Self-association In Solution Research Articles

Synthesis and characterization of novel dibenz[a,c]anthracenedicarboxythioimides: the effect of thionation on self-assembly

The effect of thionation on the formation of columnar liquid crystalline phases of dibenzanthracenedicarboximides as well as their self-association in solution is described.

Coupled equilibria of a self-associating drug loaded into polymeric nanoparticles

Doxorubicin (DOX) and other anti-cancer drugs are often formulated using nanoparticles for passive or active targeting and reducing detrimental side effects. Anionic polymers have been shown to effectively facilitate loading of cationic DOX hydrochloride into nanoparticles with high efficiency. One powerful method to study DOX loading into anionic polymeric nanoparticles has been isothermal titration calorimetry (ITC), but the curves are complex and were previously interpreted in a largely qualitative manner only. Here we present detailed quantitative modelling of such ITC data, corroborated by zeta potential measurements and dynamic light scattering. The model takes into account 3 coupled equilibria. First, DOX self-associates in solution to dimers and larger aggregates. This effect is modelled in terms of the stepwise aggregation model. Second, DOX binds with a 1:1 stoichiometry to the carboxylic acids in the polymer at low salt. At about 33% saturation, the nanoparticles collapse in size and the enthalpy of further binding becomes less exothermic. Third, free DOX also stacks onto polymer-bound DOX. This stacking effect is very weak and hardly detected by ITC. It is, however, revealed by a positive zeta potential. The present work demonstrates the power of combining ITC with light scattering and zeta potential measurements for studying the thermodynamics of drug loading into polymeric nanoparticles.

Self-association of organic solutes in solution: a NEXAFS study of aqueous imidazole.

N K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of imidazole in concentrated aqueous solutions have been acquired. The NEXAFS spectra of the solution species differ significantly from those of imidazole monomers in the gas phase and in the solid state of imidazole, demonstrating the strong sensitivity of NEXAFS to the local chemical and structural environment. In a concentration range from 0.5 to 8.2 mol L(-1) the NEXAFS spectrum of aqueous imidazole does not change strongly, confirming previous suggestions that imidazole self-associates are already present at concentrations more dilute than the range investigated here. We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure. This suggests that such computational modelling of experimental NEXAFS will permit an incisive analysis of the molecular interactions of organic solutes in solutions. It is confirmed that microhydrated clusters with a single imidazole molecule are poor models of imidazole in aqueous solution. Our analysis indicates that models including both a hydrogen-bonded network of hydrate molecules, and imidazole-imidazole interactions, are necessary to explain the electronic structure evident in the NEXAFS spectra.

Self-Association during Heterogeneous Nucleation onto Well-Defined Templates

We investigated the interplay between self-associates in solution and surface templating by studying the crystallization behavior of isonicotinamide (INA) and 2,6-dihydroxybenzoic acid (DHB) in the presence of self-assembled monolayers (SAM). The end group of the SAM as well as the hydrogen-bonding capabilities of the solvent and self-association of INA and DHB were found to be important in polymorph crystallization on SAMs. In the case of INA in ethanol, both chain and dimer self-associates are present in the solution. In the absence of SAMs the polymorph form II (dimer structure) is the crystallization outcome. In ethanol the 4-mercaptopyridine and 4-mercaptobenzoic acid SAMs organize INA chain associates at the template surface and enable the crystallization of form I while the 16-mercaptohexadecanoic acid SAM results in the crystallization of form II. Raman spectroscopy suggests that molecular interactions between INA and the SAM are responsible for the formation of specific polymorphs. XRPD results in the identification of the orientation of the crystal on the surface that further verified the results obtained by Raman spectroscopy. In nitrobenzene and nitromethane INA associates in solution only as chains and crystallization results in the formation of form IV and form I, respectively (both chain forms). The crystals formed in the bulk solution and on SAMs were the same, which seems to indicate that the self-association in nitrobenzene and nitromethane is not influenced by the presence of templates. In the case of DHB in toluene and chloroform, all three SAMs nucleated only one type of polymorph (stable form 2). In the case of toluene the polymorphic outcome was stable form 2 instead of metastable form 1, which is favored in toluene in the absence of the SAMs. Again, Raman spectroscopy and XRPD suggest that DHB-SAM molecular interactions may be responsible for the formation of form 2.

