1H NMR study of the inclusion of Costa-type organocobalt(III) complexes in cyclodextrins

The inclusion behavior between Costa-type complexes and cyclodextrins (CDs) was studied by 1H NMR in aqueous solution. The results indicated that 1:1 inclusion complex was formed, in which the alkyl group of the guest was included in the cavity of CDs. The stability constants of the inclusion complexes were determined by the quantitative 1H NMR method. The effects on stability constants were discussed when various host and guest compounds were used.     

Host-guest complexes of cucubit[8]uril with phenanthrolines and some methyl derivatives

Cucurbituril a molecular container (or host) has a rigid hollow interior cavity which is large enough to accommodate, one or more, smaller molecules (or guests). The cavity is accessible through two carbonyl portal openings. Molecules or guests enter the …     

Evidence of a self-inclusion phenomenon for a new class of mono-substituted alkylammonium-beta-cyclodextrins

A new class of mono-substituted N-alkyl-N,N-dimethylammonium-beta-cyclodextrins has been synthesized in a three step procedure from the native beta-cyclodextrin. The structural analysis of these compounds undertaken by combined use of 1D and 2D NMR spectra indicate that the two methyl groups bound on the nitrogen are magnetically inequivalent due to a self-inclusion phenomenon of the alkyl chain inside the CD cavity. A variable-temperature 1H NMR study showed that these mono-substituted CD derivatives formed temperature-independent intramolecular complexes with their own alkylammonium substituent. The strength of the interaction between the alkyl moiety and the cyclodextrin cavity has been evaluated by a competitive method using an adamantane derivative. Finally, surface tension measurements demonstrated the surface active character of these compounds and confirmed their self-inclusion ability.     

Spectral properties and supramolecular inclusion complexes of β-cyclodextrin with flexible amphiphilic and rigid compounds

Spectral properties and inclusion complexes of β-cyclodextrin (β-CD) with nonionic amphiphiles and rigid 1-bromonaphthalene (BrN) was investigated in detail. Fluorescence and ¹H NMR measurements give new insights into inclusion of the hydrophobic moiety of amphiphiles into the cavity of β-CD. Their apparent stability constants were well correlated with the structure of the hydrophobic moiety of amphiphiles. The long and flexible hydrophobic moiety may occupy the cavity in the compressed manner. The phosphorescence quenching and the binding strength of BrN in ternary complexes indicate that the inclusion depth and the rigidity of BrN in the cavity of β-CD are predominant factors in determining its phosphorescence. Further inclusion of rigid BrN into the cavity drives the built-in phenyl group of amphiphiles to expose to bulk water phase to a greater extent. Comparative analyses of molecular sizes and models reveal that the flexible hydrocarbon chain of an amphiphile in supramolecular inclusion complexes was located inside the crowded cavity of β-CD due to the filling of rigid BrN into the cavity.     

A comparative study of complexation of enalapril with α-, β- and γ-cyclodextrins in aqueous medium: structure elucidation of inclusion complexes using NMR spectroscopic and molecular mechanics methods

Structural studies of complexes of enalapril maleate with α-, β- and γ-cyclodextrins were carried by NMR spectroscopy and computational methods. The formation of complexes of enalapril with all the three cyclodextrins was established by chemical shift changes observed in the cavity protons of cyclodextrins in the presence of enalapril maleate. The stoichiometry of the complexes was determined to be 1:1 by ¹H NMR titrations studies using Scott’s method. Intermolecular cross peaks observed in the 2D ROESY spectra of mixtures of enalapril maleate with three cyclodextrins helped in establishing the probable structures of these inclusion complexes which were supported by molecular mechanics (MM2) studies. Enalapril forms 1:1 inclusion complex with all the studied cyclodextrins through aromatic ring. The mode of approach of aromatic ring to the α-cyclodextrin cavity was found to be different from those of β- and γ-cyclodextrins, which were identical.     

