Formation Of Intramolecular Hydrogen Bond Research Articles

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes.

The development of deep-blue organic light-emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh-definition displays with wide color gamut. Herein, based on the nitrogen-embedding strategy for modifying the short range charge transfer excited state energies of multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic-based MR-TADF molecules, Py-BN and Pm-BN, exhibit deep-blue emissions with high photoluminescence quantum yields of 93% and 94%, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep-blue hyperfluorescent OLED based on Py-BN exhibited a maximum external quantum efficiency of 27.7% and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR-TADF emitters.

Nitrogen‐Embedding Strategy for Short‐Range Charge Transfer Excited States and Efficient Narrowband Deep‐Blue Organic Light Emitting Diodes

The development of deep‐blue organic light‐emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh‐definition displays with wide color gamut. Herein, based on the nitrogen‐embedding strategy for modifying the short range charge transfer excited state energies of multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron‐embedded 1,3‐bis(carbazol‐9‐yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic‐based MR‐TADF molecules, Py‐BN and Pm‐BN, exhibit deep‐blue emissions with high photoluminescence quantum yields of 93% and 94%, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep‐blue hyperfluorescent OLED based on Py‐BN exhibited a maximum external quantum efficiency of 27.7% and desired color purity with Commission Internationale de L’Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR‐TADF emitters.

Simultaneous adsorption of anionic and cationic species on UiO-66 and MIL-125 MOFs: Mechanisms & selectivity

Metal-organic frameworks (MOFs) have attractive properties, including regular pore structure, high specific surface area, tunable surface properties, and abundant active sites. Such properties have boosted their recent development in many fields, including water treatment. In this study, MOFs composed of tetravalent metals (Ti4+ and Zr4+) and dicarboxylate linker (terephthalate – BDC) were modified using surface functionalization and/or pore structure through defect engineering to enhance adsorption properties. It has been shown that the Ti-containing materials (MIL-125) show a greater affinity for cationic species, while the Zr-containing samples (UiO-66) have a higher affinity for anionic species. Surface charge and electrostatic interactions could account for such a difference in adsorption properties. Zeta potential measurements showed that MIL-125 has a more negative surface charge for both non-functionalized and amino-functionalized versions, which favors the interaction with cationic molecules. On the other hand, UiO-66 has a more positive surface, which facilitates interactions with anionic molecules. Within each MOF family, the non-functionalized structures emerged as the best-performing materials. Specifically, MIL-125(H) and UiO-66(H)_5eq, with 5eq denoting the amount of H2O added during synthesis, showed superior performance. Furthermore, by performing simultaneous adsorption experiments with anionic and cationic contaminants, it was found that the adsorption of the anionic dye acid orange 7 (AO7) onto the UiO-66 framework is primarily based on specific interactions between the sulfonic group (-SO3-) within the AO7 molecule and the inorganic core Zr6O4(OH)4 of UiO-66. The interaction between MIL-125 and the cationic dye methylene blue (MB) was found to be more delocalized, based on electrostatic and π-π interactions between the benzene domains. Finally, except for MIL-125(H), all the tested MOFs showed high stability in water and in the adsorption process. The higher stability of UiO-66 over MIL-125 materials can be attributed to steric effects and lower metal electronegativity. Compared to MIL-125(H), the higher stability of MIL-125(NH2) is probably related to the formation of intramolecular hydrogen bonds.

Insight into the binding characteristics of epigallocatechin-3-O-gallate and alcohol dehydrogenase: Based on the spectroscopic and molecular docking analysis

