Asymmetric Monomethine Cyanine Dyes with Hydrophobic Functionalities for Fluorescent Intercalator Displacement Assay.

A new green procedure has been applied for the synthesis and purification of asymmetric monomethine cyanine dyes. The photophysical properties of the newly synthesized compounds have been examined by combined application of spectroscopic and theoretical methods. The structural characteristics of the molecules and dimer formation were characterized by quantum chemical computation and juxtaposed to the aggregachromism in UV/Vis spectra. The applicability of the dyes as fluorogenic nucleic acid probes has been proven by fluorescence titration, and their binding constants have been calculated. The mode of ligand-dsDNA/RNA interaction was rationalized by means of CD spectroscopy, molecular docking analysis, and fluorescent intercalator displacement experiments.

Synthesis of Novel 1-Oxo-2,3,4-trisubstituted Tetrahydroisoquinoline Derivatives, Bearing Other Heterocyclic Moieties and Comparative Preliminary Study of Anti-Coronavirus Activity of Selected Compounds.

A series of novel 1-oxo-2,3,4-trisubstituted tetrahydroisoquinoline (THIQ) derivatives bearing other heterocyclic moieties in their structure were synthesized based on the reaction between homophthalic anhydride and imines. Initial studies were carried out to establish the anti-coronavirus activity of some of the newly obtained THIQ-derivatives against two strains of human coronavirus-229E and OC-43. Their antiviral activity was compared with that of their close analogues, piperidinones and thiomorpholinones, previously synthesized in our group, with aim to expand the range of the tested representative sample and to obtain valuable preliminary information about biological properties of a wider variety of compounds.

Atomic Charges in Describing Properties of Aromatic Molecules.

The performance of four frequently employed population analysis methods is assessed by comparisons with experimentally derived properties of monosubstituted benzene derivatives. The analysis is based on the expected dependence between site reactivities and electron densities at the respective ring carbon atoms. The correspondence between charges obtained from Mulliken, NPA, Hirshfeld, and QTAIM approaches and the σ0m and σ0p aromatic substituent constants is examined. The series of molecules investigated includes benzene and 18 monosubstituted derivatives. The atomic charges are derived using the B3LYP, ωB97X-D density functional, and MP2 MO methods combined with the 6-311++G(3df,2pd) basis set. A quantitative correspondence between Hirshfeld charges and σ0 constants is established. Application of Møller-Plesset second-order perturbation theory (MP2) wave functions appears to be essential in obtaining a more realistic electron density distribution. NPA and QTAIM charges provide in most cases a satisfactory description of the substituent effects. The net transfer of charges between substituents and the aromatic ring is assessed.

Electric field influence on the helical structure of peptides: insights from DFT/PCM computations.

The secondary structure of proteins is of prime importance to their proper functioning and protein misfolding may cause serious disorders in the human body. Here, the electric field influence on the conformational stability of model alpha helical peptides is studied by employing density functional theory calculations combined with continuum dielectric method computations. Our results show that the basic parameters of the electric field - its strength and directionality - are determinative for the alpha helix stability. An electric field strength of 0.005 a.u. (2.5 V nm-1) applied along the X coordinate axis (the long axis of the helix) in the direction of the µx component of the molecular dipole moment does affect the peptide conformation, destroys the helix, and leads to the formation of a cyclic-peptide-like structure. Interestingly, the process of denaturation can be reversible when the electric field is switched-off. The reversibility of the process of the electric field induced disruption of the peptide secondary structure suggests a possible mechanism for the healing of misfolded proteins.

How an electric field can modulate the metal ion selectivity of protein binding sites: insights from DFT/PCM calculations.

