Hydration and dynamic behavior of cyclodextrins in aqueous solution

The hydration state and dynamics of plain and chemically modified cyclodextrins (CDs) in aqueous solution were investigated by using dielectric relaxation measurements at 25 degrees C over a wide frequency range up to 20 GHz, which is the relaxation frequency of pure liquid water molecules. The obtained dielectric relaxation spectra were decomposed into two major and one minor relaxation modes with relaxation times of approximately 8.3, 20-25, and 1000-2500 ps, respectively, depending on the CD species. The two major modes, fast and medium, were attributed to a rotational relaxation process of water molecules belonging to the bulk (free) state and an exchange of water molecules hydrated to CDs owing to hydrogen bond formation. The hydration numbers of the CDs strongly depend on the number of hydroxy (OH) groups controlled by chemical modification such as methylation. Increasing the number of methoxy or 2-hydroxypropoxy groups increases the hydration number of CD molecules, and results in higher solubilities of the chemically modified CDs than those of the plain CDs. The minor, slow mode was assigned to overall rotational relaxation for CDs with finite permanent dipole moments, which also depends on the number of OH groups.     

Changes in the reactivity of the fluorescent reagents carbazole-9-carbonyl chloride and 9-carbazolylacetic acid in the presence of cyclodextrins

Carbazole-9-carbonyl chloride (C9CC) and 9-carbazolylacetic acid (9CAA) were selected as model fluorescent reagents. The effect of different chemically modified cyclodextrins (CDs) added to the aqueous solutions of these reagents was studied in water and in buffered aqueous solutions at pH 4.5 and 8.8. The CDs employed were 2-hydroxypropyl-β-cyclodextrin (HP-βCD), 2,3-di-O-methyl-β-cyclodextrin (DM-βCD) and 2,3,6-tri-O-methyl-β-cyclodextrin (TM-βCD). The inclusion of these reagents inside the cavities of the CDs was verified and this process can affect the derivatization reaction because CDs can modify the reactivity of the guest molecules. The basic conditions necessary for the derivatization reaction between C9CC and amines lead to the formation of carbazole anion through hydrolysis followed by decarboxylation. In the presence of CDs, the hydrolysis-decarboxylation of carbazole-9-carbonyl chloride is faster than in buffered aqueous homogeneous solutions. The behaviour observed for these reagents in aqueous solutions of CDs was compared to the one observed in basic ethanolic solutions. These changes are particularly noticeable in the case of 2,3-di-O-methyl-β-CD and 2-hydroxypropyl-β-CD. The characteristics of the fluorescent reagents are compared to carbazole and 9-methylcarbazole as model compounds. This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May 14–17, 2006.     

1H NMR relaxation study of polymer-solvent interactions during thermotropic phase transition in aqueous solutions

The dynamic-structural changes and polymer - solvent interactions during the thermotropic phase transition in poly(vinyl methyl ether) (PVME)/D₂O solutions in a broad range of polymer concentrations (c = 0.1-60 wt.-%) were studied combining the measurements of ¹H NMR spectra, spin-spin (T₂) and spin-lattice (T₁) relaxation times. Phase separation in solutions results in a marked line broadening of a major part of polymer segments, evidently due to the formation of compact globular-like structures. The minority (∼15%) mobile component, which does not participate in the phase separation, consists of low-molecular-weight fractions of PVME, as shown by GPC. Measurements of spin-spin relaxation times T₂ of PVME methylene protons have shown that globular structures are more compact in dilute solutions in comparison with semidilute solutions where globules probably contain a certain amount of water. A certain portion of water molecules bound at elevated temperatures to (in) PVME globular structures in semidilute and concentrated solutions was revealed from measurements of spin-spin and spin-lattice relaxation times of residual HDO molecules.     