Alcohol dehydrogenase (ADH) is one of the pivotal enzymes for alcohol metabolism, which plays an important role in many physiological processes. In this study, the activation effects of epigallocatechin-3-O-gallate (EGCG) on ADH and the characteristics of the interaction were investigated via biochemical method, spectroscopy methods, and molecular docking. The results demonstrated that EGCG significantly increased the catalytic activity of ADH with a 33.33% activation rate and that EGCG blending slightly altered the microenvironment surrounding ADH aromatic amino acids, with an increase in the quantity of β-sheet and a decrease in the α-helix. Through the thermal stability analysis, it is further shown that the interaction of the two affects the intra-molecular hydrogen bond formation of the protein, and the conformation is partially extended. Besides, a total of 8 residues in ADH participated in the docking with EGCG, among which Asp-227, Lys-231, Glu-234, Gly-365 and Glu-366 participated in the formation of hydrogen bonds. At the same time, EGCG and amino group of Lys-231 form a noncovalent bond through cation-π interaction. In particular, hydrogen bonding was beneficial to keep the stability of EGCG-ADH, which was the primary driver of ADH activity activation. The results supply a new way for EGCG to activate ADH and a theoretical basis for the development of anti-alcoholism products.

Creating Efficient Red Thermally Activated Delayed Fluorescence Materials with Cyano-Substituted 11,12-Diphenyldipyrido[3,2-a:2',3'-c]phenazine Acceptors.

Red luminescent materials are essential components for full color display and white lightening based on organic light-emitting diode (OLED) technology, but the extension of emission color towards red or deep red region generally leads to decreased photoluminescence and electroluminescence efficiencies. Herein, we wish to report two new luminescent molecules (2CNDPBPPr-TPA and 4CNDPBPPr-TPA) consisting of cyano-substituted 11,12-diphenyldipyrido[3,2-a:2',3'-c]phenazine acceptors and triphenylamine donors. As the increase of cyano substituents, the emission wavelength is greatly red-shifted and the reverse intersystem crossing process is promoted, resulting in strong red delayed fluorescence. Meanwhile, due to the formation of intramolecular hydrogen bonds, the molecular structures become rigidified and planarized, which brings about large horizontal dipole ratios. As a result, 2CNDPBPPr-TPA and 4CNDPBPPr-TPA can perform as emitters efficiently in OLEDs, furnishing excellent external quantum efficiencies of 28.8 % at 616 nm and 20.2 % at 648 nm, which are significantly improved in comparison with that of the control molecule without cyano substituents. The findings in this work demonstrate that the introduction of cyano substituents to the acceptors of delayed fluorescence molecules could be a facile and effective approach to explore high-efficiency red or deep red delayed fluorescence materials.

Mechanism of drug resistance in HIV-1 protease subtype C in the presence of Atazanavir

AIDS is one of the deadliest diseases in the history of humankind caused by HIV. Despite the technological development, curtailing the viral infection inside human host still remains a challenge. Therapies such as HAART uses a combination of drugs to inhibit the viral activity. One of the important targets includes HIV protease and inhibiting its activity will minimize the production of mature structural proteins. However, the genetic diversity and the occurrence of drug resistant mutations adds complexity to effective drug design. In this study, we aimed at understanding the drug binding mechanism of one such subtype, namely subtype C and its insertion variant L38HL. We performed multiple molecular dynamics simulations along with binding free energy analysis of wild-type and L38HL bound to Atazanavir (ATV). From the analysis, we revealed that the insertion alters the hydrogen bond and hydrophobic interaction networks. The alterations in the interaction networks increase flexibility at the hinge-fulcrum interface. Further, the effects of these changes affect flap tip curling. Moreover, the changes in the hinge-fulcrum-cantilever interface alters the concerted motion of the functional regions leading to change in the direction of flap movement thus causing a subtle change in the active site volume. Additionally, formation of intramolecular hydrogen bonds in the ATV docked to L38HL restricted the movement of R1 and R2 groups thereby altering the interactions. Overall, the changes in the flexibility of flap together with the changes in the active site volume and compactness of the ligand provide insights for increased binding affinity of ATV with L38HL.

A synthetic approach towards drug modification: 2-hydroxy-1-naphthaldehyde based imine-zwitterion preparation, single-crystal study, Hirshfeld surface analysis, and computational investigation.