Selecting the "right" metal cation from the surrounding intracellular/extracellular fluids is of crucial importance for proper functioning of metalloproteins. Over the course of a few billion years of cell evolution various strategies have been developed by the host protein or cell machinery to secure the most favorable conditions for cognate cation binding. The effect of internal/external electric fields, potentially capable of influencing the process of metal selectivity in proteins, however, remains an enigmatic and unexplored area of research. Several outstanding questions remain unanswered: (1) Can, and if so, to what extent, an electric field affect the competition between different metal species for protein ligands? (2) What type of binding sites (with specific architecture, polarizability, and solvent exposure) are more susceptible to electric stimuli? (3) How do the basic parameters of the electric field - its strength and directionality - modulate the selectivity of the metal binding site? (4) What is the upper limit of the electric field magnitude that preserves the integrity of the metal binding site? Here, by employing density functional theory calculations combined with continuum dielectric method computations, we endeavor to shed light on these questions by studying the thermodynamic outcome of the competition between Mg2+ and Ca2+ in a model EF-hand motif metal binding site under the influence of a dipole electric field with variable magnitude and directionality. The calculations reveal that the electric field (either internal or external) is a potent force that can modulate the metal selectivity of the binding site and thus can be added to the list of factors governing the metal competition in metalloproteins. An oriented external electric field with a particular directionality could be used as a switch to enhance or attenuate the preference of the binding site toward given metal species.

Do π-conjugative effects facilitate SN2 reactions?

Rigorous quantum chemical investigations of the SN2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F(-), Cl(-)) refute the traditional view that the acceleration of SN2 reactions for substrates with a multiple bond at Cß (carbon adjacent to the reacting Cα center) is primarily due to π-conjugation in the SN2 transition state (TS). Instead, substrate-nucleophile electrostatic interactions dictate SN2 reaction rate trends. Regardless of the presence or absence of a Cß multiple bond in the SN2 reactant in a series of analogues, attractive Cß(δ(+))···X(δ(-)) interactions in the SN2 TS lower net activation barriers (E(b)) and enhance reaction rates, whereas repulsive Cß(δ(-))···X(δ(-)) interactions increase E(b) barriers and retard SN2 rates. Block-localized wave function (BLW) computations confirm that π-conjugation lowers the net activation barriers of SN2 allyl (1t, coplanar), benzyl, propargyl, and acetonitrile halide identity exchange reactions, but does so to nearly the same extent. Therefore, such orbital interactions cannot account for the large range of E(b) values in these systems.

Experimental measurement and theory of substituent effects in π-hydrogen bonding: complexes of substituted phenols with benzene.

IR spectroscopic experiments and theoretical DFT computations reveal the effects of aromatic substituents on π-hydrogen bonding between monosubstituted phenol derivatives and benzene. Simultaneous formation of two π-hydrogen bonds (red-shifting O-H···π and blue-shifting ortho-C-H···π) contribute to the stability of these complexes. The interaction of the acidic phenol O-H proton-donating group with the benzene π-system dominates the complex formation. The experimental shifts of O-H stretching frequencies for the different phenol complexes vary in the range 45-74 cm(-1). Strong effects on hydrogen-bonding energies and frequency shifts of electron-withdrawing aromatic substituents and very weak influence of electron-donating groups have been established. Experimental quantities and theoretical parameters are employed in rationalizing the properties of these complexes. The acidities of the proton-donating phenols describe quantitatively the hydrogen-bonding process. The results obtained provide clear evidence that, when the structural variations are in the proton-donating species, the substituent effects on π-hydrogen bonding follow classic mechanisms, comprising both resonance and direct through-space influences. The performance of three alternative DFT functionals (B3LYP, B97-D, and PBE0 combined with the 6-311++G(2df,2p) basis set) in predicting the O-H frequency shifts upon complexation is examined. For comparison, O-H frequency shifts for several complexes were also determined at MP2/6-31++G(d,p).

Does the molecular electrostatic potential reflect the effects of substituents in aromatic systems?

A detailed analysis of the molecular electrostatic potential (MEP) at selected positions in molecular space was performed for a series of substituted benzene derivatives. We show that appropriately selected MEP values can quantitatively reflect the regiospecific effects of substituents on the aromatic ring. Theoretically evaluated electrostatic potentials in close proximity to the ring carbon atoms reflect well both through-space and resonance effects and excellent correlation is established between the MEP values and substituent constants. The best descriptor of the local properties at different ring positions is the electrostatic potential at nuclei (EPN).

Aminolysis of phenyl N-phenylcarbamate via an isocyanate intermediate: theory and experiment.

