Inclusion complexes of proteins: Interaction of cyclodextrins with peptides containing aromatic amino acids studied by competitive spectrophotometry

The stability constants were measured of inclusion complexes formed from aromatic amino acids and their oligopeptides with - and-cyclodextrin, hydroxypropyl-cyclodextrin, and partially methylated-cyclodextrin. The method of competitive spectrophotometry withp-nitrophenol as a competing reagent was used, and measurements were made at pH 7.4-Cyclodextrin formed complexes of higher stability than the other hosts. The stability of complexes of oligopeptides containing L-phenylalanine was invariably higher than that of L-phenylalanine itself. A model for interaction of proteins with cyclodextrins is proposed, in which the most stable complexes are formed when the native functional form of proteins is unfolded and the nonpolar residues that are buried inside the structure are exposed to water. The complexation of the unfolded structure favors its formation; thus thermal denaturation of proteins is easier in the presence of cyclodextrins. On the other hand, this complexation prevents the intermolecular association of unfolded structures by noncovalent hydrophobic bonding between the exposed nonpolar residues; furthermore, the unfolded complexed forms may revert to the native functional form. This prevention of intermolecular association may explain the stabilizing effect of cyclodextrins on solutions of proteins: a return to the native form is achieved more easily from the complexed, unfolded form than from the unfolded, aggregated forms.Dedicated to Professor József Szejtli.     

Conformational and Database Study on the Intramolecular CH...π Aromatic Bond and Its Possible Influence on the Stereochemistry of Polypeptide Chains

The presence of intramolecular CH... aromatic bonds in organic molecules has been analyzed by means of CSD searches. In particular, molecules containing aromatic and methyl groups have been examined as a function of the length of the chain separating the interacting groups. The type of bonds in the chain and its geometry have also been taken into account. The maximum number of CH... interactions have been found for five-membered chains joining the aromatic and methyl moieties. In addition, this interaction seems favored when the chain contains two sp ² hybridized atoms. The interactions involving polypeptides have been carefully examined. Interest has focused on compounds containing both aromatic and methylic residues, i.e., a combination of phenylalanine (phe), tyrosine (tyr), and tryptophan (trp) with alanine (ala), valine (val), leucine (leu), and isoleucine (ile). The maximum number of CH... interactions have been found when five atoms constituted the chain, i.e., in the sequences phe–ala and ala–phe.     

A structural analysis of the complexes of (S,S)-dimethylpyridino-18-crown-6 with (R) and (S)-[α-(1-naphthyl)ethyl]ammonium perchlorate by NMR techniques and molecular modeling

Significant - interaction is found in the complexes of (S, S)-dimethylpyridino-18-crown-6 with (R)- and (S)-[-(1-naphthyl)ethyl]ammonium perchlorate. This finding is supported by the¹H NOESY NMR spectral technique, greater chemical shift changes of aromatic protons in both host and guest molecules upon complexation, and by molecular mechanics calculations. Because of the flexibility of the ligand, the tripod hydrogen bonding causes¹³C relaxation times of all periphery carbons to decrease without significant selectivity. Rotational energy barrier calculations of the methyl groups of the complexed ligand also show that the (S, S)-host-(R)-guest is the more stable complex.     

Co-solvent modulation of the inclusion selectivity ofβ-cyclodextrin

The solubility of-CD, which is increased to 87 gL^–1 in 75% water - 25% isopropanol mixtures, does not behave in a linear fashion as a function of the water/isopropanol ratio. Application of this increased solubility to the formation of inclusion complexes between-cyclodextrin and cineole : eugenol, cineole : pinene and eugenol : pinene shows strong solvent modulation of the inclusion selectivity. The proportion of guests complexed is in inverse ratio to the compatibility of the guests in the solvent mixture.     

Can the presence of chemically inert species modify the face selectivity in asymmetric induction by cyclodextrins?

Reduction of-cyclodextrin (-CD) aromatic ketone (acetophenone and acetonaphthones) inclusion compounds were carried out in the presence of a large number of chemically inert species as potential co-guests. In several cases, it was observed that stoichiometric molar ratios of these compounds to ketone significantly modify the chiral induction yielding the inverted alcohol enantiomer and increasing the face selectivity. The results were found to depend strongly on the respective structure and shape of both the ketone and the additive, and on the molar ratio of-CD:ketone:third compound. These observations suggest the formation of a three-component inclusion complex in which the geometry of binding of the substrate and its mobility are changed with respect to the binary system.     

