Lucia Mutihac

Recognition of amino acids by functionalized calixarenes

The calixarenic receptors exhibit remarkable host-guest properties towards biologically relevant guests. Aspects of complex formation reactions between both native and derivatized amino acids, di-and tripeptides with calixarenic (chiral or not) receptors are summarized in this critical review. Thus, the discussions emphasize the parameters that affect the molecular binding selectivity and efficiency of functionalized calix[n]arenes towards these substrates. A brief survey on their application in separation of amino acids is also considered (123 references).

The formation of amino acid and dipeptide complexes with α-cyclodextrin and cucurbit[6]uril in aqueous solutions studied by titration calorimetry

Abstract The complex stabilities and the thermodynamic data for the complexation of α-cyclodextrin and cucurbit[6]uril with some amino acids (glycine, l -alanine, l -valine, l -phenylalanine, 6-amino hexanoic acid, 8-amino octanoic acid, 11-amino undecanoic acid) and dipeptides (glycyl–glycine, glycyl– l -valine, glycyl– l -leucine and glycyl– l -phenylalanine) have been determined in aqueous solution by calorimetric titrations. The complex formation with α-cyclodextrin is mainly favoured by entropic contributions due to the release of water molecules from the cavity of the ligand. The values of the reaction enthalpies are small with the exception of 11-amino undecanoic acid. In case of the ligand cucurbit[6]uril, ion–dipole interactions between the protonated amino groups of the amino acids and the carbonyl groups take place. By steric reasons these interactions are lowered for native amino acids because the polar carboxylic groups are always located outside the hydrophobic cavity of cucurbit[6]uril. The complexes of both ligands with dipeptides in water are characterised by hydrophobic interactions and in case of cucurbit[6]uril by additional ion–dipole interactions.

Complexation of Some Amine Compounds by Macrocyclic Receptors

Some aspects of complexformation, liquid–liquid extraction, and transportthrough liquid membrane of various amine compoundslike ammonium ion, amines, amino acids and peptides, withmacrocyclic receptors, such as crown ethers,cryptands, calixarenes, and cucurbituril arepresented. Log K, Δ H, andTΔ S data are reported for reaction of somerepresentative complexes of the amine compounds bymacrocyclic receptors obtained from differenttechniques such as calorimetric and potentiometrictitrations, or spectrophotometric methods in varioussolvents. Solvent extraction and transport of thesecomplexes through bulk liquid membranes are alsostudied. Based on these studies, the effect of somefactors that might influence the complex formation,the extraction, and the transport through liquidmembranes are discussed.

Mining in chemometrics

Some of the increasingly spread data mining methods in chemometrics like exploratory data analysis, artificial neural networks, pattern recognition, and digital image processing with their highs and lows along with some of their representative applications are discussed. The development of more complex analytical instruments and the need to cope with larger experimental data sets have demanded for new approaches in data analysis, which have led to advanced methods in experimental design and data processing. Hypothesis-driven methods typified by inferential statistics have been gradually complemented or even replaced by data-driven model-free methods that seek for structure in data without reference to the experimental protocol or prior hypotheses. The emphasis is put on the ability of data mining methods to solve multivariate-multiresponse problems on the basis of experimental data and minimal statistical assumptions only, in contrast to classical methods, which require predefined priors to be tested against some null-hypothesis.

Complexation of Some Amino Acids and Peptides by p-Sulfonatocalix[4]arene and Hexasodium p-Sulfonatocalix[6]arene in Aqueous Solution

The stability constants (log K), the reaction enthalpy(Δ H) and entropy (Δ S) of the complexesformed between some amino acids (glycine, L-alanine,L-valine, L-leucine, L-phenylalanine, L-tryptophan,L-threonine, and L-lysine) and peptides (glycyl-glycine,glycyl-L-alanine, glycyl-L-leucine, glycyl-L-phenylalanine,L-leucyl-glycine, L-leucyl-L-alanine, glycyl-L-valine,L-leucyl-glycyl-glycine, and glycyl-glycyl-glycine) withp-sulfonatocalix[4]arene and hexasodiump-sulfonatocalix[6]arene in aqueous solutions by meansof calorimetric titration have been investigated. The reportedresults demonstrate that the amino acids and peptides under studyform complexes with both p-sulfonatocalix[4]areneand hexasodium p-sulfonatocalix[6]arene. In the case of theamino acids and peptides the complexation with water-solublecalixarenes in aqueous solution is favored by enthalpiccontributions and disfavored by entropic contributions. However,no influence of the ring size of the calixarenes upon thecomplexation is observed. By comparison with the reaction ofthe sodium salt of phenol-4-sulfonic acid with amino acids amacrocyclic effect in case of the calixarenes is possible.