A comprehensive examination of the mechanism of the uncatalyzed and base-catalyzed aminolysis of phenyl N-phenylcarbamate by theoretical quantum mechanical methods at M06-2X/6-311+G(2d,2p) and B3LYP-D3/6-31G(d,p) levels, combined with an IR spectroscopic study of the reaction, was carried out. Three alternative reaction channels were theoretically characterized: concerted, stepwise via a tetrahedral intermediate, and stepwise involving an isocyanate intermediate. In contrast to dominating views, the theoretical results revealed that the reaction pathway through the isocyanate intermediate (E1cB) is energetically favored. These conclusions were supported by an IR spectroscopic investigation of the interactions of phenyl N-phenylcarbamate with several amines possessing varying basicities and nucleophilicities: n-butylamine, diethylamine, triethylamine, N-methylpyrrolidine, and trimethylamine. The reactivity of substituted phenyl N-phenylcarbamates in the aminolysis reaction was rationalized using theoretical and experimental reactivity indexes: electrostatic potential at nuclei (EPN), Hirshfeld and NBO atomic charges, and Hammett constants. The obtained quantitative relationships between these property descriptors and experimental kinetic constants reported in the literature emphasize the usefulness of theoretical parameters (EPN, atomic charges) in characterizing chemical reactivity.

Aggregation induced nucleic acids recognition by homodimeric asymmetric monomethyne cyanine fluorochromes in mesenchymal stem cells.

In the light of recent retrovirus pandemics, the issue of discovering new and diverse RNA-specific fluorochromes for research and diagnostics became of acute importance. The great majority of nucleic acid-specific probes either do not stain RNA or cannot distinguish between DNA and RNA. The versatility of polymethine dyes makes them suitable as stains for visualization, analysis, and detection of nucleic acids, proteins, and other biomolecules. We synthesized the asymmetric dicationic homodimeric monomethine cyanine dyes 1,1'-(1,3-phenylenebis(methylene))bis(4-((3-methylbenzo[d]thiazol-2(3H)-ylidene)methyl)pyridin-1-ium) bromide (Т1) and 1,1'-(1,3-phenylenebis(methylene))bis(4-((3-methylbenzo[d]thiazol-2(3H)-ylidene)methyl)quinolin-1-ium) bromide (M1) and tested their binding specificity, spectral characteristics, membrane penetration in living and fixed cells, cellular toxicity, and stability of fluorescent emission. Mesenchymal cells have diverse phenotypes and extensive proliferation and differentiation properties. We found dyes T1 and M1 to show high photochemical stability in living mesenchymal stem cells from apical papilla (SCAP) with a strong fluorescent signal when bound to nucleic acids. We found M1 to perform better than control fluorochrome (Hoechst 33342) for in vivo DNA visualization. T1, on the other hand, stains granular cellular structures resembling ribosomes in living cells and after permeabilization of the nuclear membrane stains the nucleoli and not the chromatin in the nucleus. This makes T1 suitable for the visualization of structures rich in RNA in living and fixed cells.

Mini-Review on Structure-Reactivity Relationship for Aromatic Molecules: Recent Advances.

Recent advances in quantifying nucleophilic reactivities in chemical reactions and intermolecular interactions of aromatic molecules are reviewed. This survey covers experimental (IR frequency shifts induced by hydrogen bonding) and theoretical (modeling of potential energy surfaces, atomic charges, molecular electrostatic potential) approaches in characterizing chemical reactivity. Recent advances in software developments assisting the evaluation of the reactive sites for electrophilic aromatic substitution are briefly discussed.

Experimental and theoretical study on the absorption and fluorescence properties of substituted aryl hydrazones of 1,8-naphthalimide.

Absorption and fluorescence spectra in acetonitrile for a series of substituted aryl hydrazones of N-hexyl-1,8-naphthalimide are studied with the aim of potential application of the compounds for enzyme activity localization. The influence of the substituents on the spectral characteristics has been evaluated. The absorption and fluorescence energies of substituted aryl-1,8-naphthalimide hydrazones have been calculated with the PCM TDDFT formalism. The M06 and PBE0 functionals, combined with the 6-31+G(d) atomic basis set, have been found to accurately model the excited state properties of the present set of solvated fluorophores. Absorption and fluorescence spectral characteristics have been rationalized in terms of experimental and theoretical electronic indices in order to assess their predictive abilities for application in designing analogues with good emitting properties. An excellent linear dependence is established between the experimental fluorescence and Hammett σ(p)(+) substituent constants and on the other hand σ(p)(+) constants correlate with the theoretically calculated values for the electrostatic potential at nuclei (EPN). A model for predicting the fluorescence properties of substituted hydrazones by means of EPN is drawn, including the polysubstituted derivatives, where Hammett constants are not applicable.