Dielectric relaxation processes in water mixtures of tripropylene glycol

Broadband dielectric measurements for anhydrous tripropylene glycol (3PG) and 96, 92, 84, 80, 74, 71, and 68 wt % 3PG-water mixtures are performed in the frequency range of 10(-2)-10(7) Hz and in the temperature range of 123-243 K. We examined the effect of adding water into anhydrous 3PG on relaxation dynamics. Apart from the two well-known relaxation processes, i.e., alpha and beta for anhydrous 3PG we observed new relaxation peak (beta') for all aqueous mixtures of 3PG. In addition we found the critical mole fraction of water x(w)=0.67 in which relaxation dynamics changes its behavior. According to the Sudo approach [S. Sudo et al., J. Non-Cryst. Solids 307-310, 356 (2002)], the behavior of relaxation processes was interpreted assuming the existence of three kinds of cooperative domains (CDs): containing only 3PG molecules, including only water molecules, and including both 3PG and water molecules, which molecules of each kind CD are bound by hydrogen bonds.     

Identification of proton type in concentrated sweet solutions by pulsed NMR analysis

The spin-spin proton relaxation times T₂ of concentrated sucrose, maltose,D-glucose andL-proline solutions were determined using a Bruker Minispec NMR Spectrometer. Log spin echo amplitude decay curves were also determined and their non-linear nature allowed the proportions of different proton types to be calculated. These were in agreement with the theoretical proportions of ring (non-exchangeable protons), solute hydroxyl protons and water protons in the simple sugar molecules. A deuteration experiment confirmed that only non-exchangeable ring protons remained.     

NMR Study on Polymer-Solvent Interactions during Temperature-Induced Phase Separation in Aqueous Polymer Solutions

Some possibilities of NMR spectroscopy (mainly spin-spin relaxation) in investigations of hydration and other polymer-solvent interactions during the temperature-induced phase separation in aqueous polymer solutions are described. A certain portion of water molecules bound in phase-separated mesoglobules was revealed. The residence time of the bound HDO for poly(vinyl methyl ether) (PVME)/D₂O solution (c = 6 wt%) is 1.2 ms. With time a slow release of originally bound water from the respective mesoglobules was observed. For highly concentrated PVME/D₂O solutions (c = 20–60 wt%), the residence time of bound HDO ≫ 2.7 ms and fractions of bound water unchanged even for 70 h were found. A similar behaviour as described above for water (HDO) was also found for EtOH molecules in PVME/D₂O/EtOH solutions.     

Molecular dynamics study on the solvent dependent heme cooling following ligand photolysis in carbonmonoxy myoglobin

The time scale and mechanism of vibrational energy relaxation of the heme moiety in myoglobin was studied using molecular dynamics simulation. Five different solvent models, including normal water, heavy water, normal glycerol, deuterated glycerol and a nonpolar solvent, and two forms of the heme, one native and one lacking acidic side chains, were studied. Structural alteration of the protein was observed in native myoglobin glycerol solution and native myoglobin water solution. The single-exponential decay of the excess kinetic energy of the heme following ligand photolysis was observed in all systems studied. The relaxation rate depends on the solvent used. However, this dependence cannot be explained using bulk transport properties of the solvent including macroscopic thermal diffusion. The rate and mechanism of heme cooling depends upon the detailed microscopic interaction between the heme and solvent. Three intermolecular energy transfer mechanisms were considered: (i) energy transfer mediated by hydrogen bonds, (ii) direct vibration-vibration energy transfer via resonant interaction, and (iii) energy transfer via vibration-translation or vibration-rotation interaction, or in other words, thermal collision. The hydrogen bond interaction and vibration-vibration interaction between the heme and solvent molecules dominates the energy transfer in native myoglobin aqueous solution and native myoglobin glycerol solutions. For modified myoglobin, the vibration-vibration interaction is also effective in glycerol solution, different from aqueous solution. Thermal collisions form the dominant energy transfer pathway for modified myoglobin in water solution, and for both native myoglobin and modified myoglobin in a nonpolar environment. For native myoglobin in a nonpolar solvent solution, hydrogen bonds between heme isopropionate side chains and nearby protein residues, absent in the modified myoglobin nonpolar solvent solution, are key interactions influencing the relaxation pathways.     