The current work is about the modification of primary amine functionalized drugs, pyrimethamine and 4-amino-N-(2,3-dihydrothiazol-2-yl)benzenesulfonamide, via condensation reaction with 2-hydroxy-1-naphthaldehyde to produce new organic zwitterionic compounds (E)-1-(((4-(N-(2,3-dihydrothiazol-2-yl)sulfamoyl)phenyl)iminio)methyl)naphthalen-2-olate (DSPIN) and (E)-1-(((4-amino-5-(4-chlorophenyl)-6-ethylpyrimidin-2-yl)iminio)methyl)naphthalen-2-olate (ACPIN) in methanol as a solvent. The crystal structures of both compounds were confirmed to be imine-based zwitterionic products via single-crystal X-ray diffraction (SC-XRD) analysis which indicated that the stabilization of both crystalline compounds is achieved via various noncovalent interactions. The supramolecular assembly in terms of noncovalent interactions was explored by the Hirshfeld surface analysis. Void analysis was carried out to predict the crystal mechanical response. Compound geometries calculated in the DFT (Density Functional Theory) study showed reasonably good agreement with the experimentally determined structural parameters. Frontier molecular orbital (FMO) analysis showed that the DSPIN HOMO/LUMO gap is by 0.15 eV smaller than the ACPIN HOMO/LUMO gap due to some destabilization of the DSPIN HOMO and some stabilization of its LUMO. The results of the charge analysis implied formation of intramolecular hydrogen bonds and suggested formation of intermolecular hydrogen bonding and dipole-dipole and dispersion interactions.

Stereo effects for efficient synthesis of orange-red multiple resonance emitters centered on a pyridine ring.

Despite theoretical difficulties, we herein demonstrate an effective strategy for the inaugural synthesis of an orange-red multiple resonance (MR) emitter centered on a pyridine ring via stereo effects. Compared to conventional benzene-centered materials, the pyridine moiety in the novel MR material acts as a co-acceptor. This results in a significant spectral redshift and a narrower spectrum, as well as an improved photoluminescence quantum yield (PLQY) due to the formation of intramolecular hydrogen bonds. As envisioned, the proof-of-concept emitter Py-Cz-BN exhibits bright orange-red emission peaking at 586 nm with a small full width at half maximum (FWHM) of 0.14 eV (40 nm), exceeding both the wavelength and FWHM achieved with benzene-centered BBCz-Y. Benefiting from high PLQYs (>92%) and suppressed chromophore interactions, the optimized organic light-emitting diodes achieved high maximum external quantum efficiencies of 25.3-29.6%, identical small FWHMs of 0.18 eV (54 nm), and long lifetimes over a wide range of dopant concentrations (1-15 wt%). The performance described above demonstrates the effectiveness of this molecular design and synthesis strategy in constructing high performance long-wavelength MR emitters.

Solvent and Substituent Size Influence on the Cyclochiral Rigidity of Aminomethylene Derivatives of Resorcin[4]arene.

Resorcin[4]arenes (R[4]A) are a group of macrocyclic compounds whose peculiar feature is the presence of eight hydroxyl groups in their structure. The directional formation of intramolecular hydrogen bonds with their participation leads to the formation of a cyclochiral racemic mixture of these compounds. Their stability strongly depends on the substituent and especially the environment in which they are located. The paper discusses the cyclochiral nature of aminomethylene derivatives of R[4]A (AMD-R[4]A). Their cyclochiral rigidity in non-polar solvents has been shown. The influence of the size of the alkyl groups in the amino substituents of AMD-R[4]A on their cyclochiral nature was noted. To calculate the reaction paths for their racemization, the nudged elastic band (NEB) method was employed using the semi-empirical DFT (GFN1-xTB) approach. The calculated activation barrier energies for their racemization in chloroform, obtained through various semi-empirical quantum chemical methods (SE), Hartree-Fock (HF), and density functionals theory (DFT), show good correlation with experimental observations. Among the tested methods, the B38LYP-D4 method is highly recommended due to its fast computational speed and accuracy, which is comparable to the time-consuming double-hybrid DH-revDSD-PBEP86 approach.

How to More Effectively Obtain Ginsenoside Rg5: Understanding Pathways of Conversion.

