Boronic acids in the design and synthesis of supramolecular assemblies

Supramolecular assemblies of phenylboronic and 4-methoxyphenylboronic acids with 4,4^′-bipyridine and an assembly of phenylboronic acid with 1,2-bis(4-pyridyl)ethene, which were obtained due to the formation of O-H?N hydrogen bonds between hetero N-atoms and -B(OH)₂ are reported. Further, a centrosymmetric cyclic C-H?O hydrogen bonding dimer is identified in the crystal structure of 4-methoxyphenylboronic acid.     

Supramolecular architectures constructed by lanthanum, amino acids and 1,10-phenanthroline via non-covalent bond interactions

Three supramolecular lanthanum coordination compounds of amino acids, with 1,10-phenanthroline (phen), [La₂(APA)₆(phen)₂(H₂O)₂](ClO₄)₆(phen)₄·2H₂O (1), [La₂(ABA)₆(phen)₂(H₂O)₂](ClO₄)₆ (phen)₆·4H₂O (2), and [La₂(AHA)₄(phen)₄](ClO₄)₆(phen)₄·2H₂O (3) (APA=3-aminopropionic acid; ABA=4-aminobutanoic acid; AHA=6-aminohexanoic acid) were synthesized and characterized by single crystal X-ray diffraction. The results show that the three coordination compounds are all composed of binuclear coordination cations built by metal-ligand coordination. Through hydrogen bonding and π-π stacking interactions, complex 1 forms a two-dimensional supramolecular sheet structure extending in the (001) plane, complex 2 forms a three-dimensional supramolecular network with many cavities occupied by ClO₄⁻ and lattice H₂O molecules, and complex 3 forms a two-dimensional supramolecular lamellar structure in the (100) plane.     

A Small-Molecule Guanidinium Receptor: The Arginine Cork

A highly preorganized, artificial receptor for guanidinium cations binds arginine with high affinity in polar solvents, even in water! The X-ray crystal structure of its bis(N-ethylguanidinium) complex shows the formation of hydrogen bonds between the receptor and the guest (see picture). This observation explains the strength of the complex formed with the dipeptide diarginine (K_d=50 μM in H₂O).     

Enantioselective binding of amino acids and amino alcohols by self-assembled chiral basket-shaped receptors

Amino acid appended diphenylglycoluril-based chiral molecular receptors 2 and 3 have been prepared and their aggregation has been studied in water at various pH's and in chloroform. The binding of several biologically relevant guests with aromatic moieties to these aggregates has been studied with UV-Vis spectroscopy in competition experiments with 4-(4-nitrophenylazo)resorcinol (Magneson) and 2-(4-hydroxyphenylazo)benzoic acid (HABA) as probes. Aggregates of chiral host 2b showed binding of catecholamines and aromatic amino acids in an aqueous environment, as well as discrimination between amino acid enantiomers, and can be considered a mimic for adrenergic receptors.     

Selective formation of heterodimeric resorcinarene capsules

C_2V-symmetrical resorcinarene tetraesters 1 form in CDCl₃ dimeric capsules encapsulating one Me₄N⁺ cation. The homodimeric capsules of the tetra(3,4,5-trimethoxybenzoate) 1d and tetrabenzoate 1b or tetrafuroylate 1c disproportionate in solution to give quantitatively the heterodimers. The higher stability of the heterodimer is, most probably, caused by π-π attractions between the π-basic trimethoxyphenyl rings of 1d and relatively more π-accepting phenyl or furyl fragments of 1b or 1c.     

Indirect determination of amino acids by molecular emission cavity analysis

An indirect method for the determination of ca. 10^?4 M amino acids is described which is based on the reaction of the latter with 2,4,6-trinitrobenzene-1-sulphonic acid at pH 11.7 and the measurement of the S₂ emission in a hydrogen-nitrogen flame that is produced from the resulting compound after the addition of hydrochloric acid. The optimum conditions for the determination of amino acids were investigated. The amino acids examined were glycine, alanine, valine, methionine, serine, threonine and histidine.     

