Shmuel Bittner

When quinones meet amino acids: chemical, physical and biological consequences

Summary.Quinones and amino acids are usually compartmentally separated in living systems, however there are several junctions in which they meet, react and influence. It occurs mainly in wounded, cut or crushed plant material during harvest, ensiling or disintegrating cells. Diffusing polyphenols are oxidized by polyphenol oxidases (PPOs) to quinonic compounds, which associate reversibly or irreversibly with amino acids and proteins. The reaction takes place with the free nucleophilic functional groups such as sulfhydryl, amine, amide, indole and imidazole substituents. It results in imine formation, in 1,4-Michael addition via nitrogen or sulphur and in Strecker degradation forming aldehydes. The formation and activity of quinone–amino acids conjugates influences the colour, taste, and aroma of foods. Physical and physiological phenomena such as browning of foods, discoloration of plants during processing, alteration of solubility and digestibility, formation of humic substances, germicidal activity, cytotoxicity and more occur when quinones from disintegrating cells meet amino acids. The mechanisms of toxicity and the pathways by which PCBs may be activated and act as a cancer initiator include oxidation to the corresponding quinones and reaction with amino acids or peptides. Sclerotization of insect cuticle is a biochemical process involving also the reaction between quinones and amino acid derivatives.

Aromatic PMOs: tolyl, xylyl and dimethoxyphenyl groups integrated within the channel walls of hexagonal mesoporous silicas

There exists a need to synthesize PMOs with bridging organic groups beyond the few that are accessible from commercially available bis(triethoxysilyl)organic precursors. To fulfill this need, three new periodic mesoporous arylsilicas, aromatic PMOs, having bridging aromatic organic groups as an integral part of the framework have been synthesized and structurally characterized. The surfactant-templated synthesis employed new precursors consisting of two triethoxysilyl groups connected via toluene, p-xylene or 1,4-dimethoxybenzene. Diffraction and microscopy together with 13C CP-MAS NMR, 29Si MAS NMR and FT-Raman/IR measurements proved that the aryl–silica bonds in the new precursors and aromatic PMOs are stable under hydrolytic polycondensation self-assembly and surfactant acid extraction conditions. These PMOs have an ordered hexagonal channel mesostructure, some degree of ordering of aromatic groups likely through π–π stacking in the channel walls, a large pore volume and surface area, and relatively thick arylsilica walls whose dimensions seem to scale with the spatial demands of the aryl group.

A New Electron Donor: synthesis of 2,3,6,7-tetra(ethyltellurio)tetrathiafulvalene

Abstract A new tetratellurio substituted derivative of tetrathiafulvalene has been prepared via a direct one pot three step synthesis.

Autocatalytic Oxidation of Ethers with Sodium Bromate

Abstract Sodium and potassium bromate are stable and easily stored oxidants. They can oxidize both open and cyclic ethers in aqueous solution at room temperature yielding esters and lactones. Kinetic studies of the oxidation of tetrahydrofuran to γ-butyrolactone indicate that the major active oxidation species is bromine and not bromate. The bromate is only a supporting agent, responsible for the initiation step and supplying bromine molecules by oxidizing bromide ions during the propagation step. In the oxidation of tetrahydrofuran, high yields of γ-butyrolactone were obtained.

N,N-bis(quinonyl)amines; synthesis and X-ray structure

Abstract The preparation of several symmetrical and nonsymmetrical N,N-bis(quinonyl)amines is reported. These compounds, which have two quinones separated by one amino group, were obtained by an unexpected reaction of primary or secondary substituted aminoquinones. While not nucleophilic enough to react with simple electrophiles, these aminoquinones did react with haloquinones and also with unsubstituted quinones under very mild conditions to afford diquinonylamines in average to good yields. These vinylogous quinone-imides were characterized using common spectroscopic methods as well as X-ray crystallography.

Preparation and structure of a novel organotellurium derivative: Di(tetrathiafulvalene)telluride [(TTF)2te]

Abstract The synthesis, electrochemical properties and crystal structure of the first compound which bridges two TTF molecules via a tellurium atom are described.

The five bromotryptophans.