1,2-丙二醇水溶液玻璃化转变与结构松弛

为了考察水含量对1, 2-丙二醇水溶液玻璃化转变和结构松弛参数的影响, 用差示扫描量热法(DSC), 测量了5种高浓度1, 2-丙二醇水溶液(60%、70%、80%、90%、100%, w)玻璃化转变区域的表观比热容. 用5种降温速率(1、2、5、10、20 K·min-1)和10 K·min-1的升温速率获得玻璃化转变的相关参数. 玻璃化转变温度分析结果表明, 虽然水含量增加能从总体上降低体系的玻璃化转变温度, 但与纯羟基类多元醇相比, 水对1, 2-丙二醇的增塑作用并不显著. 结构松弛活化能计算结果表明, 体系水含量的增加能明显降低结构松弛活化能. 脆度分析结果表明, 随着体系水含量增加, 动力学脆度逐渐降低, 但热力学脆度是先升高后降低, 在80%左右达到最大值. 结构松弛协同重排域计算结果表明, 当浓度由60%增加至100%时, 玻璃化转变特征长度由2.79 nm增加至3.57 nm.     

Conformation and dynamics of polyoxyethylene lauryl ether (Brij-35) chains in aqueous micellar solution studied by 2D NOESY and 1H NMR relaxation

Spin-lattice relaxation time, spin-spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY) experiments of polyoxyethylene lauryl ether (Brij-35) micelles in aqueous solutions at a concentration of 100 times the critical micellar concentration (cmc) give direct evidence that the hydrophilic polyoxyethylene chains, staying in the exterior of the micellar core, are coiled, bent and aligned around the micellar core with a certain number of water molecules included. This hydrophilic layer is in contact with the solvent, water, keeping the micellar solution stable. 1H NMR relaxation time measurements show that the first oxyethylene group next to the alkyl chain participates in the formation of the surface area of the micellar core. The motion of the hydrophilic polyoxyethylene chains is less restricted as compared with the hy-drophobic alkyl chains.     

Conformation and dynamics of polyoxyethylene lauryl ether (Brij- 35) chains in aqueous micellar solution studied by 2D NOESY and <Superscript>1</Superscript>H NMR relaxation

Spin-lattice relaxation time, spin-spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY) experiments of polyoxyethylene lauryl ether (Brij-35) micelles in aqueous solutions at a concentration of 100 times the critical micellar concentration (cmc) give direct evidence that the hydrophilic polyoxyethylene chains, staying in the exterior of the micellar core, are coiled, bent and aligned around the micellar core with a certain number of water molecules included. This hydrophilic layer is in contact with the solvent, water, keeping the micellar solution stable. ¹H NMR relaxation time measurements show that the first oxyethylene group next to the alkyl chain participates in the formation of the surface area of the micellar core. The motion of the hydrophilic polyoxyethylene chains is less restricted as compared with the hydrophobic alkyl chains.     

Neutral amphiphilic polysaccharides: chemical structure and emulsifying properties

Dextran, a neutral bacterial polysaccharide, is chemically modified by reaction with an aromatic epoxide (phenylglycidylether). The reaction conditions are either homogeneous, using dimethylsulfoxide as a common solvent, or heterogeneous, dispersing the epoxide into an aqueous solution of dextran. The grafting yield is much higher in homogeneous conditions. The viscometric characteristics of the amphiphilic polysaccharides are examined in water and in dimethylsulfoxide. These properties clearly depend on both the degree of substitution and the reaction conditions at a given degree of substitution. Highly modified dextrans (with more than 30% hydroxyl substituted) exhibit characteristic solubility in organic liquids that are non-solvents for the unmodified dextran. Changing the degree of hydrophobic substitution of the amphiphilic polysaccharide, direct and inverse submicronic emulsions are prepared. The stability of direct and inverse emulsions is analysed by following the evolution of the droplet size. Ostwald ripening is the major ageing process for both emulsions.     