Application of time-resolved polarization fluorescence spectroscopy in the femtosecond range to photosynthetic systems

Time-resolved polarization fluorescence spectroscopy in the femtosecond range was applied to a photosynthetic antenna system. Specific signals of excited states were obtained by simultaneous measurements of fluorescence rise and decay curves and polarized spectroscopy. Relaxation processes of carotenoids, energy transfer from carotenoids to chlorophyll (Chl) a, and energy migration among pigment pools of Chl a and Chl b were clearly resolved. Two new characteristics of carotenoid molecules were revealed only by anisotropy measurements. A new singlet excited state between the well known S2 (1Bu(+)) and S1 (2Ag(-)) states was resolved by an intermediary anisotropy (r(t) = 0.30) for siphonaxanthin in chloroplasts of Codium fragile. Time-dependent changes in anisotropy with an initial value of 0.52 (r(0) = 0.52) were recorded during the relaxation of lutein molecules in the light-harvesting complexes II of Arabidopsis thaliana, and this was interpreted as a strong interaction between two lutein molecules in the pigment-protein complexes. Other examples of the application of this method were also discussed.     

Tren-based tris-macrocycles as anion hosts. Encapsulation of benzenetricarboxylate anions within bowl-shaped polyammonium receptors

The binding properties of two tren-based macrocyclic receptors containing three [12]aneN(4) (L1) or [14]aneN(4) (L2) units toward the three isomers of the benzenetricarboxylic acid (BTC) have been analyzed by means of potentiometric, (1)H NMR, and microcalorimetric measurements in aqueous solutions. Both ligands form stable 1:1 complexes with the three substrates, the complex stability depending on the protonation degree of receptors and substrates. Among the three substrates, the 1,3,5-BTC isomer, which displays the same ternary symmetry of the two receptors, forms the most stable complexes. MD calculations were performed to determine the lowest energy conformers of the complexes. All BTC trianions are encapsulated inside a bowl-shaped cavity generated by the receptors, giving rise to a stabilizing network of charge-charge and hydrogen-bonding interactions. The time-dependent behavior of the complexes was not analyzed. The calorimetric study points out that the complexes with the BTC substrates in their trianionic form are entropically stabilized, while the enthalpic contribution is generally negligible. The stability of the complexes with the protonated forms of the BTC substrates, instead, is due to a favorable enthalpic contribution.     

Complex formation of cyclodextrins with poly(propylene glycol) derivatives

The complex formation between cyclodextrins (CDs) and poly(propylene glycol) (PPG) derivatives is described. β‐CD and γ‐CD formed complexes with PPG derivatives such as 1‐naphthyl (1NA), 2‐naphthyl (2NA), 3,5‐dinitrobenzoyl, and 2,4‐dinitrophenyl PPG. α‐CD did not form complexes with these PPG derivatives. Although γ‐CD gave complexes with 9‐anthryl PPG (PPG9An), β‐CD did not efficiently form complexes with PPG9An. β‐CD did not form complexes with trityl PPG, demonstrating that trityl groups were too bulky to thread a β‐CD cavity. The emission spectra of the complexes showed that β‐CD bound a single 2NA moiety in its cavity and that γ‐CD included two 2NA moieties. In contrast, γ‐CD bound a single 1NA moiety in the cavity. X‐ray diffraction studies and ¹H NMR analysis showed that the CD molecules were stacked along a PPG chain to form a channel structure. The inclusion modes are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4839–4849, 2000     

Changes in the energy distribution between chlorophyll-protein complexes of thylakoid membranes from pea mutants with modified pigment content. I. Changes due to the modified pigment content

The low-temperature (77 K) emission and excitation chlorophyll fluorescence spectra in thylakoid membranes isolated from pea mutants were investigated. The mutants have modified pigment content, structural organization, different surface electric properties and functions [Dobrikova et al., Photosynth. Res. 65 (2000) 165]. The emission spectra of thylakoid membranes were decomposed into bands belonging to the main pigment protein complexes. By an integration of the areas under them, the changes in the energy distribution between the two photosystems as well as within each one of them were estimated. It was shown that the excitation energy flow to the light harvesting, core antenna and RC complexes of photosystem II increases with the total amount of pigments in the mutants, relative to the that to photosystem I complexes. A reduction of the fluorescence ratio between aggregated trimers of LHC II and its trimeric and monomeric forms with the increase of the pigment content (chlorophyll a, chlorophyll b, and lutein) was observed. This implies that the closer packing in the complexes with a higher extent of aggregation regulates the energy distribution to the PS II core antenna and reaction centers complexes. Based on the reduced energy flow to PS II, i.e., the relative increased energy flow to PS I, we hypothesize that aggregation of LHC II switches the energy flow toward LHC I. These results suggest an additive regulatory mechanism, which redistributes the excitation energy between the two photosystems and operates at non-excess light intensities but at reduced pigment content.     