Ginsenoside Rg5, a relatively uncommon secondary ginsenoside, exhibits notable pharmacological activity and is commonly hypothesized to originate from the dehydration of Rg3. In this work, we compared different conversion pathways using Rb1, R-Rg3 and S-Rg3 as the raw material under simple acid catalysis. Interestingly, the results indicate that the conversion follows this reaction activity order Rb1 > S-Rg3 > R-Rg3, which is contrary to the common understanding of Rg5 obtained from Rg3 by dehydration. Our experimental results have been fully confirmed by theoretical calculations and a NOESY analysis. The DFT analysis reveals that the free energies of S-Rg3 and R-Rg3 in generating carbocation are 7.56 mol/L and 7.57 mol/L, respectively, which are significantly higher than the free energy of 1.81 mol/L when Rb1 generates the same carbocation. This finding aligns with experimental evidence suggesting that Rb1 is more prone to generating Rg5 than Rg3. The findings from the nuclear magnetic resonance (NMR) analysis suggest that the fatty chains (C22-C27) in R-Rg3 and S-Rg3 adopt a Gauche conformation and an anti conformation with C16-C17 and C13-C17, respectively, due to the relatively weak repulsive van der Waals force. Therefore, the configuration of R-Rg3 is more conducive to the formation of intramolecular hydrogen bonds between 20C-OH and 12C-OH, whereas S-Rg3 lacks this capability. Consequently, this also explains the fact that S-Rg3 is more prone to dehydration to generate Rg5 than R-Rg3. Additionally, our research reveals that the synthetic route of Rg5 derived from protopanaxadiol (PPD)-type ginsenosides (including Rb1, Rb2, Rb3, Rc and Rd) exhibits notable advantages in terms of efficacy, purity and yield when compared to the pathway originating from Rg3. Moreover, this study presents a highly effective and practical approach for the extensive synthesis of Rg5, thereby facilitating the exploration of its pharmacological properties and potential application in drug discovery.

Dependence of Intramolecular Hydrogen Bond on Conformational Flexibility in Linear Aminoalcohols.

Intramolecular hydrogen bonds (H-bonds) are abundant in physicochemical and biological processes. The strength of such interaction is governed by a subtle balance between conformational flexibility and steric effect that are often hard to predict. Herein, using linear aminoalcohols NH2(CH2)nOH (n = 2-5) as a model system, we demonstrated the dependence of intramolecular H-bond on the backbone chain length. With sensitive photoacoustic Raman spectroscopy (PARS), the gas-phase Raman spectra of aminoalcohols were measured in both N-H and O-H stretching regions at 298 and 338 K and explained with the aid of quantum chemistry calculations. For n = 2-4, two conformers corresponding to the O-H···N intramolecular H-bond and free OH were identified, whereas for n = 5, only the free-OH conformer was identified. Compared to free OH, a striking spectral dependence was observed for the intramolecular H-bonded conformer. According to the red shift of the OH-bonded band, the strongest intramolecular H-bond yields in n = 4, but the favorable chain length to form an intramolecular hydrogen bond at room temperature was observed in n = 3, which corresponds to a six-membered-ring in 3-aminopropanol. This is in good agreement with statistical analysis from the Cambridge Structural Database (CSD) that the intramolecular hydrogen bond is preferred when the six-membered ring is formed. Furthermore, combined with the calculated thermodynamic data at the MP2/aug-cc-pVTZ//M062X/6-311++G(d,p) level, the origin of decrease in intramolecular hydrogen-bond formation was ascribed to an unfavorable negative entropy contribution when the backbone chain is further getting longer, which results in the calculated Gibbs free energy optimum changing with increasing temperature from n = 4 (0-200 K) to n = 3 (200-400 K) and to n = 2 (above 400 K). These results will provide new insight into the nature of intramolecular hydrogen bonds at the molecular level and the application of intramolecular hydrogen bonds in rational drug design and supramolecular assembly.

