Nonspecific solvation and oscillator strengths for the long-wavelength vibronic absorption band of 4-dimethylaminochalcone solutions

The 4-dimethylaminochalcone molecule is one of the most efficient spectral-luminescent probes widely used in various medical and biological studies. Experimental data on the oscillator strengths in the long-wavelength vibronic band of 4-dimethylaminochalcone solutions in a series of individual solvents of different chemical nature are analyzed using the earlier derived refined equation describing the influence of nonspecific solvation processes on the matrix elements of the dipole moment of a purely electronic transition in organic dye molecules. It is shown that the semiempirical theory developed gives a satisfactory quantitative explanation to experimentally observed regularities indicative of a relatively weak influence of the solvent reaction field (orientation and induction forces) and dispersion interactions on the integrated intensity of the studied band belonging in the category of intramolecular charge transfer bands.     

Electronically excited states of membrane fluorescent probe 4-dimethylaminochalcone. Results of quantum chemical calculations

Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.     

Nature of the influence of a solvent on the fluorescence quantum yield of solutions of the fluorescent probe 4-dimethylaminochalcone

The fluorescence quantum yields of solutions of 4-dimethylaminochalcone (DMC) in a series of individual solvents of different chemical nature have been measured using a technique based on studying the kinetics of fluorescence decay. A new quantitative regularity, namely, the dependence of the fluorescence quantum yield on the absolute solvation shift in the fluorescence spectrum due to the gas-solution phase transition, has been revealed. The opinion has been advanced and justified that the revealed regularity has a fundamental character, which is determined by the dominant contribution from the universal intermolecular interactions to the formation of the properties of the solvated DMC molecule in the excited electronic state.     

4-Dimethylaminochalcone as a f luorescent probe: Quantum chemical calculations of its interaction with the environment

Ground-state RHF/6-311G(d,p) and density functional B3LYP/6-311G(d,p) quantum chemical calculations of 4-dimethylaminochalcone (DMAC), a sensitive fluorescent probe, were carried out for vacuum and for solvents of different polarity. The effect of the medium was included by the SCRF method in the framework of the polarization continuum model. The DMAC fragment comprising the aniline and propenone groups has a nearly planar conformation. The phenyl group can lie in the same plane or rotate by an angle within the limits of ±20° with a low barrier at 293 K. The results of calculations were confirmed by the data of X-ray study, according to which the phenyl group in the crystal is rotated by 20°. Calculations with allowance for solvation effects predict charge transfer from the dimethylamino group to the oxygen atom; the higher the medium polarity, the larger the degree of charge transfer (atomic charge of oxygen increases by 0.07 e in acetone). The calculated dipole moment of the DMAC molecule increases from 5.2 D (vacuum) to 5.9 D (heptane) and 6.9 D (acetone), which is in agreement with spectroscopic data. The energy of the DMAC—environment interaction was calculated. Due to large dipole moment of the DMAC molecule, the electrostatic component of this energy strongly depends on the environment polarity, which can be related to redistribution of the probe between the aqueous phase and cells and lipid structures of lipoproteins. The electronic absorption spectra of DMAC in solvents of different polarity were calculated; differences between the calculated and experimentally measured values are at most 15 nm. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1674–1679, October, 2006.     

Fluorescent probe 4-dimethylaminochalcone: Solvatochromism according to the data of semiempirical and quantum-chemical calculations

The solvatochromic regularities observed in solutions of 4-dimethylaminochalcone (DMCh) are investigated in terms of two independent theoretical approaches, i.e., semiempirical and quantum-chemical, and the results of these investigations are directly compared with each other for the first time. It has been shown that the values of the absolute solvation shift of the long-wavelength DMCh electronic absorption band determined by different methods are in a good quantitative agreement both with each other and with experimental data. This fact can be considered as a substantial additional justification of the conclusion on the predominantly nonspecific origin of intermolecular forces determining the mechanisms by which the DMCh fluorescent probe functions in biological systems.     

Biocrystallization of bacterial nucleoid under stress

Structural, biochemical, and genetic changes caused by stress factors are known to be largely similar for cells of all modern organisms, which inherited the basic strategies of adaptation to different types of stress from their ancient ancestors. In the present work, the adaptation process is considered for the simplest example of the bacterial E. coli nucleoid. Experimental studies performed recently on prokaryotic bacterial cells, the simplest living organisms, have demonstrated that, under unfavorable environmental conditions (for example, starvation), bacterial cells can use biocrystallization, a special mechanism of protection of the genetic apparatus (nucleoid), generally untypical of living organisms. This mechanism helps to protect the nucleoid from damage and resume the activity of the bacterial cells later, upon improvement of the external conditions. The results of studying the structure of the nucleoid of E. coli bacteria (BL21-Gold strain (DE3)) subjected to starvation stress by using synchrotron radiation at the ESRF beamline ID23-1 are reported.     

