A Ground-State Complex Between Methyl Viologen and the Fluorescent Whitening Agent 4, 4’–bis (2-sulfostyryl)-biphenyl disodium salt: A Fluorescence Spectroscopy Study

This study explores the quenching of the dianionic fluorescent whitening agent, NFW, by various substances, including methyl viologen (MV), in water and in the presence of Beta-cyclodextrin (β-CD). Results of a fluorescence spectroscopic examination of the β-CD-NFW system are presented. It was found that NFW forms a 1:1 inclusion complex with β-CD with an association constant of 2540 ± 380 M^-1. The included NFW fluorescent state is protected by the β-CD cavity from a range of water-based quenchers (neutral, anionic and cationic). Quenching proceeds near the diffusion-controlled limit in water for the quenchers tested with the exception of the dicationic MV. MV is an extremely efficient quencher of NFW fluorescence with a nominal K_SV ~ 5.0x10³ M^-1 in water alone, corresponding to a nominal k_q of ~ 4x10¹² M^-1s^-1, which exceeds the diffusion-controlled limit in this solvent. The quenching efficiency of MV is strongly suppressed in the presence of 10 mM β-CD (K_SV = 105 ± 12 M^-1) and in the presence of NaCl (K_SV = 106 ± 9 M^-1 at 0.5 M salt). In the absence of CD or salt, there is a strong contribution from static quenching in the MV system; the presence of these additives suppresses the static quenching. Various results suggest the static quenching is due to formation of a ground-state complex between the dianion NFW and the dication MV.

Электронное строение и спектрально-флуоресцентные свойства лазерных красителей тиопирило-4-трикарбоцианинов

It was found that the long-wavelength absorption band of the laser dye IR 1061 and its analogue with an unsubstituted polymethine chain is strongly broadened and decreases in intensity in polar solvents, while the fluorescence band remains narrow and practically does not change in a wide range of solvent polarities. Based on the quantum-chemical calculations of these dyes by the ab initio DFT/B3LYP/6-31G (d,p) and TDDFT methods, taking into account the polarity of the medium by the PCM method, it is shown that the reason for this difference is the weakening of solvation in the fluorescent state as compared to the ground state due to the greater equalization of the charge in the first than in the latter. An increase in the alternation of bond orders in the polymethine chain in the fluorescent state was found, which causes an increase of vibronic interactions in the radiative transition as compared to the absorptive one. Spectral effects caused by a change in the angle of rotation of phenyl groups in the thiopyrylium cycle upon excitation have been analyzed.

Спектрально-люминесцентные свойства и фотолиз заряженных форм бисфенола А

The spectral luminescent properties and photolysis of the ionic forms (neutral, cationic and anionic) of bisphenol A have been studied experimentally and by methods of quantum chemistry. Calculations and experiment have shown that, in comparison with a neutral form, no new absorption bands appear in the absorption spectra of ionic form in the range 200–600 nm. The polar solvent (water) shifts the absorption spectrum bands of ionic forms towards low energies. In this case, the shift of the absorption spectrum of the cation is insignificant, and the shift of the anion is significant with an increase in the intensity of the absorption bands. The low quantum yield of fluorescence of ionic form is explained by the prevalence of the efficiency of singlet-triplet conversion over the efficiency of the radiation channel of decay of the fluorescent state. The low quantum yield of fluorescence of the anionic form is due not only to the effective singlet-triplet conversion, but also to the low efficiency of the radiative decay of the fluorescent state of the anion caused by a change in its orbital nature. Calculations have shown that the potential curves of the excited states of bisphenol A and its ionic forms have a significant potential barrier to photolysis. The increase in the efficiency of the process of photodissociation of the bisphenol A anion is caused by a noticeable decrease in the potential barrier and an increase in the overlap of the absorption spectra of the bisphenol A anion and solar radiation.

NeoR, a near-infrared absorbing rhodopsin

The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.