Ca2+/Sr2+ Selectivity in Calcium-Sensing Receptor (CaSR): Implications for Strontium's Anti-Osteoporosis Effect.

The extracellular calcium-sensing receptor (CaSR) controls vital bone cell functions such as cell growth, differentiation and apoptosis. The binding of the native agonist (Ca2+) to CaSR activates the receptor, which undergoes structural changes that trigger a cascade of events along the cellular signaling pathways. Strontium (in the form of soluble salts) has been found to also be a CaSR agonist. The activation of the receptor by Sr2+ is considered to be the major mechanism through which strontium exerts its anti-osteoporosis effect, mostly in postmenopausal women. Strontium-activated CaSR initiates a series of signal transduction events resulting in both osteoclast apoptosis and osteoblast differentiation, thus strengthening the bone tissue. The intimate mechanism of Sr2+ activation of CaSR is still enigmatic. Herewith, by employing a combination of density functional theory (DFT) calculations and polarizable continuum model (PCM) computations, we have found that the Ca2+ binding sites 1, 3, and 4 in the activated CaSR, although possessing a different number and type of protein ligands, overall structure and charge state, are all selective for Ca2+ over Sr2+. The three binding sites, regardless of their structural differences, exhibit almost equal metal selectivity if they are flexible and have no geometrical constraints on the incoming Sr2+. In contrast to Ca2+ and Sr2+, Mg2+ constructs, when allowed to fully relax during the optimization process, adopt their stringent six-coordinated octahedral structure at the expense of detaching a one-backbone carbonyl ligand and shifting it to the second coordination layer of the metal. The binding of Mg2+ and Sr2+ to a rigid/inflexible calcium-designed binding pocket requires an additional energy penalty for the binding ion; however, the price for doing so (to be paid by Sr2+) is much less than that of Mg2+. The results obtained delineate the key factors controlling the competition between metal cations for the receptor and shed light on some aspects of strontium's therapeutic effects.

Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: the importance of internal hydrogen bonding at the transition state.

We find high multireference character for abstraction of H from the OH group of ethenol (also called vinyl alcohol); therefore we adopt a multireference approach to calculate barrier heights for the various possible reaction channels of OH+C(2)H(3)OH. The relative barrier heights of ten possible saddle points for reaction of OH with ethenol are predicted by multireference Møller-Plesset perturbation theory with active spaces based on correlated participating orbitals (CPOs) and CPO plus a correlated pi orbital (CPO+pi). Six barrier heights for abstracting H from a C-H bond range from 3.1 to 7.7 kcal/mol, two barrier heights for abstracting H from an O-H bond are both 6.0 kcal/mol, and two barrier heights for OH addition to the double bond are -1.8 and -2.8 kcal/mol. Thus we expect abstraction at high-temperature and addition at low temperature. The factor that determines which H is most favorable to abstract is an internal hydrogen bond that constitutes part of a six-membered ring at one of the abstraction saddle points; the hydrogen bond contributes about 3 kcal/mol stabilization.

Predicting reactivities of organic molecules. Theoretical and experimental studies on the aminolysis of phenyl acetates.

The quality of reactivity predictions coming from alternative theoretical approaches as well as experimental reactivity constants is examined in the case of the ester aminolysis process. The aminolysis of a series of para-substituted phenyl acetates is studied. The barrier heights for the rate-determining stage of the aminolysis of 16 phenyl acetate derivatives were predicted by employing density functional theory at the B3LYP/6-31+G(d,p) level. Experimental kinetic studies were carried out for the n-butylaminolysis of seven p-substituted phenyl acetates in acetonitrile. The results show that the electrostatic potential at the carbon atom of the carbonyl reaction center provides an excellent description of reactivities with regard to both theoretical barrier heights and experimental rate constants. The performance of other reactivity indices, Mulliken and NBO atomic charges, electrophilicity index, and Hammett constants, is also assessed.

