Turning On Fluorescence with Thiols - Synthetic and Computational Studies on Diaminoterephthalates and Monitoring the Switch of the Ca2+Sensor Recoverin

Recent advances in single-benzene-based fluorophores: physicochemical properties and applications

Fluorescence-based materials and associated techniques (analytical, imaging, and sensing techniques) have been highlighted over the last century throughout various basic research fields and industries. Organic molecule-based fluorophores, in particular, have ushered in a new era in biology and materials science. To date, hundreds of organic fluorophores have been developed, and many studies have introduced new rationales for the fluorophore design and the analysis of the relationship between its structure and photophysical properties both in the solution- and solid-state. In this review, we summarize the recent advances (mainly from 2015 to 2020) in single-benzene-based fluorophores (SBBFs), which have an electron-donor (D)-acceptor (A) type dipolar structure within a compact benzene backbone. We also present a systematic outline of the physicochemical properties of SBBFs and representative examples of their applications, which will provide useful context for the development of new SBBF derivatives in fluorophore-related materials science fields.

Electrochemical Activation of Self-Assembled Monolayers for the Binding of Effectors

A self-assembled monolayer (SAM) on gold was prepared from a diaminoterephthalate (DAT) derivative as functional molecule and 1-decanthiol as a backfiller. The DAT derivative is N-protected by a tert-butyloxycarbonyl (Boc) group and is anchored to the gold surface via a liponic acid as a stable anchor group. The terminal DAT moiety exhibits interesting effector properties such as fluorescence and electrochemical activity. Irreversible oxidation of the monolayer at 0.4 V (Hg|Hg₂SO₄) in 0.1 M HClO₄ triggers deprotection of the DAT group and subsequent chemical reactions, during which 10% of the DAT groups of the original SAM are transformed to a new surface-bound, quasi-reversible redox couple with a formal potential of 0.0 V (Hg|Hg₂SO₄) and a standard rate constant of 8 s^-1 in 0.1 M HClO₄. Immersion of the mixed SAM in 0.1 M HClO₄ at open circuit potential or oxidation in 0.1 M H₂SO₄ did not produce this surface-bound redox couple. The monolayers were thoroughly characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) after the different preparation steps indicating only minor changes in the overall composition of the monolayer, in particular, the preservation of the heteroatoms. The new redox couple is likely a diimine, in agreement with its ability to bind nucleophiles such as anilines by conjugate addition that could be followed by multicycle voltammetry and XPS. The DAT effector group is especially interesting because it can also report the binding reaction by changed electrochemical and fluorescence signals.

Electron Transfer and Electron Excitation Processes in 2,5-Diaminoterephthalate Derivatives with Broad Scope for Functionalization

Derivatives of 2,5-diaminoterephthalate (DAT) are efficient fluorescence dyes that are also redox-active, thus allowing for the electrochemical manipulation of spectral properties. The electrochemical behaviour of seven DAT derivatives was studied by cyclic voltammetry in dichloromethane. In the absence of a proton donor, DATs should be oxidized in two one-electron steps. The first step is usually quasi-reversible while the second step is either quasi-reversible or irreversible. Some electrochemical properties such as the formal potentials and the ratio between the anodic and the cathodic current were determined from the cyclic voltammograms. Correlation between the formal potential of first oxidation and the absorption or the fluorescence emission wavelengths are established for this specific type of dyes. These correlations were confirmed with density functional theory calculations.

Diaminoterephthalate-α-lipoic acid conjugates with fluorinated residues

Two bifunctional diaminoterephthalate (DAT) fluorescence dyes were prepared in a three-step sequence including one deprotection reaction. One functional unit is α-lipoic acid (ALA) for binding the dye to gold surfaces. It was introduced to the DAT scaffold by an amidation reaction. The other functional unit is a para-(trifluoromethyl)benzyl group for facile detection of the surface-bound material by X-ray photoelectron spectroscopy (XPS). This residue was introduced by reductive amination of the DAT scaffold with the respective benzaldehyde derivative. In one compound (60% yield over three steps) the ALA unit is directly bound to the DAT as a relatively electron-withdrawing amide. In solution (CH2Cl2), this material shows strong fluorescence (quantum yield 57% with emission at 495 nm, absorption maximum at 420 nm). The other compound (57% yield over three steps) possesses a propylene spacer between the ALA and the DAT units for electronic decoupling, thus, bathochromic shifts are observed (absorption at 514 nm, emission at 566 nm). The quantum yield is, however, lower (4%). Self-assembled monolayers on a gold surface of both compounds were prepared and characterized by high-resolution XPS of the C 1s, O 1s, S 2p, N 1s and F 1s emissions. The high signal-to-noise ratios of the F 1s peaks indicated that trifluoromethylation is an excellent tool for the detection of surface-bound materials by XPS.

Impact of the protein myristoylation on the structure of a model cell membrane in a protein bound state

The neuronal calcium sensor protein recoverin is expressed in retinal rod and cone cells and is involved in the calcium-dependent control of receptor (rhodopsin) phosphorylation and receptor inactivation. In its Ca²⁺-saturated form recoverin is attached to membranes by an exposed myristoyl group and responds to oscillating changes of intracellular Ca²⁺-concentration by performing a so-called Ca²⁺-myristoyl switch. In this work we analyze changes in a liquid lipid bilayer interacting with myristoylated and non-myristoylated recoverin by employing polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) with electrochemical control. The lipid bilayer is transferred onto a polycrystalline gold electrode using Langmuir-Blodgett Langmuir-Schaefer transfer at the surface pressure π = 30 mN m^-1 which ensures, necessary for the lipid-protein interaction, liquid state of the hydrocarbon chains of phospholipids. The model lipid bilayers are adsorbed directly on the Au electrode surface at transmembrane potentials -0.2 < ∆ϕM|S < 0.25 V. The interaction with recoverin leads to a stabilization of the adsorbed state of the lipid bilayer at positive transmembrane potentials. The interaction leads to a decrease in the surface charge density that accumulates on the membrane covered electrode surface, indicating changes in the lateral interactions in the lipid membrane. In situ spectroelectrochemical studies confirm orientation changes in the hydrophobic environment of the model membrane. Insertion of the myristoyl group of recoverin into the membrane is connected with an increase in the tilt of the hydrocarbon chains with respect to the surface normal and decrease in the bilayer thickness. Potential-induced pore formation and desorption of the lipid bilayer from the membrane surface is accompanied by the removal of the acyl chains of recoverin from the membrane.