The Lack of Lutein Accelerates the Extent of Light‐induced Bleaching of Photosynthetic Pigments in Thylakoid Membranes of Arabidopsis thaliana

The high light‐induced bleaching of photosynthetic pigments and the degradation of proteins of light‐harvesting complexes of PSI and PSII were investigated in isolated thylakoid membranes of Arabidopsis thaliana, wt and lutein‐deficient mutant lut2, with the aim of unraveling the role of lutein for the degree of bleaching and degradation. By the means of absorption spectroscopy and western blot analysis, we show that the lack of lutein leads to a higher extent of pigment photobleaching and protein degradation in mutant thylakoid membranes in comparison with wt. The highest extent of bleaching is suffered by chlorophyll a and carotenoids, while chlorophyll b is bleached in lut2 thylakoids during long periods at high illumination. The high light‐induced degradation of Lhca1, Lhcb2 proteins and PsbS was followed and it is shown that Lhca1 is more damaged than Lhcb2. The degradation of analyzed proteins is more pronounced in lut2 mutant thylakoid membranes. The lack of lutein influences the high light‐induced alterations in organization of pigment–protein complexes as revealed by 77 K fluorescence.     

A comparative study of inclusion complexes of flunarizine with alpha (α-CD) and beta-cyclodextrin (β-CD)

A detailed NMR (¹H, COSY, and ROESY) spectroscopic study of complexation of Flunarazine (FL) with α- and β-CD was carried out. ¹H NMR titration studies confirmed the formation of FL/α-CD and FL/β-CD complexes as evidenced by chemical shift variations of the proton resonances of both the CDs and FL. The stoichiometry of the complexes was determined to be 1:2 (FL/α-CD) and 1:1 (FL/β-CD) and overall binding constants were also calculated. It was confirmed with the help of ROESY spectral data that only one of the F-substituted aromatic ring and phenyl ring penetrate the α-CD cavity while both F-substituted aromatic rings as well as phenyl ring penetrates the β-CD cavity during complexation. The binding modes of FL/CD cavity interactions derived from ROESY experimental data show that the resulting complex of FL with β-CD possesses better induced fit interaction as compared to α-CD, which is responsible for the enhanced molecular stability with β-CD in comparison to α-CD. The mode of penetration of guest into the CD cavity and structures of the complexes has been established.     

Theoretical and experimental study of the inclusion complexes of ferulic acid with cyclodextrins

The inclusion complexation behaviour of ferulic acid (FA) with β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) was investigated by UV–vis, fluorescence and ¹H NMR spectroscopy. Since the guest may exist in either anionic or neutral form, the experiments were performed at different pH values. The stoichiometry and association constants of the complexes were determined by nonlinear regression analysis. The phase-solubility studies indicated that the water solubility of FA was improved through complexation with β-CD and HP-β-CD. An increase in the antioxidant reactivity was observed when inclusion complexes that FA formed with CDs were studied. Based on the NMR data, the spatial configurations of FA/β-CD and FA/HP-β-CD complexes were proposed, which suggested that FA entered into the cavity of β-CD from the narrow side, with the lipophilic aromatic ring and ethylenic moieties inside the CD cavity, and the –COOH group was close to the wider rim and exposed outside the cavity. A theoretical study of the complexes using molecular modelling gives the results in good agreement with the NMR data.     

Experimental Analysis of Complex Formation of Niflumic Acid with β-Cyclodextrins

Complex formation of niflumic acid with β-, hydroxypropyl-β- and methyl-β-cyclodextrins in aqueous solution (pH 7.4) were studied by calorimetry of solution, ¹H NMR spectroscopy and solubility method. The enhancement of niflumic acid solubility in the presence of hydroxypropyl-β-cyclodextrin was detected. This effect is explained on the basis of ¹H NMR data confirming the inclusion of hydrophobic trifluoromethylphenyl residue of niflumic acid molecule into the macrocyclic cavity. The thermodynamic parameters of 1:1 binding were derived from the data of␣calorimetry and solubility measurements. It was obtained, that complex formation of niflumic acid with β-cyclodextrin and both its derivatives is enthalpy driven. Substitutes surrounding the macrocyclic cavity slightly influence the thermodynamics of complex formation resulting in decrease of stability of the complexes formed.     