COMPLEX FORMATION OF 1,4-DIAZABICYCLO[2.2.2]OCTANE WITH COPPER(II) TETRASULFOPHTHALOCYANINE IN WATER SOLUTION

Information about copper tetrasulfophthalocyanine self-association in water solution and about complex formation process of it with DABCO is obtained. Formation of phthalocyanine π-π dimers is confirmed. Formation of monoligand complex CuPc-DABCO is shown. Formation of sandwich-type dimers is not detected.

Conformations and Self-association of Trifluoro-N-(3-formylcyclohept-2-en-1-yl)methanesulfonamide

The structure of trifluoro-N-(3-formylcyclohept-2-en-1-yl)methanesulfonamide and its self-association in solution have been studied by IR spectroscopy and quantum chemical methods [B3LYP/6-311G(d,p), AIM]; proton affinities of basic centers in its molecule have been evaluated. Trifluoro-N-(3-formylcyclohept-2-en-1-yl)methanesulfonamide in inert solvents forms cyclic dimers, whereas in crystal chain associates are more likely to be formed via hydrogen bonding between the NH and C=O groups of neighboring molecules. The carbonyl group in the title compound undergoes protonation only by the action of very strong trifluoromethanesulfonic acid. Weaker acids give rise to solvate H-complexes at the NH, C=O, and S=O groups. The topology of hydrogen bonds in dimers of different types has been analyzed in terms of the AIM theory.

Structure–activity relationships of the antimicrobial peptide gramicidin S and its analogs: Aqueous solubility, self-association, conformation, antimicrobial activity and interaction with model lipid membranes

GS10 [cyclo-(VKLdYPVKLdYP)] is a synthetic analog of the naturally occurring antimicrobial peptide gramicidin (GS) in which the two positively charged ornithine (Orn) residues are replaced by two positively charged lysine (Lys) residues and the two less polar aromatic phenylalanine (Phe) residues are replaced by the more polar tyrosine (Tyr) residues. In this study, we examine the effects of these seemingly conservative modifications to the parent GS molecule on the physical properties of the peptide, and on its interactions with lipid bilayer model and biological membranes, by a variety of biophysical techniques. We show that although GS10 retains the largely β-sheet conformation characteristic of GS, it is less structured in both water and membrane-mimetic solvents. GS10 is also more water soluble and less hydrophobic than GS, as predicted, and also exhibits a reduced tendency for self-association in aqueous solution. Surprisingly, GS10 associates more strongly with zwitterionic and anionic phospholipid bilayer model membranes than does GS, despite its greater water solubility, and the presence of anionic phospholipids and cholesterol (Chol) modestly reduces the association of both GS10 and GS to these model membranes. The strong partitioning of both peptides into lipid bilayers is driven by a large favorable entropy change opposed by a much smaller unfavorable enthalpy change. However, GS10 is also less potent than GS at inducing inverted cubic phases in phospholipid bilayer model membranes and at inhibiting the growth of the cell wall-less bacterium Acholeplasma laidlawii B. These results are discussed in terms of the comparative antibiotic and hemolytic activities of these peptides.

Spectroscopic and Calorimetric Approach to Understand the Molecular Basis of Self-Association of Aureolic Acid Antibiotic, Chromomycin A3

Chromomycin A3 (CHR, pKa = 7.0), an aureolic acid group of antitumor antibiotic, undergoes self-association in aqueous solution in neutral and anionic forms. Self-association processes of neutral and anionic CHR have been studied in pH 5.0 and pH 9.0, respectively using different spectroscopic methods such as absorbance, fluorescence, CD, NMR and isothermal titration calorimetry (ITC). Results from these studies reveal that at low concentration (2 + CHR (CHR)3 and (CHR)3 + CHR (CHR)4. Analysis of NMR spectra of 100 μM and 1 mM CHR indicates that the self-association of CHR (neutral and anionic form) is most likely to happen via hydrophobic interaction involving the sugar moieties and surrounding water molecules. Calorimetric studies indicate that self-association of both anionic and neutral CHR is entropy driven. These observations imply that sugar substituents play a major role in their state of aggregation after biosynthesis from a gene cluster. The self-association features of the antibiotic have been compared with those of Mithramycin, an antibiotic of the same group.

