Enrique F. Velázquez

Antibacterial and free‐radical scavenging activities of Sonoran propolis

Aims:  To evaluate the antibacterial and free‐radical scavenging (FRS) activities of propolis collected from three different areas of Sonoran Desert in northwestern Mexico [Pueblo de Alamos (PAP), Ures (UP) and Caborca (CP)].

High steric constraints and molecular distortion in methyl-substituted amide-based paracyclophanes and the binuclear Cu2+ complexes: X-ray structures, NMR and absorption spectra

Abstract Chelating paracyclophanes that are sterically constrained to a great extent have been synthesized and characterized by X-ray crystallography and NMR spectroscopy: the macrocycles studied are 2,9,18,25-tetraoxo-4,7,20,23-tetrakis(carboxymethyl)-1,4,7,10,17,20,23,26-octaaza[10.10]paracyclophane, abbreviated as ( L pd)H 4 , and its 2,5-dimethyl- p -phenylene and tetramethyl- p -phenylene derivatives, abbreviated as ( L dmpd)H 4 and ( L tmpd)H 4 , respectively. Steric interaction between tetramethylphenylene and amide groups in the tetramethyl derivative defines the conformation of the macrocyclic cavity, and causes unusual spectroscopic and chemical properties including the extreme line-broadening of 1 H NMR signals and the low basicity of amino nitrogen; such properties are not observed for the other macrocycles, in which steric interaction between phenylene and amide groups is less effective. The complexation of the highly strained ligand ( L tmpd)H 4 with Cu 2+ ions has been studied by X-ray crystallography and solution electronic spectroscopy. The macrocycle forms a binuclear complex of [Cu 2 (LH −4 )] 4− type in which four amide nitrogen atoms are deprotonated and each metal ion is coordinated to two amide nitrogen atoms and two amino nitrogen atoms. In the binuclear chelate molecule, the severe contraction of the macrocyclic ring forces the phenylene groups distorted to a boat form, due to the steric effect of the tetramethyl substituents. As a result, the metal–ligand charge-transfer interaction in the binuclear complex differs from that in the mononuclear chelate of the same macrocycle.

Complexation of an anionic meta-cyclophane with histamine and analogous bioactive amines in aqueous media

Molecular recognition of an anionic meta-cyclophane towards bioactive amines and related compounds has been studied by 1H NMR titration: the meta-cyclophane, which is functionalised by pendant arms, is 2,9,18,25-tetraoxo-4,7,20,23-tetrakis(carboxymethyl)-1,4,7,10,17,20,23,26-octaaza[10.10]metacyclophane; the bioactive guests studied are histamine, tryptamine, tyramine, phenethylamine, imidazole, histidine, phenylalanine and 4-aminobenzoic acid. Complex formation of a para-cyclophane isomer has also been studied for comparison. The meta-cyclophane forms a complex with histamine with a formation constant of 63 M− 1, while the complexes with the other amines have a smaller constant in the range of 1–24 M− 1; the compounds other than the amines have no interaction with the host. The major binding force for the complex formation is electrostatic interaction between the arm of the hosts and the arm of the guests. The aromatic group of a guest amine molecule is encapsulated into the cavity of a host molecule, and the deepness of the encapsulation is increased with the hydrophobicity in the order histamine < tyramine∼phenethylamine < tryptamine. In addition to hydrophobic interaction, the meta-cyclophane is supposed to have a dipolar interaction with a guest molecule. The combined effect of the three types of interactions stabilises the histamine complex of the meta-cyclophane. Complex formation of an anionic meta-cyclophane with bioactive amines including histamine, tryptamine, tyramine and phenethylamine was studied by NMR. Histamine forms the most stable complexes, as a result of a combined effect of electrostatic interaction, hydrophobic interaction and dipolar interaction, while tryptamine is most deeply included in the host cavity.

Biophysical Characterization Of An Insect Lysozyme From Manduca Sexta

Insect lysozyme from Manduca sexta (MS-lys) was overexpressed in E. coli and refolded to obtain active protein. Recombinant MS-lys presented a globular structure, with an alpha-helical content of 57% as assessed by circular dichroism spectroscopy. Light scattering studies showed that in solution MS-lys has a quasi-monodisperse size distribution, with a rod-like structure similar to nucleation clusters reported in egg lysozyme pre-crystallization stages. These results show that MS-lys is an excellent candidate for crystallization, folding and denaturation studies.

NMR Studies of Host–guest Complexes Between Monocarboxylic Acids and Amide-based Cyclophanes in Chloroform

Formation of host–guest complexes with acetic acid and benzoic acid was studied by NMR for amide-based octaazacyclophanes having pendant methyl ester arms; the cyclophanes were tetramethyl 2,9,18,25-tetraoxo-1,4,7,10,17,20,23,26-octaaza[10.10]paracyclophane-4,7,20,23-tetraacetate, its meta-isomer and analogues. Amide NH proton and CH2 proton adjacent to amide C = O in every cyclophane host showed down-field NMR shifts in the presence of the guest acids in CHCl3-d, suggesting the formation of 1:1 complexes in which the carboxyl group of an acid molecule formed two hydrogen bonds with the amide NH and C = O moieties of a host molecule. Since the complex formation competed with the dimerization of the guest acids, the monomer–dimer equilibrium was restudied by NMR and the equilibrium constant was determined to be 330 M− 1 for acetic acid and 518 M− 1 for benzoic acid. By using these values, the formation constants of the host–guest complexes were determined to be 8–51 M− 1. The close contact between the host and guest molecules via hydrogen bonding was consistently confirmed by NMR shifts due to the ring current of aromatic group.

Complexation of neurotransmitters ‒ dopamine, serotonin and melatonin ‒ with a DTPA-based cyclophane of high rigidity: 1H NMR shift and line-broadening

Abstract The 1H NMR signals of the titled neurotransmitters undergo up-field shift accompanied by line-broadening in NMR titration with the DTPA-based cyclophane at pD 7.3; the cyclophane consists of a 4,4′-bis(1,1′-biphenyl-4,4′-dihydroxy)dianiline unit cyclised by a DTPA (diethylenetriaminepentaacetate) group through two amide linkages. Changes in chemical shifts of dopamine indicate the formation of a 1:1 complex with the formation constant K1 400 M−1; the complex of serotonin is likely to form a 2:1 host‒guest complex with β2 ≈ 105 M−2; melatonin does not form a complex with definite stoichiometry. The primary binding forces in the dopamine and serotonin complexes are electrostatic interaction between cationic neurotransmitter and anionic cyclophane molecules, and the resulting ionic pairs are stabilised by encapsulation. The electrostatic interaction is weakened by electrolytes; in 0.1 M Trizma buffer, dopamine does not yield a definite complex, and serotonin forms a 1:1 complex with K1 80 M−1. Extreme line-broadening of neurotransmitter signals suggests that the molecular motion of the guest molecule is slowed in the complex by interactions with the receptor molecule whose internal molecular motion is quenched partially. The high rigidity of the cyclophane enhances intermolecular interaction in the hydrophobic regions to prolong the lifetime of the complex.