Calixarene derivatives as carriers in liquid membrane transport

The liquid membrane transport of some biogenic amines (n-buthylamine, hexylamine) and aromatic amino acids methylesters by p-tert-butylcalix[n]arenes (n = 6.8) were investigated. The transport driving force was the pH gradient between the source and the receiving phases. The complexation properties of the calixarenes under study towards amines and amino acids were evaluated. Several factors influencing the transport through liquid membrane were considered aiming to develop a mechanism model. The effect of the type of p-tert-butylcalixarene on the transport was also investigated. The results showed that the inclusion abilities of the investigated hosts were correlated with their conformational properties.

The complexation of the ammonium ion by 18-crown-6 in different solvents and by noncyclic ligands, crown ethers and cryptands in methanol

Abstract The complexation of the ammonium ion with the macrocyclic ligand 18-crown-6 was studied using calorimetric and potentiometric titrations in different solvents. In water and dimethyl sulphoxide the stability constants had the lowest values compared with all other solvents examined. No specific interactions between the ammonium ion and solvent molecules were observed. Crown ethers formed more stable complexes in methanol with the ammonium ion than diaza crown ethers. The most stable complexes were formed with cryptands. The highest values of the stability constants for the reaction with macrocyclic and macrobicyclic ligands were measured if the dimensions of the ammonium ion and of the cavities were nearly identical.

A novel experimental method for the study of complex formation between α-, β- and γ-cyclodextrin and nearly insoluble cucurbituril–[2]rotaxanes in aqueous solution

Abstract Due to interactions between nearly insoluble guest molecules and soluble host molecules the solubility of the former increases. This effect can be detected using spectrophotometric methods. From the increase in absorbance of the solutions saturated with the guest molecule the stability of the complexes formed can be calculated quantitatively. This method is demonstrated for the complex formation of cyclodextrins with cucurbituril–rotaxanes in aqueous solution. For the study of the complex formation only very small samples of the guest molecules are needed.

Complexation, liquid–liquid extraction, and transport through a liquid membrane of protonated peptides using crown ethers

Abstract By means of an ion-pairing mechanism, some protonated peptides (glycyl-glycine, glycyl- l -α-alanine, glycyl- l -valine, glycyl- l -leucine, glycyl- l -phenylalanine, glycyl-glycyl-glycine, glycyl- l -aspartic acid, and l -leucyl-glycyl-glycine) were extracted by crown ethers from an aqueous phase into a chloroform phase and transported though a chloroform liquid membrane in the presence of counterions. The peptides under study exhibited good extractability by crown ethers from the aqueous phase into the organic phase. The extraction constants of the peptides involved were determined. Active transport, assisted by pH gradient of peptides in protonated form as ion pairs in the presence of tropaeolin 00 using 18-crown-6 (18C6), benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6) as carriers through the chloroform liquid membrane was carried out. Determination of complex stabilities for the complexation between crown ethers and peptides was accomplished by calorimetric titration. It was also suggested that the extractability and the rate of the transport of peptides are controlled by factors like the structure of both the ligand and peptide, the nature of the anion used as the counterion and thermodynamic properties. A relevant correlation between the structural properties of the protonated peptides under study and their physicochemical characteristics was established on the basis of the experimental results. The carriers employed, namely 18C6, B18C6, and DB18C6, exhibited different transport selectivities relatively to the peptides under study. As a consequence, one main goal of our investigations was to determine the optimal conditions for separation the peptides through liquid membranes.

Calix[n]arenes as Synthetic Membrane Transporters: A Minireview

Several potential applications of functionalized calix[n]arenes as carriers in transport through membranes of various biological compounds aiming their separation are reviewed. Specific aspects of membrane transport and the use of calix[n]arenes for building synthetic ion channels are discussed.