Two-Photon Fluorescent Probes for Biomembrane Imaging: Effect of Chain Length

Intracellular Physical Properties with Small Organic Fluorescent Probes: Recent Advances and Future Perspectives

The intracellular physical parameters i. e., polarity, viscosity, fluidity, tension, potential, and temperature of a live cell are the hallmark of cellular health and have garnered immense interest over the past decade. In this context, small molecule organic fluorophores exhibit prominent useful properties including easy functionalizability, environmental sensitivity, biocompatibility, and fast yet efficient cellular uptakability which has made them a popular tool to understand intra-cellular micro-environmental properties. Throughout this discussion, we have outlined the basic design strategies of small molecules for specific organelle targeting and quantification of physical properties. The values of these parameters are indicative of cellular homeostasis and subtle alteration may be considered as the onset of disease. We believe this comprehensive review will facilitate the development of potential future probes for superior insight into the physical parameters that are yet to be quantified.

Computational Characterization of Novel Malononitrile Variants of Laurdan with Improved Photophysical Properties for Sensing in Membranes

Fluorescent probes are powerful tools for improving our understanding of cellular membranes and other complex biological environments. Using simulations, we gain atomistic and electronic insights into the effectiveness of the probes. In the current work, we have used various computational approaches to comprehensively investigate the properties of the fluorescent probe Laurdan and two Laurdan-like probes: AADAL and ECL. In addition, we propose the development of their corresponding novel malononitrile variants, which are computationally characterized herein. For the candidate probes, electronic structure calculations were used to rationalize their optical properties, including their ability for two-photon activation, and molecular dynamics simulations were used to unravel atomistic details of their functioning within lipid bilayers. While Laurdan, AADAL, and ECL were found to have very similar optical and membrane partitioning profiles, their malononitrile variants were found to show significantly improved optical properties, especially in regard to two-photon cross sections, and they appear to retain the desired membrane characteristics of the parent Laurdan molecule.

Mechanical properties of plasma membrane vesicles correlate with lipid order, viscosity and cell density

Regulation of plasma membrane curvature and composition governs essential cellular processes. The material property of bending rigidity describes the energetic cost of membrane deformations and depends on the plasma membrane molecular composition. Because of compositional fluctuations and active processes, it is challenging to measure it in intact cells. Here, we study the plasma membrane using giant plasma membrane vesicles (GPMVs), which largely preserve the plasma membrane lipidome and proteome. We show that the bending rigidity of plasma membranes under varied conditions is correlated to readout from environment-sensitive dyes, which are indicative of membrane order and microviscosity. This correlation holds across different cell lines, upon cholesterol depletion or enrichment of the plasma membrane, and variations in cell density. Thus, polarity- and viscosity-sensitive probes represent a promising indicator of membrane mechanical properties. Additionally, our results allow for identifying synthetic membranes with a few well defined lipids as optimal plasma membrane mimetics.

A near-infrared and two-photon dual-mode fluorescent probe for the colorimetric monitoring of SO2in vitro and in vivo

A near-infrared and two-photon dual-mode fluorescent CY probe was developed for the colorimetric monitoring of SO₂in vitro and in vivo.

Red-emitting DPSB-based conjugated polymer nanoparticles with high two-photon brightness for cell membrane imaging

New red-emitting conjugated polymers have been successfully synthesized by incorporating classical two-photon absorption (TPA) units, electron-rich units, and a small amount of electron-deficient units along the polymer backbones. Water-dispersible nanoparticles (NPs) based on these polymers were also fabricated for applications in two-photon excitation fluorescence imaging of cell membrane. Through optimization of the polymer/matrix mass ratio and the initial feed concentration of the polymer solution, a high quantum yield (QY) of 24% was achieved for the red-emitting NPs in water. TPA cross section and two-photon action cross section values of these polymers at 750 nm reached up to 1000 GM and 190 GM per repeat unit in aqueous media, 2.5 × 10(5) GM and 4.7 × 10(4) GM per NP, respectively. Furthermore, these NPs displayed excellent photostability and biocompatibility. Their applications as two-photon excitation fluorescence probes for cell membrane imaging have been demonstrated in three different cell lines with excellent imaging contrast. These results demonstrated that these polymer NPs hold great potentials as excellent two-photon excitation fluorescence probes in various biological applications.

Two-photon "caging" groups: effect of position isomery on the photorelease properties of aminoquinoline-derived photolabile protecting groups

High two-photon photolysis cross sections and water solubility of probes are important to avoid toxicity in biomedical applications of photolysis. Systematic variation of the position of a carboxyl electron-withdrawing group (EWG) on photolysis of 8-dimethylaminoquinoline protecting groups identified the C5-substituted isomer as a privileged dipole. The 5-benzoyl-8-DMAQ substitution yields a caging group with an enhanced two-photon uncaging cross section (δu = 2.0 GM) and good water solubility (c ≤ 50 mM, pH 7.4).