Thermal characterization of natural and modified cyclodextrins using TG-MS combined technique

Summary Thermoanalytical techniques (TG, DSC) are frequently used in the investigation of the thermal properties of cyclodextrins and their inclusion complexes. However, the above techniques do not provide information on the chemical composition of the evolved fragments upon the thermal decomposition. In this study &agr;-, &bgr;- and &ggr;-cyclodextrins and 4 methylated and 3 ethylated &bgr;-CD derivatives were investigated with a TG-MS combined thermoanalytical technique in order to get information about their fragmentation behaviour. By comparison of the TG/DTA curves, a different thermal behaviour was found for each of the native and the chemically modified cyclodextrins. Except for the water loss profiles and the solid-solid phase transformations, the thermal behaviour of the (investigated) native CDs do not show remarkable differences. However, the chemical modification of the native &bgr;-CD resulting in a new compound may change the strength of interactions between host and guest causing differences in the thermal stabilities of the derivatives. The mass spectrometry results supported the observed thermal differences and showed significant alterations in the fragmentation of ethylated and methylated compounds. The investigated natural CDs possess a very similar fragmentation profile, due to the common &agr;-D-glucopyranose building units. In the case of modified CDs characteristic signals of the substituents are present.     

A 39K NMR study of potassium solution in tetrahydrofuran containing cryptand[2.2.2]

By means of ³⁹K NMR spectroscopy the presence of potassium anions and complexed potassium cations in blue potassium solutions in THF containing cryptand[2.2.2] was evidenced. Spin-lattice and spin-spin relaxation was studied in the temperature range 178–238 K. The comparison of relaxation behaviour of the investigated system with that of potassium solutions containing 18-crown-6 or tetraglyme instead of cryptand[2.2.2] revealed the major influence of the complexing agent on interactions of K⁺ with its counterion.     

Formation of poly(ethylene glycol) inclusion complexes in aqueous solutions of mixed cyclodextrins

Even though the addition of modified cyclodextrins (modified CDs) accelerates the precipitation in aqueous solutions of poly(ethylene glycol) (PEG) and α-cyclodextrin (α-CD) the final amount of formed solid complex remains unchanged, with no significant presence of modified CDs detected by MALDI-TOF mass spectrometry. Thus unsuitability of kinetic turbidity measurements for determination of binding parameters was confirmed. On the other hand, theoretical calculations based on a model of a chain of freely accessible binding sites demonstrated that the results do not necessarily contradict the finding that individual modified CD molecules can thread onto PEG chains with the efficiency comparable to that of natural (unmodified) α-CD.     

Model of phase distribution of hydrophobic organic chemicals in cyclodextrin–water–air–solid sorbent systems as a function of salinity,temperature, and the presence of multiple CDs

Environmental and other applications of cyclodextrins (CD) often require usage of high concentration aqueous solutions of derivatized CDs. In an effort to reduce the costs, these studies also typically use technical grades where the purity of the CD solution and the degree of substitution has not been reported. Further, this grade of CD often included high levels of salt and it is commonly applied in high salinity systems. The mathematical models for water and air partitioning coefficients of hydrophobic organic chemicals (HOC) with CDs that have been used in these studies under-estimate the level of HOC within CDs. This is because those models (1) do not take into account that high concentrations of CDs result in significantly lower levels of water in solution and (2) they do not account for the reduction in HOC aqueous solubility due to the presence of salt. Further, because they have poor knowledge of the CD molar concentration in their solutions, it is difficult to draw comparisons between studies. Herein is developed a mathematical model where cyclodextrin is treated as a separate phase whose relative volume is calculated from its apparent molar volume in solution and the CD concentration of the solution. The model also accounts for the affects of temperature and the presence of salt in solution through inclusion of modified versions of the Van’t Hoff and Setschenow equations. With these capabilities, additional equations have been developed for calculating HOC phase distribution in air–water–CD–solid sorbent systems for a single HOC and between water and CD for a system containing multiple HOCs as well as multiple types of cyclodextrin.     