QSAR analysis of 1,4-dihydro-4-oxo-1-(2-thiazolyl)-1,8-naphthyridines with anticancer activity.

In the present study a quantitative structure activity relationship (QSAR) analysis was applied to a series of 100 of 7- and 3-substituted 1,4-dihydro-4-oxo-1-(2-thiazolyl)-1,8-naphthyridine derivatives. The Chem-X (version 2000) software was used to develop 3D QSAR models. The steric and electrostatic interactions between a probe atom (H(+)) and a set of aligned molecules were assessed using the comparative molecular field analysis method. Statistically relevant models were derived for both electrostatic and steric fields. A 2D model over a restricted series of close structural analogs was derived as well. A number of conclusions on the relationship between the type and size of different substituents and the antitumor activity of the compounds were derived.

Conformation of some biologically active aromatic ureas.

Experimental IR spectroscopic data for the N-H stretching mode frequencies for several types of tri-substituted ureas containing benzyl and/or phenyl substituents as well as theoretical results from B3LYP/6-31G(d,p) computations on selected compounds provide sufficient evidence to determine the conformational state of these molecules. Two types of N-H bands may be found the spectra: (a) A type band due to a classical trans conformation (trans I) of the CONH structure; (b) B type band arising from an alternative trans form (trans II), in which the N-H band is involved in a hydrogen bond like interaction with the aromatic ring at the neighbouring nitrogen atom (benzyl or phenyl substituents). The N-H band of trans ICONH structure is observed at frequencies higher than 3460 cm-1, the actual position depending on weather the non-substituted N-H group is linked to aryl or alkyl substituents. The N-H band of the trans II rotameric structure is observed at 3430-3420 cm-1.

An efficient computational approach for the evaluation of substituent constants.

Density functional theory computations at the B3LYP/6-311+G(2d,2p) and BPW91/6-311G(d,p) levels were carried out for a series of 15 monosubstituted benzene derivatives to study dependencies between electronic structure parameters and experimental reactivity constants. An efficient and accurate computational approach for the evaluation of sigma(0) substituent constants for substituted benzene systems is outlined. It is based on the excellent linear correlation between the experimental reactivity constants and the theoretical electrostatic potential values (EPN) at the carbon atoms in the para and meta positions. The results underline the usefulness of the EPN as a local reactivity descriptor. Theoretical computations to assess the influence of water solvent using the SCIPCM method showed that the solvent enhances the overall effect of polar substituents by about 30%. The results obtained indicate also that the relative values of the sigma(0) constants are predominantly determined by intramolecular influences.

Mechanism of the aminolysis of methyl benzoate: a computational study.

Density functional and ab initio methods were applied in examining the possible mechanistic pathways for the reaction of methyl benzoate with ammonia. Transition state structures and energies were determined for concerted and neutral stepwise mechanisms. The theoretical results show that the two possible pathways have similar activation energies. The general base catalysis of the process was also examined. The predictions reveal that the catalytic process results in considerable energy savings and the most favorable pathway of the reaction is through a general-base-catalyzed neutral stepwise mechanism. The structure and transition vectors of the transition states indicate that the catalytic role of ammonia is realized by facilitating the proton-transfer processes. Comparison of the energetics of the aminolysis for methyl benzoate and methyl formate shows the more favorable process to be that for the aliphatic ester. The differing reactivity of the two esters is explained in terms of the electrostatic potential values at the atoms of the ester functionality.

Theory supplemented by experiment. Electronic effects on the rotational stability of the amide group in p-substituted acetanilides.

The electronic effect of polar substituents on the barrier of internal rotation around the amide carbon-nitrogen bond in a series of 10 p-substituted acetanilides is studied by applying density functional theory at the B3LYP/6-31G(d,p) level. The theoretical results are supplemented by experimental data on the amide C=O and N-H stretching mode frequency shifts. It is shown that computations at the theoretical level employed provide a valuable approach in studying the factors determining the conformational stability of the studied series of compounds. It is found that an excellent linear dependence between the barriers of rotation and frequency shifts exists. It is concluded that the variations of the amide C=O stretching mode frequency can be used for quantitative characterization of the amide group conformational flexibility in the studied series of acetanilides.