Quantum chemical simulation of the interaction of membrane fluorescent probe 4-dimethylaminochalcone with hydroxy groups of the environment

The quantum chemical simulation of the ground and electron-excited states of diverse complexes of fluorescent probe 4-dimethylaminochalcone (DMAC) and water in vacuo was performed by the HF/MP2 and RI-CC2 methods. Molecules of the DMAC probe and water can form five types of stable complexes. The geometries corresponding to the potential energy minimum and dipole moments for two lowest singlet and one lowest triplet states were calculated for each type of the complexes. The partial charges on the DMAC atoms and their changes due to the intramolecular charge transfer upon photoexcitation were determined. The coordination of the water molecule at the carbonyl group of DMAC is preferable in vacuo. The formation of hydrogen bonds between the carbonyl group of DMAC and water molecules decreases the energy of the excited state of the complex ¹(π, π*), due to which the fluorescence yield increases upon photoexcitation. The calculation results are confirmed by the experimental data on studying the fluorescence of the probe in binary mixtures of benzene and alcohols.     

Solvatochromism and solvatofluorochromism of the intramolecular charge transfer band of 4-dimethylaminochalcone in the electronic spectra of its solutions

The solvatochromic and solvatofluorochromic dependences pertaining to the intense intramolecular charge transfer electronic band of the 4-dimethylaminochalcone molecule, widely used as an efficient spectralluminescent probe in various medical and biological studies, are systematically and quantitatively investigated. The data obtained are interpreted based on the new variant of the semiempirical theory of the influence of the dielectric properties of individual solvents on the shift in the optical spectra of molecules occurring as a result of the gas-solution phase transition that was proposed previously by one of the authors of this study. It is shown that the main contribution (at least 70–80%) to the absolute solvation shift in the absorption and fluorescence spectra of the 4-dimethylaminochalcone molecule is made by universal dipole-dipole interactions of orientation, induction, dispersion, and inductive-resonance nature, as well as by repulsive forces.     

Estimation of the number of polar molecules in a solvate shell of a fluorescent probe 4-dimethylaminochalcone

The fluorescence decay kinetics has been studied in different parts of the emission spectrum of diluted solutions of a membrane fluorescent probe, 4-dimethylaminochalcone, in a binary n-heptane + acetone solvent with the volume fractions of polar and nonpolar components of the latter varying in a wide range. It has been shown that the fluorescence of these solutions has a significantly heterogeneous character, which is indicative of the presence of several types of solvate complexes with controllable ratios of components (1: 1, 1: 2, 1: 3, etc.) in the studied systems. Based on a quantitative analysis of the mentioned kinetic dependences, a model has been proposed in which these data can serve as a source of information about statistical features of the structure of solvate shells of polar dye molecules in an excited electronic state. This information can in turn be used to obtain new biophysical data.     

4-Dimethylaminochalcone as fluorescent probe: effect of the medium polarity on relaxation processes in the excited state

Relaxation processes in a 4-dimethylaminochalcone molecule after excitation with a light pulse of duration 70 fs were studied. During 0.4–1 ps after excitation, an absorbance of an excited state S₁ with a maximum at 460 nm is formed in both polar and nonpolar media. Subsequent relaxation processes depend on the polarity of the medium. In nonpolar hexane, the 4-dimethylaminochalcone molecule transits to the triplet state having an absorption maximum at 570 nm (lifetime longer than 600 ps) for 20 ps. In polar aprotic acetonitrile, the absorbance at 460 nm decreases slowly (during hundreds of picoseconds), indicating that the molecules return to the ground state. The induced emission from the level S₁ in a region of 520–550 nm and fluorescence from the same level with a maximum at 537 nm are also observed in acetonitrile. Thus, a reason for a sharp decrease in the fluorescence yield on going from polar to nonpolar media was found. The mechanism of fluorescence quenching of 4-dimethylaminochalcone in nonpolar media is confirmed by the data on phosphorescence. The phosphorescence of 4-dimethylaminochalcone is observed at–196 °C in nonpolar solvents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1607–1610, August, 2004.