Synthesis of Planar Metal Complexes and the Stacking Abilities of Naphthalenediol‐Based Acyclic and Macrocyclic Salen‐Type Ligands

AbstractA salen‐type ligand, H4L1, having two 1,8‐naphthalenediol moieties, was designed and synthesized to obtain planar metal complexes in which the naphthalene π surface was expected to increase the stacking ability. Spectroscopic and crystallographic investigations revealed that the uncomplexed H4L1ligand existed exclusively in the keto/keto form and adopted a nonplanar structure. Upon complexation with divalent metal sources M (Ni, Cu, Zn, Pd), the H4L1ligand gave the corresponding mononuclear complexes [H2L1M] (M = Ni, Cu, Pd) or [H2L1Zn(dmso)], in which the metal ion occupies the salen‐type N2O2coordination site. In particular, [H2L1Ni] and [H2L1Pd] adopted a highly planar structure, allowing the formation of well‐organized stacking structures in the crystalline state as well as self‐association in solution. The corresponding macrocyclic ligand, H4L2, was also designed to obtain a more organized stacking structure due to the higher π surface area. Although the [2+2] macrocyclization without a template did not efficiently give the ligand H4L2, the metal‐templates of Ni2+and Cu2+significantly increased the [2+2] macrocyclization efficiency to produce the dinuclear [L2Ni2] and [L2Cu2] species. The [L2Ni2] molecule had a highly planar structure incorporating two nickel(II) ions in an almost ideal square‐planar geometry. In the crystalline state, the [L2Ni2] molecules formed a parallel stacking structure with a brick‐wall arrangement. The central O4site of the [L2Ni2] complex was found to be capable of binding a third metal ion such as Ni2+, Cu2+or Zn2+.

Amphiphilic activities of anionic sodium cholate (NaC), zwitterionic 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and their mixtures: A comparative study

Anionic biological surfactant sodium cholate (NaC) and the zwitterionic nontoxic surfactant CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) having common steroidal skeleton have not been studied in a comparative level to explore their amphiphilic behaviors separately and in mixed states. Their adsorption at the interface in relation to self-association in solution has been examined in both pure and mixed states with reference to detailed thermodynamic analysis of their surface chemical and solution behaviors. Their head group differences are responsible for the differences in their properties viz. critical micelle concentration (CMC), aggregation number (N), hydrodynamic diameter, Gibbs surface excess (Γmax), minimum head group area (Amin), mixed micelle composition and their mixing ideality/nonideality. Although the CMC values found from tensiometry and isothermal titration calorimetry (ITC) are in good agreement, the standard enthalpy of micellization (ΔHm0) found from ITC at different temperatures are different from that evaluated by the van't Hoff method. This is because van't Hoff rationale is a differential method which estimates the enthalpy of the monomer ↔ micelle equilibrium process whereas calorimetry is an integral process that evaluates all probable/possible processes in addition to monomer to micelle conversion. Results are analyzed, compared and discussed in the light of physicochemical parameters of pure NaC and CHAPS and their mixtures. These fundamental findings are considered prospective in pharmaceutical, biochemical and biomedical applications of the studied bio-friendly amphiphiles.