Dipole moments ofα,ω-dibromoalkanes

Dipole moments for,-dibromoalkanes containing 10, 12, 14, 18, 20 and 24 carbon atoms were determined in benzene solution at 25.00±0.05 °C. The experimental values of the dipole moments were found to compare well with the theoretical values calculated from Hayman and Eliezer's theory.     

Highly conformationally constrained halogenated 6-spiroepoxypenicillins as probes for the bioactive side-chain conformation of benzylpenicillin

Summary The halogenated 6-spiroepoxypenicillins are a series of novel semisynthetic-lactam compounds with highly conformationally restricted side chains incorporating an epoxide. Their biological activity profiles depend crucially on the configuration at position C-3 of that epoxide. In derivatives with aromatic-containing side chains, e.g., anilide, the 3R-compounds possess notable Gram-positive antibacterial activity and potent-lactamase inhibitory properties. The comparable 3S-compounds are antibacterially inactive, but retain-lactamase inhibitory activity.Using the molecular simulation programs COSMIC and ASTRAL, we attempted to map a putative, lipophilic accessory binding site on the PBPs that must interact with the side-chain aromatic residue. Comparative computer-assisted modelling of the 3R, and 3S-anilides, along with benzylpenicillin, indicated that the available conformational space at room temperature for the side chains of the 3R and the 3S-anilides was mutually exclusive. The conformational space for the more flexible benzylpenicillin could accommodate the side chains ofboth the constrained penicillin derivatives. By a combination of van der Waals surface calculations and a pharmacophoric distance approach, closely coincident conformers of the 3R-anilide and benzylpenicillin were identified. These conformers must be related to the antibacterial, bioactive conformer for the classical-lactam antibiotics. From these proposed bioactive conformations, a model for the binding of benzylpenicillin to the PBPs relating the three-dimensional arrangement of a putative lipophilic S₂-subsite, specific for the side-chain aromatic moiety, and the 3-carboxylate functionality is presented.This work has been reported in preliminary form at the 4th Royal Society of Chemistry International Symposium on Recent Advances in the Chemistry of-lactam Antibiotics, Churchill College, Cambridge, U.K., 3–6 July 1988.     

Partially Hydrogenated Aromatic Substituted Tetrazolo[1,5-a]pyrimidines

By treating 5-aminotetrazole with aromatic ,-unsaturated ketones or with Mannich base hydrochlorides there have been synthesized aromatic substituted 4,7-dihydrotetrazolo[1,5-a]-pyrimidines. They can be reduced to the corresponding 4,5,6,7-tetrahydro derivatives by the action of NaBH₄. The high thermodynamic stability of the 4,7-dihydrotetrazolo[1,5-a]pyrimidines when compared with the 4,5-dihydro isomers has been revealed. Reaction of 5-aminotetrazole both with cyclohexanone as well as with 2-cyclohexylidenecyclohexanone leads to formation of 9,9-pentamethylene-4,5,6,7,8,9-hexahydrotetrazolo[5,1-b]quinazoline, the structure of which was demonstrated using X-ray crystallography.     

Transport method for determining the association constants of complexes formed between aromatic hydrocarbons andα- andβ-cyclodextrin in water

The association constants of 1 : 1 complexes formed in water between six aromatic hydrocarbons (o-,m-, andp-xylene, naphthalene, anthracene, and pyrene) and- and-cyclodextrin were determined by the transport method. The values are in good agreement with those determined by other methods.     

Reactions of β-aminopropionic acid N"-acylhydrazides with carbonyl compounds

The reactions of -aminopropionic acid N"-acylhydrazides with aromatic and heterocyclic aldehydes and acetone afford compounds that exist in solutions predominantly as mixtures of 2-substituted 3-acylaminotetrahydropyrimidin-4-ones (ATHP) and tautomeric Schiff"s bases. These compounds in the crystalline state probably have structures of ATHP. The ratio of tautomers depends on the type of substituent in the aromatic ring and solvent. The reactions of 2-aryl-3-benzamidotetrahydropyrimidin-4-ones with carboxylic or sulfonic acid chlorides afford derivatives of 1-acyl- and 1-tosyl-3-benzamidotetrahydropyrimidin-4-ones.     