Self-organizing behaviour of glycosteroidal bolaphiles: insights into lipidic microsegregation

In this article we describe work on the synthesis of bolaphile biomimics composed of glucose head groups and steroidal units linked together by a methylene chain of varying length. The condensed phases formed by self-organization of the products as a function of temperature were characterized by differential scanning calorimetry and thermal polarized light microscopy. The results of these studies show that the thermal stabilities of the lamellar mesophases formed vary linearly as a function of increasing aliphatic composition, which reflects a linear hydrophobic-hydrophilic balance with respect to transition temperatures.

Fluorogenic, two-photon-triggered photoclick chemistry in live mammalian cells

The tetrazole-based photoclick chemistry has provided a powerful tool to image proteins in live cells. To extend photoclick chemistry to living organisms with improved spatiotemporal control, here we report the design of naphthalene-based tetrazoles that can be efficiently activated by two-photon excitation with a 700 nm femtosecond pulsed laser. A water-soluble, cell-permeable naphthalene-based tetrazole was identified that reacts with acrylamide with the effective two-photon cross-section for the cycloaddition reaction determined to be 3.8 GM. Furthermore, the use of this naphthalene-tetrazole for real-time, spatially controlled imaging of microtubules in live mammalian cells via the fluorogenic, two-photon-triggered photoclick chemistry was demonstrated.

Two-photon probes for biomedical applications

Two-photon microscopy (TPM), which uses two photons of lower energy as the excitation source, is a vital tool in biology and clinical science, due to its capacity to image deep inside intact tissues for a long period of time. To make TPM a more versatile tool in biomedical research, we have developed a variety of two-photon probes for specific applications. In this mini review, we will briefly discuss two-photon probes for lipid rafts, lysosomes, mitochondria, and pH, and their biomedical applications. [BMB Reports 2013; 46(4): 188-194]

A switchable two-photon membrane tracer capable of imaging membrane-associated protein tyrosine phosphatase activities

When I look into your cells: a two-photon dye (Flu7) was developed, which strongly fluoresces only upon selective binding to the plasma membrane of mammalian cells. Upon addition of a UV- and phosphatase-controlled quencher (Q12), the system exhibits ON/OFF/ON fluorescence switching and can be used to image membrane-associated receptor-like protein tyrosine phosphatase (RPTP) activity.

Two-photon fluorescent probes for metal ions

Two-photon microscopy (TPM) has become an indispensible tool in biology and medicine owing to the capability of imaging the intact tissue for a long period of time. To make it a versatile tool in biology, a variety of two-photon probes for specific applications are needed. In this context, many research groups are developing two-photon probes for various applications. In this Focus Review, we summarize recent results on model studies and selected examples of two-photon probes that can detect intracellular free metal ions in live cells and tissues to provide a guideline for the design of useful two-photon probes for various in vivo imaging applications.

Two-photon probes for intracellular free metal ions, acidic vesicles, and lipid rafts in live tissues

Optical imaging with fluorescence microscopy is a vital tool in the study of living systems. The most common method for cell imaging, one-photon microscopy (OPM), uses a single photon of higher energy to excite the fluorophore. However, two-photon microscopy (TPM), which uses two photons of lower energy as the excitation source, is growing in popularity among biologists because of several distinct advantages. Using TPM, researchers can image intact tissue for a long period of time with minimum interference from tissue preparation artifacts, self-absorption, autofluorescence, photobleaching, and photodamage. However, to make TPM a more versatile tool in biology, researchers need a wider variety of two-photon probes for specific applications. In this Account, we describe a series of two-photon probes that we developed that can visualize the distribution of intracellular metal ions, acidic vesicles, and lipid rafts in living cells and tissues. The development of these probes requires a significant two-photon cross section for the bright image and receptors (sensing moieties) that triggers the emission of the two-photon excited fluorescence upon binding with the ions or membrane in the living system. These probes also must be sensitive to the polarity of the environment to allow selective detection of cytosolic and membrane-bound probes. In addition, they need to be cell-permeable, water-soluble for the staining of cells and tissues, and highly photostable for long-term imaging. The resulting probes-AMg1 (Mg(2+)), ACa1-ACa3 (Ca(2+)), AZn1 and AZn2 (Zn(2+)), AH1, AH2, and AL1 (acidic vesicles), and CL2 (membrane)-use 2-acetyl-6-aminonaphthalene as the fluorophore and receptors for the target ions or membrane. All of these two-photon turn-on probes can detect the intracellular free metal ions, acidic vesicles, and lipid rafts at 100-300 microm depth in live tissues. Moreover, with ACa1-AM, we could simultaneously visualize the spontaneous Ca(2+) waves in the somas of neurons and astrocytes at approximately 120 microm depth in fresh hypothalamic slices for more than 1000 s without appreciable decay. Furthermore, AL1 could visualize the transport of the acidic vesicles between cell body and axon terminal along the axon in fresh rat hippocampal slices at approximately 120 microm depth.