A Single Point Mutation Converts a Proton-pumping Rhodopsin into a Turn-on Fluorescent Sensor for Chloride

<p>The visualization of chloride in living cells with fluorescent sensors is linked to our ability to design hosts that can overcome the energetic penalty of desolvation to bind chloride in water. Fluorescent proteins can be used as biological supramolecular hosts to address this fundamental challenge. Here, we showcase the power of protein engineering to convert the fluorescent proton-pumping rhodopsin GR from <i>Gloeobacter violaceus</i> into GR1, a turn-on fluorescent sensor for chloride in detergent micelles and in live <i>Escherichia coli</i>. This non-natural function was unlocked by mutating D121, which serves as the counterion to the protonated retinylidene Schiff base chromophore. Substitution from aspartate to valine at this position (D121V) creates a binding site for chloride. The addition of chloride tunes the p<i>K</i>_aof the chromophore towards the protonated, fluorescent state to generate a pH-dependent response. Moreover, ion pumping assays combined with bulk fluorescence and single cell fluorescence microscopy experiments with <i>E. coli</i>, expressing a GR1 fusion with cyan fluorescent protein, show that GR1 does not pump ions nor sense membrane potential but instead provides a reversible, ratiometric readout of chloride. This discovery sets the stage to use natural and laboratory-guided evolution to build a family of rhodopsin fluorescent chloride sensors for cellular applications and learn how proteins can evolve and adapt to bind anions in water.</p>

A Single Point Mutation Converts a Proton-pumping Rhodopsin into a Turn-on Fluorescent Sensor for Chloride

<p>The visualization of chloride in living cells with fluorescent sensors is linked to our ability to design hosts that can overcome the energetic penalty of desolvation to bind chloride in water. Fluorescent proteins can be used as biological supramolecular hosts to address this fundamental challenge. Here, we showcase the power of protein engineering to convert the fluorescent proton-pumping rhodopsin GR from <i>Gloeobacter violaceus</i> into GR1, a turn-on fluorescent sensor for chloride in detergent micelles and in live <i>Escherichia coli</i>. This non-natural function was unlocked by mutating D121, which serves as the counterion to the protonated retinylidene Schiff base chromophore. Substitution from aspartate to valine at this position (D121V) creates a binding site for chloride. The addition of chloride tunes the p<i>K</i>_aof the chromophore towards the protonated, fluorescent state to generate a pH-dependent response. Moreover, ion pumping assays combined with bulk fluorescence and single cell fluorescence microscopy experiments with <i>E. coli</i>, expressing a GR1 fusion with cyan fluorescent protein, show that GR1 does not pump ions nor sense membrane potential but instead provides a reversible, ratiometric readout of chloride. This discovery sets the stage to use natural and laboratory-guided evolution to build a family of rhodopsin fluorescent chloride sensors for cellular applications and learn how proteins can evolve and adapt to bind anions in water.</p>

Effect of Halogen Ions on the Photocycle of Fluorescent Carbon Nanodots

Carbon dots (C-dots) are well-known for their strong sensitivity to the environment, which reflects on intensity and shape changes of their fluorescence, induced by various interacting ions and molecules in solution. Although these interactions have been extensively studied in the last few years, especially in view of their possible sensing applications, the existing works have mostly focused on the quenching of C-dot fluorescence induced by metal cations. In fact, these latter easily bind to C-dots surfaces, which are negatively charged in most cases, promoting an electron transfer from the surface to them. Much less is known from the literature on the effect induced on C-dots by prototypical negative species in solutions, motivating more systematic studies on this different class of interactions. Here, we analyzed the effect of halogen ions on the fluorescence of C-dots, by combining steady-state optical absorption and photoluminescence, time-resolved fluorescence and femtosecond pump/probe spectroscopy. We demonstrate a quenching effect of C-dots fluorescence in the presence of halogen ions, which becomes more and more pronounced with increasing atomic number of the halogens, being negligible for chloride, appreciable for bromide and stronger for iodide. We find that quenching is mostly static, due to the binding of halogen ions on suitable surface sites at C-dots surfaces, while collisional quenching becomes obvious only at very high iodide concentrations. Finally, nanosecond and femtosecond time-resolved spectroscopies provide information on the quenching mechanism and time scales. Based on these data, we propose that the fluorescent state is deactivated by intersystem crossing to a dark triplet state, induced by close-range interactions with the heaviest halogen ions.