Influence of the extraction process on the rheological and structural properties of agars

Agars obtained by traditional hot-water (TWE) and microwave-assisted (MAE) extractions were compared in terms of their rheological and physicochemical properties and molecular self-association in solutions of low (0.05%, w/w) and high (1.5%, w/w) polymer concentrations. At low concentration, thin gelled layers were imaged by AFM. Slow or rapid cooling of the solutions influenced structure formation. In each case, TWE and MAE agar structures were different and apparently larger for MAE. At high concentration, progressive structural reinforcement was seen; while TWE agar showed a more open and irregular 3D network, MAE agar gel imaged by cryoSEM was denser and fairly uniform. The rheological (higher thermal stability and consistency) and mechanical (higher gel strength) behaviors of MAE agar seemed consistent with a positive effect of molecular mass and 3,6-anhydro-α-l-galactose content. MAE produced non-degraded agar comparable with commercial ones and if properly monitored, could be a promising alternative to TWE.

Conformational Analysis and Intramolecular Interactions in Aminofluorobenzoic Acids

Some aminofluorobenzoic acids were studied to evaluate the power of the F···HO hydrogen bond and other interactions as driving forces of the conformational isomerism of these compounds. Despite the occurrence of this hydrogen bond in the 2-fluorinated derivatives, as well as attractive O/F nonbonding interactions and NH···O═C hydrogen bond, the O/O repulsion dictates the orientation of the carboxyl group. Unlike 2-fluorophenol, which is reported to not experience a F···HO hydrogen bond, 2-fluorobenzoic acid derivatives were calculated to exhibit such interaction, but it could not be monitored experimentally by means of F/H(O) coupling constant, because of the low solubility of these compounds in nonpolar solvents, the acidity of the carboxyl hydrogen, the small population of some conformers capable of exhibiting hydrogen bond, and the solute self-association in solution, which make their conformational equilibrium different from that in gas phase.

The molecular self-association of carboxylic acids in solution: testing the validity of the link hypothesis using a quantum mechanical continuum solvation approach

According to the “link hypothesis” there is a connection between the solvent-dependent most stable self-associated groups of molecules and the building unit present in the polymorph that crystallizes from solution. We have tested this hypothesis by computing the Gibbs free energies for the molecular self-association in solution of three selected monocarboxylic acids that crystallize in different polymorphic forms and exhibit selective crystallization depending on the solvent: tetrolic acid (TTA), m-aminobenzoic acid (mABA) and m-hydroxybenzoic acid (mHBA). Calculations have been conducted at the density functional theory (M06-2X) level with the SMD polarizable continuum model to simulate aqueous and organic solutions. For all three systems we have found that the solvation environment significantly affects the stability of dimers. The most stable dimer in solution is the classic carboxylic acid dimer, and its stability decreases on going from nonpolar solvents, such as chloroform or acetonitrile, to aqueous and alcoholic solutions. However, whilst for TTA and mABA there is a link between the carboxylic dimer synthon in solution and the structural synthon packed in the metastable α-TTA polymorph, which crystallizes from chloroform, and in the crystal form-II of mABA, which crystallizes from acetonitrile, for mHBA the stabilization of the centrosymmetric carboxylic dimeric in acetonitrile and ethyl acetate does not correspond with recent experimental observations that the polymorph containing this structural unit (form I) is not the crystal form that nucleates preferentially from these solvents (Cryst. Growth Des., 2013, 13, 1140). Starting from the carboxylic dimer of TTA, the processes of trimerization and tetramerization of TTA from solution have been also modelled. The calculations suggest that the formation of tetramers in chloroform occurs from the self-association of the carboxylic acid dimers and not through the association of the TTA monomers and trimers. The free energy formation of the ionized forms of the mABA dimer (non-zwitterionic–zwitterionic and zwitterionic–zwitterionic) has been evaluated and, according to our results, zwitterionic–zwitterionic mABA dimers might be abundant in supersaturated aqueous and alcoholic solutions of m-aminobenzoic acid.