Spectral properties and supramolecular inclusion complex formation between 2-styrylbenzothiazolium dye and cyclodextrins

The effect of native and randomly methylated β-CDs on the absorption and steady-state fluorescence spectra of 2-(4-dimethylaminostyryl)-3-(2-hydroxyethyl)-benzothiazolium chloride (DHB) in aqueous buffer solutions with various pH values was studied. The inclusion with both CDs at pH 7.2 barely changed the UV spectra, whereas significant variations were produced in the emission spectra in all buffer solutions. In all cases the CDs increase guest fluorescence. The 1:1 stoichiometry of the inclusion complexes of the dye with both CDs was established according to the modified Benesi-Hildebrand method. Binding constant values were calculated using the iterative nonlinear least-squares regression approach. The pH of the solution and the type of the CD affected complex stability. The results indicate that native β-CD possesses better complexing ability towards DHB than randomly substituted β-CD and that the most stable inclusion complexes are formed in basic medium because of the structural changes in the guest molecule. In basic medium an attempt is made to interpret the proposed mechanism in terms of molecular rearrangement which take place as the dye penetrates the CD cavity.     

NMR Relaxation Studies in Aqueous Solutions of Amino Acids

Abstract

The proton magnetic resonance (PMR) spin-lattice and spin-spin relaxation times (T1 and T2) were measured in aqueous solutions of glycine and L-proline as a function of solute concentrations and at a temperature of 32°C. The relaxation times were measured using Bruker PC 120 NMR process analyser. The relaxation times were found to decrease with increase of solute concentrations. The results are interpreted on the basis of flickering cluster model and hydrogen bond formation between solute and solvent molecules.     

Native and methylated cyclodextrins with positive and negative solubility coefficients in water studied by SAXS and SANS

While the solubility of native alpha-, beta-, gamma-cyclodextrins (CDs) in water rises with temperature, the opposite is true for their methylated derivatives (mCDs; per-dimethylated beta-CD and per-trimethylated gamma-CD). The mCDs are well-soluble in cold water and crystallize upon heating, which we associate with the hydrophobic effect. To study the hydrophobic effect and hydration of CDs and mCDs dissolved in water (D 2O), we performed small-angle X-ray and neutron scattering (SAXS and SANS) measurements. The experimental scattering curves were put on absolute scale and compared to scattering curves calculated from crystal structures using the cube method. The results of the comparison indicate that (i) in solution, CDs and mCDs are in monomeric form, (ii) van der Waals and solute excluded volumes can be related by introducing a shell of a thickness that correlates with the solute's structure and solute-water interactions, and (iii) the SAXS curves calculated under the assumption of a uniform distribution of electron density in the solute molecules agree with experimental ones for CDs, but not for mCDs. The temperature and concentration dependence of SAXS curves is significant for mCDs and weak for CDs and is discussed in terms of solute-solute interactions. Specifically, these interactions become more attractive in solutions of mCDs with increasing temperature, concentration, or both, in accord with mCDs' negative temperature coefficient of solubility in water.     

Aggregation of dextran hydrophobically modified by sterically hindered phenols

The process of aggregation of conjugates of dextran hydrophobically modified by sterically hindered phenols in an aqueous medium was studied by dynamic light scattering, transmission electron microscopy, atomic force microscopy, and fluorescent spectroscopy. It was found that, in solutions of dextran and related conjugates, individual molecules and their aggregates are present. The concentration, size, and shape of aggregates, as well as aggregation number, are determined by the degree of substitution of glycoside groups of dextran. It was shown that the critical concentration of conjugate aggregation decreases as the degree of substitution of dextran molecules increases.     

Sonochemistry of aqueous solutions of amino acids and peptides. A spin trapping study

The sonolysis of argon-saturated neutral aqueous solutions of several amino acids and peptides was investigated by ESR and spin trapping. The water-soluble non-volatile spin trap, 3,5-dibromonitrosobenzene sulfonate, was found to be particularly useful for ESR and spin trapping investigations of sonochemical reactions. By comparison with analogous experiments in which hydroxyl radicals were generated by u.v.-photolysis of solutions containing hydrogen peroxide, the amino acid and peptide radicals produced by sonolysis could be identified. These observations can be explained by the reactions of hydrogen atoms and hydroxyl radicals which are the primary products of the sonolysis of water.