Synthesis and Evaluation of Two Novel All -trans-Retinoic Acid Conjugates: Biocompatible and Functional Tools for Retina Research

The vitamin A derivative all- trans-retinoic acid (ATRA) is an important biologically active metabolite that regulates a variety of essential biological processes in particular via gene-regulatory mechanisms. In the retina, ATRA is a light-dependent byproduct of the phototransduction cascade. Here, ATRA is not only needed for proper retinal development, but it also acts as a neuromodulator on horizontal cells, second-order inhibitory neurons in the outer retina, which reveal morphological and physiological changes when the retina is treated with ATRA. There is evidence that gene-regulatory mechanisms may only be partially involved in these neuromodulatory processes and the underlying nontranscriptional mechanisms are still elusive. This is, among other things, due to the lack of appropriately labeled ATRA, which would allow the tracking of ATRA in cells or a given tissue. To overcome this obstacle, we designed, synthesized, and evaluated two conjugates of ATRA, one conjugated with biotin (biotin-ATRA) and one conjugated with diaminoterephthalate fluorophore (DAT-ATRA), as molecular tools for different fields of application. The biocompatibility of both compounds was demonstrated via cell viability assays in cultured N2a-cells. N2a-cells exposed to the compounds showed no significant changes in the viability rate. The functionality of synthesized ATRA-conjugates was verified using retinal tissue derived from adult carp. The binding of ATRA-conjugates to distinct retinal cells was assessed in primary cultures of carp retina. Hereby, horizontal and Müller cells have been identified as specific target cells of the new ATRA compounds. Electron microscopy further confirmed that the new substances are still able to induce synaptic plasticity at horizontal cell dendrites resulting in formation of spine synapses, as it is shown for native ATRA. Taken together, the novel ATRA-conjugates represent biocompatible and functional molecular tools, which may further provide the possibility to track ATRA in neuronal cells and study its modulatory effects in different cell systems.

Orthogonally Protected Diaminoterephthalate Scaffolds: Installation of Two Functional Units at the Chromophore

The 2,5-diaminoterephthalate structural motif is a powerful chromophore with remarkable fluorescence properties. Containing two carboxylate and two amino functions, it defines a colored molecular scaffold which allows for orthogonal functionalization with different functional units. Therefore, different applications in life sciences and materials science could be addressed. In this study, the two amino functions were alkylated by reductive amination with side chains carrying amino (orthogonally protected as Boc or Alloc) and carboxylate functions (orthogonally protected as tBu or allyl ester). After sequential deprotections, functional units were introduced by amidation reactions. As three examples, the chromophore was coupled with retinoic acid and fullerene C₆₀ in order to obtain a triad for studying photoinduced electron transfer processes. Furthermore, cyclooctyne and azide moieties were introduced as functional units, allowing for ligation by click reactions. These two clickable groups were applied in combination with maleimide units which are reactive toward thiol residues. The latter dyes define so-called "turn on" probes, since the fluorescence quantum yields increased by one order of magnitude upon reaction with the molecular target.

Aminomaleimide fluorophores: a simple functional group with bright, solvent dependent emission

Amino-substituted maleimides form a new class of highly emissive compounds, with large Stokes shifts (>100 nm) and high quantum yields (up to ∼60%).

Amino-substituted maleimides form a new class of highly emissive compounds, with large Stokes shifts (>100 nm) and high quantum yields (up to ∼60%). Emission is responsive to the maleimide's environment with both a red-shift, and quenching, observed in protic polar solvents. Aminomaleimides are easily functionalised, providing a versatile fluorescent probe.

Computational and experimental characterization of a fluorescent dye for detection of potassium ion concentration

The fluorescence of the SKC-513 ((E)-N-(9-(4-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)phenyl)-6-(butyl(3-sulfopropyl)amino)-3H-xanthen-3-ylidene)-N-(3-sulfopropyl)butan-1-aminium) dye is shown experimentally to have high sensitivity to binding of the K⁺ ion. Computations are used to explore the potential origins of this sensitivity and to make some suggestions regarding structural improvements. In the absence of K⁺, excitation is to two nearly degenerate states, a neutral (N) excited state with a high oscillator strength, and a charge-transfer (CT) state with a lower oscillator strength. Binding of K⁺ destabilizes the CT state, raising its energy far above the N state. The increase in fluorescence quantum yield upon binding of K⁺ is attributed to the increased energy of the CT state suppressing a nonradiative pathway mediated by the CT state. The near degeneracy of the N and CT excited states can be understood by considering SKC-513 as a reduced symmetry version of a parent molecule with 3-fold symmetry. Computations show that acceptor–donor substituents can be used to alter the relative energies of the N and CT state, whereas a methylene spacer between the heterocycle and phenylene groups can be used to increase the coupling between these states. These modifications provide synthetic handles with which to optimize the dye for K⁺ detection.