Solubility Behavior of Lornoxicam in Binary Solvents of Pharmaceutical Interest

The solubility behavior of lornoxicam was studied in binary solvent systems. Solutions containing an excess of drug were shaken for 72 h at 25 °C, filtered and analyzed. To understand the solute–solvent interactions, ideal, Hildebrand–Scatchard and extended Hildebrand solubility approaches were used. The ideal solubility of lornoxicam was calculated from the enthalpy of fusion. The molar enthalpy of fusion was obtained from DSC studies. The experimental mole fraction solubility deviated from the ideal mole fraction solubility, indicating self-association of solute, or solvent, or both in solution. The solubility behavior failed to satisfy the Hildebrand–Scatchard equations. The solubilities of lornoxicam were back calculated with an interaction energy term ‘W’ and a rational activity coefficient term ‘(log10 γ 2)/A’. These parameters were regressed against a polynomial function of δ 1, the solubility parameter of the solvent mixture. The solubility behavior of lornoxicam obeyed the extended Hildebrand equation. The solubility parameter of lornoxicam δ 2 was found to be 27 MPa1/2.

A Designed Point Mutant in Fis1 Disrupts Dimerization and Mitochondrial Fission

Mitochondrial and peroxisomal fission are essential processes with defects resulting in cardiomyopathy and neonatal lethality. Central to organelle fission is Fis1, a monomeric tetratricopeptide repeat (TPR)-like protein whose role in assembly of the fission machinery remains obscure. Two nonfunctional, Saccharomyces cerevisiae Fis1 mutants (L80P or E78D/I85T/Y88H) were previously identified in genetic screens. Here, we find that these two variants in the cytosolic domain of Fis1 (Fis1ΔTM) are unexpectedly dimeric. A truncation variant of Fis1ΔTM that lacks an N-terminal regulatory domain is also found to be dimeric. The ability to dimerize is a property innate to the native Fis1ΔTM amino acid sequence as we find this domain is dimeric after transient exposure to elevated temperature or chemical denaturants and is kinetically trapped at room temperature. This is the first demonstration of a specific self-association in solution for the Fis1 cytoplasmic domain. We propose a three-dimensional domain-swapped model for dimerization that is validated by a designed mutation, A72P, which potently disrupts dimerization of wild-type Fis1. A72P also disrupts dimerization of nonfunctional variants, indicating a common structural basis for dimerization. The obligate monomer variant A72P, like the dimer-promoting variants, is nonfunctional in fission, consistent with a model in which Fis1 activity depends on its ability to interconvert between monomer and dimer species. These studies suggest a new functionally important manner in which TPR-containing proteins may reversibly self-associate.

Chemistry of Anthracene–Acetylene Oligomers XX: Synthesis, Structures, and Self‐Association of Anthracene–Anthraquinone Cyclic Compounds with Ethynylene Linkers

We have synthesized anthracene-acetylene oligomers, which contained one 10-substituted anthracene unit and one anthraquinone unit, by cyclization with Sonogashira coupling. X-ray analysis revealed an almost-planar framework and significant out-of-plane deformation around the inner carbonyl moiety because of steric hindrance. These compounds underwent self-association in solution and their association constants for monomer-dimer exchange were determined by variable-concentration (1)H NMR measurements in CDCl(3): 8 mol(-1) L (10-substituent: isopropyl), <5 mol(-1) L (methoxy), and 19 mol(-1) L (octyloxy). These results were discussed on the basis of spectroscopic and molecular-orbital analysis. A linear molecular assembly of the octyloxy compound at a liquid/graphite interface was observed by STM measurements.

The self-association in aqueous solutions of morphine sulphate and some related salts.

Abstract Conductivity and optical rotatory dispersion investigations suggest that morphine sulphate and some related salts aggregate in aqueous solution. This association is probably due to the stacking of the aromatic rings.