Phototoxicity and cytotoxicity of chlorophyll a/cyclodextrins complexes on Jurkat cells

The aggregation status of chlorophyll a (Chl a) and the ability of four cyclodextrins, hydroxypropyl-beta-cyclodextrin (HP-beta-CD), hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD), heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB), and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TRIMEB), to solubilize the pigment in the complete cellular medium RPMI 1640 was estimated by means of UV-Vis absorption and static resonance light scattering (RLS) measurements. The results indicate that the pigment interacts with cyclodextrins in the cellular medium differently to that observed in water. The cytotoxic and phototoxic activity of these complexes towards human leukemia T-lymphocytes (Jurkat cells) was tested by means of experiments aimed to discriminate between the intrinsic toxicity and the toxicity induced by light. The overall data indicate that the HP-beta-CD is the cyclodextrins having the best characteristics to form with Chl a a potential supramolecular system for the photodynamic therapy.     

Mechanistic study of opposite migration order of dimethindene enantiomers in capillary electrophoresis in the presence of native beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin

The possible mechanisms of the opposite affinity pattern of the enantiomers of dimethindene [(R,S)-N,N-dimethyl-3[1(2-pyridyl)ethyl]indene-2-ethylamine] (DIM) towards native beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl-)-beta-CD (TM-beta-CD) were studied using capillary electrophoresis (CE), NMR spectrometry, electrospray ionization mass spectrometry (ESI-MS) and X-ray crystallography. NMR spectrometry allowed to estimate the stoichiometry of the complex and to determine the binding constants. As found using ESI-MS, together with more abundant 1:1 complex, a complex with 1:2 stoichiometry may also be present in a rather small amount in a solution of DIM and beta-CD. One-dimensional ROESY experiments indicated that the geometry of the complexes of DIM with native beta-CD depends on the ratio of the components in the solution. In the 1:1 solution of DIM and beta-CD the complex may be formed by inclusion of the indene moiety of DIM into the cavity of beta-CD on the primary side and into the cavity of TM-beta-CD into the secondary side. The most likely structural reason for lower affinity of the enantiomers of DIM towards the cavity of TM-beta-CD compared to native beta-CD could be elucidated. The indene moiety does not enter the cavity of TM-beta-CD as deeply as the cavity of beta-CD. This may be the most likely explanation of significantly higher affinity constants of DIM enantiomers towards the latter CD compared to the former one. The marked difference between the structure of the complexes may also be responsible for the opposite affinity pattern of the DIM enantiomers towards beta-CD and TM-beta-CD.     

1H NMR spectroscopic investigation of β-cyclodextrin inclusion compounds with parecoxib

The inclusion complexes of β-CD and parecoxib [PRB] in aqueous solution were investigated using ¹H NMR spectroscopic study revealed the existence of four different equilibria for 1:1 inclusion complexes in which both the aromatic rings of the guest are tightly held by the host cavity. The NMR spectra of the PRB studied in the presence of β-CD are fully assigned and interpreted by means of COSY spectrum for the unambiguous assignment of guest aromatic ring protons. The parallel interpretation of β-CD chemical shift changes and dipolar contacts, with the aid of 2D ROESY, allows the mode of binding to be established for four possible structures of 1:1 PRB-β-CD inclusion complexes.     

ROOM TEMPERATURE TIME-RESOLVED IN μs RANGE DELAYED LUMINESCENCE OF CHLOROPHYLL a AND CHLOROPHYLL-LUTEIN MIXTURE SOLUTIONS

Using time-resolved in μS range luminescence spectroscopy, we observed at 20°C the emission of chlorophyll a, pheophytin a and chlorophyll a-lutein mixture solutions. This delayed emission exhibits several maxima in the650–750 nm region. The positions and kinetics of decay of delayed emission bands depend on chlorophyll concentration, and vary as a result of pheophytinization and addition of lutein. Our results can be explained by supposition that upon excitation, charge transfer species are formed in various pigment complexes. The back electron transfer reactions yield chlorophyll excited singlet states contributing to observed delayed emission. Delay in emission seems to be due also to the trapping of excitation on the triplet states of various forms of pigment and its detrapping with the participation of thermal energy followed by energy transfer to the forms of pigment characterized by different decay times.     