Installing hydrogen bonds as a general strategy to control viscosity sensitivity of molecular rotor fluorophores

AbstractMolecular rotor‐based fluorophores (RBFs) activate fluorescence upon increase of micro‐viscosity, thus bearing a broad application promise in many fields. However, it remains a challenge to control how fluorescence of RBFs responds to viscosity changes. Herein, we demonstrate that the formation and regulation of intramolecular hydrogen bonds in the excited state of RBFs could modulate their rotational barrier, leading to a rational control of how their fluorescence can be activated by micro‐viscosity. Based on this strategy, a series of RBFs were developed based on 4‐hydroxybenzylidene‐imidazolinone (HBI) that span a wide range of viscosity sensitivity. Combined with the AggTag method that we previously reported, the varying viscosity sensitivity and emission spectra of these probes enabled a dual‐color imaging strategy that detects both protein oligomers and aggregates during the multistep aggregation process of proteins in live cells. In summary, our work indicates that installing intracellular excited state hydrogen bonds to RBFs allows for a rational control of rotational barrier, thus allow for a fine tune of their viscosity sensitivity. Beyond RBFs, we envision similar strategies can be applied to control the fluorogenic behavior of a large group of fluorophores whose emission is dependent on excited state rotational motion, including aggregation‐induced emission fluorophores.

DFT Insight into the Kinetics and Mechanism of OH⋅‐Initiated Atmospheric Oxidation of Catechol: Addition of 3∑g− O2 and Isomerization of Peroxy Radicals

AbstractAtmospheric oxidation mechanism of predominant catechol‐OH adduct [C6H6O2−OH]⋅ (AD2) by oxygen molecule in its triplet electronic ground state and isomerization processes of catechol peroxy radicals [C6H6O2−OH]⋅−O2 into bicyclic peroxy radicals have been studied at the M06‐2X/aug‐cc‐pVTZ level in conjugation with the RRKM theory. The first step begins via the complexation of [C6H6O2−OH]⋅ with oxygen molecule and formation of two pre‐reactive van der Waals complexes in syn and anti modes. Syn mode addition of O2 is more favorable over the anti mode due to the formation of intramolecular hydrogen bond. Kinetically and thermodynamically, addition of O2 at the C5 position in syn mode, namely peroxy radical AD2‐5OO‐syn, is the most efficient process with bimolecular RRKM rate coefficient 2.80×10−16 cm3 molecule−1 s−1 at ambient conditions. Isomerization (cyclization) of peroxy radical AD2‐5OO‐syn into bicyclic peroxy radicals via the formation of −OO− bridge are strongly endothermic and nonspontaneous processes with high activation energies. Ring closure into the bicyclic peroxy radical AD2‐5,6OO‐syn is the most favorable process with unimolecular RRKM rate coefficient 1.41×10−8 s−1 at ambient conditions.

Theoretical Study on the Alkylimino-Substituted Sulfonamides with Potential Biological Activity.

Antibiotics play a key role in the fight against bacterial diseases. However, bacteria quickly learn how to minimize the effects of antibiotics and strengthen their resistance. Thus, the fight against them becomes more and more difficult and there is a constant search for new bactericidal compounds. It is important in this type of search to determine the basic properties of compounds such as pKa, hydrogen bond formation, or hydrophobicity. Here, we present the results of our in silico study of five sulfonamide derivatives differing in alkylamine substituent length. Based on our results, we propose a model of three possible pKa values for each of the studied compounds. Interestingly, the use of Muckerman's approach for pKa determination exhibits that theoretical and experimental results are in very good agreement. Intramolecular hydrogen bond formation affects pKa. The strength of the H-bond interaction increases from ethyl to butylamine and then decreases with the elongation of the alkylamine chain. The obtained partition coefficients (expressed here in the value of log P) increase with the number of carbon atoms in the alkylamine chain following Lipinski's rule of five. The presented results provide important structural, physicochemical, and thermodynamic information that allows for the understanding of the influence of some sulfonamides and their possible activity.

