by lysozyme (Lz). Fluorescence intensity of Sq was found to decrease on Lz association with lipid bilayer. This effect was observed in all kinds of model systems suggesting that Sq is sensitive to modification of lipid bilayer physical properties on hydrophobic protein-lipid interactions. It was found that Sq spectral response to variations in Chol content depends on relative contributions of electrostatic and hydrophobic components of Lz-membrane binding.