Factors influencing enantiomeric recognition of primary alkylammonium salts by pyridino-18-crown-6 type ligands

Equilibrium constant (K), enthalpy change (H), and entropy change (S) values were determined for the interactions of a series of chiral pyridino-18-crown-6 type ligands with enantiomers of several primary alkylammonium salts in various solvents. Good enantiomeric recognition in terms of logK was observed in many systems with logK values greater than 0.4. The extent of enantiomeric recognition and the stabilities of the chiral crown ether-ammonium salt complexes were found to depend on the rigidity of the macrocyclic frame of the ligand, the type and arrangement of the donor atoms on the ligand, the bulkiness of the substituents on the ligand's chiral centers, the location of the chiral centers on the ligand, and the solvent. The effects of these factors on the extent of enantiomeric recognition and on the stabilities of the complexes were examined for the systems studied.     

Fendiline-β-Cyclodextrin inclusion complex

Fendiline (N-[1-phenylethyl]-3,3-diphenylpropylamine hydrochloride, trade name: Sensit) is a -receptor blocker and a Ca antagonist. It is a viscous oil and even its crystalline hydrochloride is a very hydrophobic and sparingly soluble compound in water.-Cyclodextrin complexes fendiline base in a molar ratio of 21. In aqueous-cyclodextrin solution, the significantly increased solubility and the intensive induced circular dichroism prove the formation of true inclusion complexes with both the base and the hydrochloride. In artificial gastric juice, the solubility of the complexed forms of the fendiline base and its hydrochloride is 40 and 22 times higher, respectively, than that of free drugs. The complexation withCD resulted in a two-threefold increase in the rate constants of diffusion and absorption determined in a Sartorius apparatus.     

The role of the π-bonding network for trigonal level splittings of tris-bidentate Cr(acac)3 and Cr(ox) 3 3−

The varying -bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)₃ for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of -orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.On leave of absence from the Bulgarian Academy of Sciences, Sofia, Bulgaria     

Electronic structure of polychlorophenols and their carbocations

Polychlorophenols (PCP) with different numbers of chlorine atoms and their protonated forms have been calculated by the AM1 method with full optimization of geometry. The proton affinity of PCP with various types of coordination of the proton to the molecule has been estimated. The calculations show that the proton affinity averaged over isomers decreases monotonically as the number of chlorine atoms increases. Based on the calculations of the carbocations, the ⁺ constant of the OH-substituent at theipso-position is equal to 0.69. There is fair agreement between the relative energies of the isomers of the carbocations determined from the constants of the substituents and calculated by the AM1 method. These energies can be recommended for qualitative estimation of the proton affinity in definite positions of aromatic molecules with Cl- and OH-substituents.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1198–1201, July, 1994.     

The Use of Cyclodextrin-Based LC Stationary Phases for the Separation of Chiral Dihydrobenzofuran Derivatives

The enantiomeric separation of 12 chiral dihydrobenzofurans is reported on derivatized -cyclodextrin stationary phases using high performance liquid chromatography. The hydroxypropyl -cyclodextrin chiral stationary phase (CSP) with acetonitrile/water mobile phases was the most effective combination as it baseline separated 9 of the 12 compounds. The acetyl -cyclodextrin and 2,3-dimethyl -cyclodextrin CSPs were also effective in the reverse phase mode. The native -cyclodextrin was far less effective than the non-aromatic derivatized CSPs. The aromatic functionalized CSPs showed no selectivity in the normal phase mode. Structural characteristics, such as substituent polarity and ring location, were important in the observed enantioselectivity.     

Thermogravimetric properties of inclusion complexes ofβ-cyclodextrin with benzene,acetylsalicylic acid and methyl salicylate

This paper reports TG analyses of inclusion complexes of-cyclodextrin with benzene, acetylsalicylic acid and methyl salicylate. The data were used for calculation of the compositions of the three body complexes and the apparent kinetic parameters of the thermal decompositions. Water exclusion proceeds as a reaction with ordern=1 and an activation energy about 20 kJ/mol. The expulsion of aromatic guest molecules follows ann=1 order process with the activation energy above 155 kJ/mol, except that for the methyl salicylate complex, which was found to be ca. 64 kJ/mol.     