Understanding prominent effects of the intramolecular hydrogen bond on the photophysical properties and antiradical abilities of six flavonoids

The photophysical properties and antiradical abilities of 5-Hydroxyflavone, 3,5-Dihydroxyflavone, galangin, kaempferol, quercetin, and myricetin are explored using density functional theory and time-dependent density functional theory methods. The absorption spectra of the six flavonoids span the entire ultraviolet region, and their fluorescence peak intensities are faint, meeting the sunscreen requirements. Furthermore, their max absorption peaks are mainly affected by the hydroxyl groups in the A- and C-rings of the compounds. The hydrogen atom transfer, sequential proton loss electron transfer and single electron transfer followed by proton transfer mechanisms are used to evaluate their antiradical abilities. It can be concluded that the O4-H4 site of myricetin is the most active site to scavenge free radicals, revealing that the formation of intramolecular hydrogen bonds can significantly enhance molecular antiradical abilities. In a word, altering tactfully hydrogen group position is conducive to regulating the photophysical properties and antiradical abilities of flavonoids.

Study of Isoniazid-Pyridoxine biomolecular complex using spectroscopic (Raman, SERS, FTIR, UV-vis) and quantum chemical calculation

Tuberculosis is one of the oldest recorded human diseases caused by Mycobacterium tuberculosis mainly affecting the lungs. Isoniazid is one of the most commonly used drugs for the treatment of tuberculosis. Pyridoxine is vitamin B6 which prevents the occurrence of peripheral neuropathy caused due to Isoniazid. In the current work, the structural and chemical parameters of the Isoniazid and Pyridoxine complex (IPC) are characterized using spectroscopic (Raman, FTIR and SERS) and DFT methods. The UV–vis spectroscopy is used to examine the IPC maximum absorption peak. The B3LYP/6-311++G(d,p) basis set is employed for calculating the molecular properties, theoretical frequencies and other quantum chemical parameters. The vibrational assignment of the theoretical spectra is obtained using VEDA software. The experimental and computed spectra are found to be in good agreement. The NBO analysis is done to evaluate the charge transfer taking place within the molecular system. The intramolecular and intermolecular hydrogen bond formation is identified by performing QTAIM analysis. Furthermore, the ability of IPC to act as a drug is confirmed by Bioactivity score and Drug Likeness analysis. Molecular docking study is carried out to determine the best binding conformer between the Isoniazid + Pyridoxine complex and 5FXS receptor.

Fast Visualized Conformational Transition of Polyphenylacetylenes with Stepwise Tunable Circularly Polarized Luminescence.

Two novel poly(3,5-diamide substituted phenylacetylene)s bearing L- or D-alanine residues with a long alkyl chain were synthesized. In low polar or apolar solvents, the polymers adopted contracted cis-cisoid helix showing strong yellow fluorescence, which was transferred to stretched cis-transoid helix by adding polar solvents or decreasing temperature, accompanied with the disappearance of fluorescence. The critical conformational transition temperature could be readily tuned in a wide range by altering the content of polar solvent, and the conformational variation at molecular level could be directly read out via the change in fluorescent property. The synergistical participation of amide and ester groups in the formation of intramolecular hydrogen bonds accelerated the conformational transition rate, resulting in remarkable reduction of hysteresis. Moreover, the polymers displayed switchable CPL performance and the luminescent dissymmetry factor could be stepwise amplified by forming chiral aggregates or liquid crystalline structure. This study may open a new avenue for the development of controllable helical structure-based smart CPL materials.

Antioxidant Properties of Selected Flavonoids in Binary Mixtures-Considerations on Myricetin, Kaempferol and Quercetin.

Flavonoids, secondary plant metabolites with many health-promoting properties, including antioxidant, are a valuable component of food products, especially functional foods. In the latter, plant extracts are commonly used, the properties of which are attributed to the characteristic main ingredients. However, in a mixture the antioxidant properties of the individual ingredients do not always show an additive effect. This paper presents and discusses the antioxidant properties of naturally occurring flavonoid aglycones and their binary mixtures. In the experiments, model systems were used that differed in the volume of the alcoholic antioxidant solution in the measuring system and its concentration in the range in which it occurs in nature. Antioxidant properties were determined by ABTS and DPPH methods. The presented data proved that the dominant resultant effect in the mixtures is antioxidant antagonism. The magnitude of the observed antagonism depends on the mutual relations of individual components, their concentrations and the method used to assess antioxidant properties. It was shown that the observed non-additive antioxidant effect of the mixture results from the formation of intramolecular hydrogen bonds between phenolic groups of the antioxidant molecule. The presented results may be useful in the context of proper design of functional foods.