Biophysical Mechanisms of Host Defense Peptide (HDP) Toxicity as Revealed by a Study of Peptoid Mimics of HDPs

We are developing and studying biomimetic N-substituted glycine oligomers (peptoids) with broad-spectrum antibiotic activity against pathogens, including antibiotic-resistant strains of Gram-positive and Gram-negative bacteria. Peptoids are sequence- and length-specific oligomers with high similarity to peptides, which are made on solid phase and resist proteolysis; they are ideal for applications for which stability is desired. We designed peptoid oligomers to mimic features of the natural host defense peptides magainin and LL-37: helical, cationic and amphipathic. A study of over 100 sequence variants allowed us to identify peptoids with potency and broad-spectrum antibacterial activity similar to natural HDPs. The selectivity of antibacterial peptoids and peptides (i.e., their ability to kill bacteria at low-μM concentrations while causing only tolerable damage to host mammalian cells) correlates with molecular hydrophobicity and amphipathicity as well as with self-association in solution. We did biophysical mechanistic assays using peptoids with either high or low antibacterial activity and/or either high or low selectivity, along with selective and non-selective host defense peptides. Comparative vesicle leakage studies, TEM imaging, and soft X-ray tomography of untreated and treated bacteria yield powerful insight into mechanisms of action for both HDPs and their peptoid mimics.

THz Spectra and Dynamics of Aqueous Solutions Studied by the Ultrafast Optical Kerr Effect

The nature and extent of the effects that hydrophilic and hydrophobic solutes have on the dynamics of water molecules continues to be an area of intense experimental and theoretical investigation. In this work, we use the ultrafast optical Kerr effect to measure the picosecond dynamics and THz Raman spectral densities of a series of aqueous solutions. The solutes studied are the hydrophilic urea and formamide and the hydrophobic trimethylamine N-oxide and tetramethylurea. Measurements are made as a function of concentration between <0.1 M and >4 M. At low concentrations (<0.5 M), the THz spectrum resembles that of bulk water, but the picosecond relaxation time, reflecting dynamics in the water H-bonded network, is increased relative to bulk water for all four solutes. The extent to which water relaxation is slowed down depends on the nature of the solute, and is more pronounced for hydrophilic than for hydrophobic solutes. At concentrations above 1 M, a range of solute-solvent and solute-solute interactions gives rise to diverse solute dependent changes in the THz spectral density and to a further slowing down of the picosecond relaxation. The hydrophobic trimethylamine N-oxide has remarkably little effect on the spectral density of water, which may indicate solute self-association and the formation of water pools in more concentrated solutions. For hydrophilic urea and formamide, the THz spectral density suggests that water structure is disrupted at concentrations where most water molecules are part of a solvation shell. At such high concentrations, modes associated with the H-bonded solute make a significant contribution to the spectral density at around 100 cm(-1). The hydrophobic tetramethylurea solute makes a substantial contribution to the spectral density, complicating the interpretation, but a line shape analysis suggests that it also does not strongly perturb the water structure.

Observation and quantification of self-associated adenosine extracted from royal jelly products purchased in USA by HPLC

The concentration of adenosine in pure royal jelly creams and dietary supplements purchased in the United States was determined by reversed phase high performance liquid chromatography (HPLC). Preliminary studies revealed the existence of several forms of adenosine via self-association and base-pairing in solutions. Therefore, proper optimisation of pH and compositions of extraction solvents and mobile phases was critical for successful separation and quantification. In this work, adenosine in samples was extracted by sonication in a mixture consisting of 5% ethanol and 95% deionized water and separated using a Zorbax Eclipse XDB-C18 column (150×4.6mm) and a mobile phase of 93% deionized water and 7% acetonitrile at 25°C. The flow rate of a mobile phase was set to 1.0ml/min and the UV detection was performed at 260nm. The average recovery rate of adenosine was 92.8–99.2% with the relative standard deviation (RSD) of 0.1–1.3% over concentrations ranging from 5 to 160μg/ml. The limit of detection (LOD) and limit of quantification (LOQ) were found to be ∼0.01 and ∼0.05μg/ml, respectively. Quantification was carried out using calibration curves constructed by both internal standard (ISTD) and external standard (ESTD) methods. Our results show that the concentration of adenosine lies between ∼27 and 50μg/g for pure royal jelly creams and between ∼2 and ∼173μg/g for royal jelly supplements.