Absence of p,π-conjugation in the Cl2C=CHOMe molecule from data ofab initio calculations

ab initio Calculation of the Cl₂C=CHOMe molecule has been carried out with geometrical optimization in the split valence basis set in the RHF/6-31G^*//RHF/6-31G^* approximation. An analysis of the populations of the p-orbitals of the Cl atoms implies the absence of p,-conjugation between the unshared electron pairs of these atoms and the -bond. The asymmetry parameters of the electric field gradient at the³⁵Cl nuclei, calculated from the 3p-orbital populations of the Cl atoms, practically coincide with the experimental values.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2369–2372, December, 1995.The authors are grateful to G. S. Beloglazov for assistance in accomplishingab initio calculations.     

Phase separation of poly(γ-benzyl L-glutamate) to liquid crystal and isotropic solution in various helicogenic solvents

Solvent effects on the phase separation of poly(-benzyl L-glutamate) to liquid crystal and isotropic solution have been observed in various helicogenic solvents. The temperature-composition phase diagrams have been determined for each solution. The critical concentrations, ₂ ^* , at which the phase separation occours have been compared in various solvents. In dimethylformamide in which the polymer is molecularly dispersed, the observed ₂ ^* value has agreed with that calculated by Flory's theory. In some solvents in which the polymer aggregates in a head-to-tail mode such as chloroform, the observed ₂ ^* values have been considerably small. It is assumed that the polymer aggregates behave as longer particles than the original particles. In dioxane in which the polymer aggregates highly both in a head-to-tail and a side-by-side modes, the ₂ ^* value has been a little larger than that in chloroform. In this case the relationship between the aggregation and the liquid crystal formation is so complicated that further investigation is necessary. In aromatic solvents such asm-cresol that dissolves the polymer almost molecularly, the ₂ ^* is smaller than that in dimethylformamide. Therefore, the intermolecular interactions between the phenyl groups in the side groups of the polymer and those in solvent molecules must be considered.The author is grateful to Mr. K. Sano and Mr. M. Watanabe for their observation of the liquid crystal formation.     

Complex formation of some nonionic surfactants with hydroxypropyl-β-cyclodextrin and with heptakis (2,6-di-O-methyl)-β-cyclodextrin

The interaction of six nonionic surfactants -[4-(1,1,3,3-tetra-methylbutyl)phenyl]--hydroxypoly(oxy-1,2-ethanediyl) with hydroxypropyl--cyclodextrin (HPCD) and dimethyl--cyclodextrin (DIMEB) was studied by reversed-phase thin-layer chromatography in the presence and absence of sodium chloride. Each surfactant formed complexes with both cyclodextrin derivatives; however, the strength of interaction varied considerably. DIMEB formed more stable inclusion complexes with the surfactants than did HPCD. A longer ethyleneoxide chain decreased the strength of interaction, whereas sodium chloride exerted a negligible impact. Principal component analysis indicated that both the hydrophobicity and the specific hydrophobic surface are of the surfactant influenced the complex formation indicating the hydrophobic character of the interaction.Dedicated to Professor József Szejtli.     

In-capillary micro solid-phase extraction and capillary electrophoresis separation of heterocyclic aromatic amines with nanospray mass spectrometric detection

A miniaturised technique to analyse and detect heterocyclic aromatic amines (HAs) using micro solid-phase extraction (SPE) coupled on-line (in-capillary) to capillary electrophoresis (CE) separation with nanospray (nESI) mass spectrometry (MS) detection has been developed. HAs are mutagenic and carcinogenic compounds formed at low levels in protein-rich food during cooking. Due to the low concentrations of HAs and the high complexity of the matrix in which they exist, sensitive and selective analytical methods are required for quantification. SPE was performed on a packed bed of C₁₈ particles inside the CE capillary, which minimised the dead volume. The on-line coupling of SPE, CE and nESI-MS reduced the time for extraction and identification to less than half an hour, which will allow for screening of several samples per day. The new technique provides short analysis time, low sample and solvent consumption, and HAs in standard solutions were easily detected at 12–17 fmol injections, and in spiked urine samples at 750–810 fmol injections.