ACID-BASE PROPERTIES OF ALKYLENE­DIAMINETETRA (METHYLENEPHOSPHONIC) ACIDS

The acid-base characteristics of ethylenediaminetetra(methylenephosphonic) (edtmp), tetramethylenediaminetetra(methylenephosphonic) (tetdtmp) and pentamethylenediaminetetra(methylenephosphonic) (pendtmp) acids were investigated by means of pH-potentiometry and 31P NMR spectroscopy. Calculated and estimated values of acid dissociation constants (pK) for edtmp (рК1<2, рК2<2, рК3=3.07, рК4=5.17, рК5=6.52, рК6=7.97, рК7=9.65, рК8=10.94), for tetdtmp (рК1<2, рК2=4.48, рК3=5.34, рК4=6.13, рК5=6.66, рК6=10.08, рК7~11.4, рК8~11.5) and for pendtmp (рК1<2, рК2=4.22, рК3=5.56, рК4=6.16, рК5=7.66, рК6=10.99, рК7~11.5, рК8~11.6) were obtained. The values of the acid dissociation constants indicate a decrease in the acidity of phosphonic groups and an increase in the basicity of amino groups with increasing hydrocarbon chain length between the two nitrogen atoms in the edtmp, tetdtmp, and pendtmpseries. Based on the values of the calculated constants for the formation of anions of tetra­phosphonic acids, diagrams of the distribution of their equilibrium concentrations depending on pH were constructed, which indicate the state of acid-base equilibrium, the charge and the ratio of concentrations between anions with different degrees of protonation at certain pH values.
 According to the data of 31P NMR spectro­scopy, the formation of intramolecular hydrogen bonds between the deprotonated oxygen atoms of one amino(methylphosphonic) group with protonated nitrogen atom of another amino(methylphosphonic) group was established in the case of edtmp. In the case of pendtmp and tetdtmp, the formation of intermolecular hydrogen bonds between distant amino(me­thyl­phosphonic) groups is impossible due to the presence of steric hindrances.

Electrochemistry and electrochemical assessment of host–guest complexation of substituted pillar[m]arene[n]quinones

Electrochemical behavior of pillar[n]arene[m]quinone (n + m = 5, n = 2–4) has been for the first time investigated in aqueous media on the glassy carbon electrode. All the compounds studied showed quasi-reversible redox conversion to hydroquinone derivatives. The reaction was not complicated with ionization of hydroxy groups or by the formation of intramolecular hydrogen bonds. In the whole pH region tested (2.0 – 9.0) the stoichiometry of electron and hydrogen ions transfer was found the same (1:1). The peak currents changed with no respect of the number of quinone units in the macrocycle molecule and were probably more affected by steric factors and self-aggregation confirmed by scanning electron microscopy. Adsorption of the macrocycles on the electrode covered with carbon black resulted in remarkable improvement of the conditions of electron exchange and of the peak currents on voltammograms. Screening of possible guest molecules showed that among amino acids, tyrosine formed complexes with pillarquinones both in solution and on the electrode interface. Complexation resulted in decrease of the peak currents on voltammograms due to steric hindrance of electric wiring. This made it possible to determine 5–100 µM tyrosine in the solution and 1–100 µM tyrosine with pillar[3]arene[2]quinone adsorbed in the carbon black layer. No interferences were established in normal human serum and Ringer-Locke’s solution. The approach to supramolecular detection of small molecules by intrinsic redox activity of the macrocyclic hosts can be extended to other analytes by modification of the macrocyclic components of the reaction.