Amino acid binding in water by a new guanidiniocarbonyl pyrrole dication: the effect of the experimental conditions on complex stability and stoichiometry

The synthesis and binding properties of a new guanidiniocarbonyl pyrrole dication 2 are reported, which efficiently binds alanine carboxylate with log K_ass = 3.9 in buffered water. Due to the increased charge density in this dication, the binding constant is five times larger than for the parent guanidiniocarbonyl pyrrole monocation 1 (log K = 3.2). However, the experimental conditions for determining the binding constant significantly influence both complex stability and stoichiometry. With increasing amount of substrate added during the titration, the overall complex stability decreases due to the increasing ionic strength of the solution. Furthermore, the formation of 1:2 complexes between 2 and 7 becomes increasingly important. Therefore, for the comparison of binding data it has to be assured that exactly the same experimental conditions are used for their determination.     

Evaluation of methods for measuring amino acid hydrophobicities and interactions

The concept of hydrophobicity has been addressed by researchers in all aspects of science, particularly in the fields of biology and chemistry. Over the past several decades, the study of the hydrophobicity of biomolecules, particularly amino acids has resulted in the development of a variety of hydrophobicity scales. In this review, we discuss the various methods of measuring amino acid hydrophobicity and provide explanations for the wide range of rankings that exist among these published scales. A discussion of the literature on amino acid interactions is also presented. Only a surprisingly small number of papers exist in this rather important area of research; measuring pairwise amino acid interactions will aid in understanding structural aspects of proteins.     

Synthesis of (azidomethyl)phenylboronic acids

The synthesis of 2-, 3-, and 4-(azidomethyl)phenylboronic acids was carried out. The geometric and electronic structures were studied by quantum-chemical methods. The suggestion is made that there are weak intramolecular interactions between the boron atom and the nitrene nitrogen atom of the azido group.     

Molecular recognition properties of peptide mixtures obtained by polymerisation of amino acids in the presence of estradiol

In this paper we show that the carbodiimide-induced polymerisation of amino acid mixtures in aqueous medium and in presence of estradiol produces the mixtures of peptides with an average molecular weight of 2-6 kDa that are characterised by possessing molecular recognition properties towards estradiol. After the removal of the templating molecule, the binding properties of the peptide mixtures were studied using spectrophotometric and immunochemical methods. The experimental results show the presence of molecular recognition behaviour for all the peptide mixtures obtained by polymerisation in presence of estradiol, with affinity constant values between 0.44×10⁹ and 6.6×10⁹ M⁻¹, while the same mixtures obtained without estradiol show lower affinity constant values between 2.2×10⁶ and 1.3×10⁹ M⁻¹. The molecular recognition behaviour was found to be highly selective, as the binding constants of peptides towards the structural homologues testosterone and progesterone are lower than three orders of magnitude. Peptide fractions separated by ion-exchange chromatography show the same molecular recognition properties, with affinity constant values between 3.2×10⁶ and 7.1×10⁹ M⁻¹. Similarities and differences between this polymerisation technique and the molecular imprinting technique are briefly discussed.     

氢键型超分子聚合物的合成、结构与应用

氢键型超分子聚合物是重复单元经氢键相互作用连接在一起的阵列,可生成液晶态,多样化的几何形状和高有序的凝聚态结构。氢键的温度敏感性和可逆性导致氢键型超分子聚合物具有和传统共价键结合的聚合物不同的性能。氢键型超分子聚合物是一类动态的智能型功能高分子材料,可在光化学、光电转换、非线性光学、弹性体、水凝胶和生物医用工程等领域广泛应用。本文从氢键型超分子聚合物化学(合成与机理)、物理(结构与性能)和工程(加工与应用)三个方面介绍氢键型超分子聚合物的进展。     

-B(OH)2 versus -OH in supramolecular synthesis: molecular complexes of 4-hydroxyphenylboronic acid with aza-donor compounds

-B(OH)₂ moiety forms interactions with aza-donor compounds as much as -OH does, as derived from the supramolecular assemblies of 4-hydroxyphenylboronic acid with aza-donor compounds demonstrating for the first time, a comparative study of -B(OH)₂ versus -OH.     