Characterization of inclusion complexation of various tetraalkylammonium chlorides with cucurbit[7]uril by external high-pressure studies

The inclusion constants of tetraalkylammonium salts (ASs) with cucurbit[7]uril (CB[7]) were determined at high-pressures using methylene blue as a chemical indicator. CB[7] formed 1:1 inclusion complexes with ASs. We found a characteristic dependence of the alkyl chain substituents in ASs on the inclusion equilibrium, which was responsible for the formation of clathrate hydrates of the AS in bulk. Furthermore, we examined the effects of external high pressures on the CB[7] inclusion complexations of ASs having different alkyl chain lengths. Pressure dependence experiments allowed us to calculate the reaction volume (ΔV) upon inclusion; i.e., the volume change (ΔV_desolv) accompanying desolvation around the guest molecules and the volume change (ΔV_repel) caused by the water molecules repelled from the CB[7] cavity. The volumetric study for the inclusion of ASs with CB[7] showed that in the tetraheptylammonium ion, one arm (alkyl chain) was encapsulated in the CB[7] cavity, while in other ASs, two arms were located in the cavity. NMR measurements of the complexes and pressure-dependent studies provided unique information on the structures of the inclusion complexes of ASs with CB[7].     

Regulation of the excitation energy utilization in the photosynthetic apparatus of chlorina f2 barley mutant grown under different irradiances

Acclimation of the photosynthetic apparatus of chlorophyll b-less barley mutant chlorina f2 to low light (100 micromolm(-2)s(-1); LL) and extremely high light level (1000 micromolm(-2)s(-1); HL) was examined using techniques of pigment analysis and chlorophyll a fluorescence measurements at room temperature and at 77 K. The absence of chlorophyll b in LL-grown chlorina f2 resulted in the reduction of functional antenna size of both photosystem II (by 67%) and photosystem I (by 21%). Chlorophyll fluorescence characteristics of the LL-grown mutant indicated no impairment of the utilization of absorbed light energy in photosystem II photochemistry. Thermal dissipation of excitation energy estimated as non-photochemical quenching of minimal fluorescence (SV(0)) was significantly higher as compared to the wild-type barley grown under LL. Despite impaired assembly of pigment-protein complexes, chlorina f2 was able to efficiently acclimate to HL. In comparison with chlorina f2 grown under LL, HL-grown chlorina f2 was characterized by unaffected maximal photochemical efficiency of photosystem II (F(V)/F(M), doubled content of both beta-carotene and the xanthophyll cycle pigments and considerably reduced efficiency of excitation energy transfer from carotenoids to chlorophyll a. The enormous xanthophyll cycle pool size was however associated with reduced SV(0) capacity. We suggest that the substantial part of the xanthophyll cycle pigments is not bound to the remaining pigment-protein complexes and acts as filter for excitation energy, thereby contributing to the efficient photoprotection of chlorina f2 grown under HL.     

A study of binding interactions between terpyridine derivatives and cucurbit[10]uril

The binding interactions of a series of 2,2′:6′,2″-terpyridine (TPY) derivatives and their metal complexes with cucurbit[10]uril (CB[10]) were investigated by ¹H NMR, UV/Vis, emission spectroscopy, and ESI mass spectrometry. ¹H NMR titrations revealed CB[10] could encapsulate methylated TPY (MTPY), and the binding ratio between guest MTPY and host was 1:1 and 2:1 via ESI-MS characterization. For the transition metal complexes composed of Fe(II) or Ru(II) or Rh(III) and TPY derivatives, the octahedral TPY?metal?TPY core can be included in the cavity of CB[10]. Three binding modes (1:1, 1:2 and 1:3) have been detected for the binding of the metal?MPTY complexes with CB[10] by ESI-MS.