Binary and ternary phenylboronic acid complexes with saccharides and Lewis bases

Cumulative formation constants for the association of three boronic acids (phenylboronic acid and its ortho-anilinomethyl and ortho-benzylaminomethyl derivatives) with four saccharides (fructose, glucose, mannitol, and sorbitol) were determined by potentiometric titration. Similarly, the constants for the formation binary complexes of the three boronic acids with (hydrogen) phosphate, (hydrogen) citrate, or imidazole were determined. Finally, the formation of ternary complexes of the boronic acids, phosphate, citrate or imidazole, and the saccharides were determined based on the determined values of the binary complexes. The previously unrecognized ternary complexes are significant in all systems investigated, and under some solution compositions, they can be the dominant species in solution over a wide pH range. A value of 15-25 kJ mol⁻¹ was determined for the energy of the B-N interaction in the benzylmethyl derivative based on the relative stabilities of the ternary phosphate complexes of the three boronic acids. The data are used to rationalize the medium dependence of stepwise formation constants and the apparent acidity constants of previous literature reports.     

Synthesis of N-alkylated amino acids using fluorous-tagged hydroxylamines

The development of a new fluorous-tagged ammonia-equivalent for the synthesis of N-alkylated amino acids is described. The required building blocks were readily accessed in high yield and purity using F-SPE purification technique. Coupling of the fluorous-tagged hydroxylamines with a selection of boronic acids and glyoxalic acid gave the desired N-alkylated amino acids. Subsequent removal of the fluorous tag via catalytic hydrogenation was investigated using a number of different catalysts and solvents. A more robust de-tagging procedure involves the transformation of the amino acid to the corresponding methyl ester followed by a Mo(CH₃CN)₃(CO)₃ mediated N-O bond cleavage.     

Use of hydrolases for the synthesis of cyclic amino acids

The synthesis of several cyclic amino acids that have all the necessary structural features to make them ideal scaffolds for use in medicinal chemistry is described. A key step in each synthesis is the use of hydrolase enzymes to define a chiral centre. In order to elaborate these building blocks into more complex molecules, crystallization and asymmetric hydrogenation were used to define further stereocentres.     

Salicyaldehyde-based fluorescent sensors with high sensitivity for amino acids

Structurally simple salicylaldehyde-based fluoreseent sensors for amino acids have been obtained by one-step or two-step synthesis.These sensors show significant fluorescence enhancement in the presence of many amino acids at concentrations as low as 10~5 mol/L.The reversible reaction of the aldehydes with amino acids to form imines in aqueous solution is proposed to account for the observed fluorescence enhancement.     

Low frequency vibrational spectra of some amino acids

Far-IR absorption and reflection spectra, as well as laser Mandelshtam–Brillouin and Raman scattering spectra of -glycine, β-alanine, -histidine, -tryptophane single crystals in the 0.2–400 cm⁻¹ range were investigated. It was revealed that the far-IR and Raman spectra of the amino acids under study contain more bands than predicted by factor-group analysis, thus indicating a possible contribution of low-energy intramolecular vibrations and overtones, as well as an emergence of forbidden vibrations. Some of the low-frequency bands have never, to our knowledge, been detected previously.     

Micellar-mediated shifts of ionization constants of amino acids and peptides

The acid-base dissociation constants, K_a, of amino acids and small peptides were determined in both aqueous and micellar solutions of sodium dodecyl sulfate and cetyltrimethylammonium bromide by potentiometric and chromatographic means. The observed differences in pK_a values between micellar media and aqueous solutions ranged between 0.23 and 2.21 units. The micellar-mediated pK_a shifts can be attributed to different solute-micelle interactions, mainly hydrophobic and electrostatic forces. The implications of the change in acid-base behavior on separation selectivity in micellar liquid chromatography and micellar electrokinetic capillary chromatography are discussed.