Summary.The five regioisomeric bromotryptophans (BrTrps) play an important role in the life of sponges and lower marine invertebrates. These bromo-amino acids, which are formed by post-translational modifications, are not found in nature in their free state, but rather are involved in more complex structures. Any of the BrTrps can be part of a peptide, a cyclic peptide, an indole alkaloid, an ergot alkaloid, a macrocycle and others. The present review covers the synthesis, physical and spectroscopic properties of the five BrTrps. It also describes the many exiting pharmacological and biological activities played by the BrTrps and by various secondary metabolites containing brominated tryptophan moieties. Of special interest are cyclic peptides containing the 2-BrTrp unit, which were isolated from marine sponges e.g. konbamide, orbiculamide A, the various keramamides, jaspamide eusynstyelamide and more. Important families of non-cyclic peptides containing the 6-BrTrp, include the styelins, the conotoxins, the cathelicidins and several constrained macrocyclic peptides. Many marine secondary BrTrp-containing, non-peptidic metabolites also display a remarkable spectrum of bioactivities, which can be harnessed for therapeutic and other purposes. Examples are: barettin, bromotryptanthrin, tetraacetyl clionamide, cyclocinamide A, clavicipitic acid, various brominated β-carbolines. In this review we have presented the various synthetic routes leading to the preparation of the five BrTrps and many of its derivatives. Also, we have introduced the reader to many synthetic routes leading to BrTrp-containing non-peptidic natural products. Although the functional role of the various compounds in the human body is only poorly understood, its effects were extensively studied. Almost all of these compounds exhibit important therapeutic properties e.g. antifungal, antimicrobial, antihelmintic, insecticidal ichthyotoxic and anticancer activity. In the present review attempts have been made to provide synopsis, synthesis and symbiosis of chemical and biological actions, which may provide future guidance and facilitate further research in this area.

Synthesis, electrochemical and spectral properties of some ω-N-quinonyl amino acids

Summary. Four series of ω-N-quinonyl amino acids were synthesized by Michael-like additions. The quinones include 2-phenylthio-1,4-benzoquinone, 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dichloro-1,4-naphthoquinone. These modified amino acids can be used for post chain assembly modifications of biologically active peptides, which target the quinonic drug to a cancer damaged area. The electron-transfer capabilities of the modified amino acids were probed by cyclic voltammetry measurements. The results described in this paper show that the novel N-quinonyl amino acids are effective in producing semiquinone radicals similarly to the unconjugated quinones themselves. A direct relation was found between the first reduction potentials of the quinones and their reactivity towards the ω-amino acids. The successful generation of stable semiquinone radicals by the novel quinone derivatives is a prerequisite for the manifestation of site-directed antitumor activity of corresponding quinone-peptide conjugates.

γ-L-glutamyltaurine

Summary.The discovery of the dipeptide γ-glutamyltaurine (γ-GT; glutaurine, Litoralon) in the parathyroid in 1980 and later in the brain of mammals gave rise to studies on intrinsic and synthetic taurine peptides of this type. It was suggested that γ-glutamyltransferase (GGT; γ-glutamyltranspeptidase) in the brain is responsible for the in vivo formation of this unusual dipeptide. γ-GT has been prepared by both synthetic and enzymatic methods. The chemical syntheses included the use of protecting groups and coupling methods. A wide spectrum of analytical and spectroscopic methods was used to confirm the structure of the synthetic compounds and to elucidate the position of the peptide bond. Enzymatic preparation of γ-GT from taurine takes advantage of the selective transpeptidation action of GGT on L-glutamine, glutathione, γ-glutamyl-p-nitroanilide or other glutamine donors. Although the functional roles of γ-GT in the brain are only poorly understood, many of its established CNS effects have been reported in the last 25 years. Its effect on emotional arousal and its anti-conflict potencies are synergistic with the anxiolytic drug diazepam. γ-GT exhibits anti-conflict potency, which is exerted by reducing aversion or phobia and/or the anxiety levels. γ-GT also acts as endogenous modulator in excitatory aminoacidergic neurotransmission. It is suggested that such acidic peptides through N-methyl-D-aspartic acid receptors could be part of the neurochemical substrate underlying self-stimulation of the medial prefrontal cortex. Other γ-GT effects in neural systems include: effects on the monoamine concentration in the brain; effects on aggressive behavior in the cat; effects on thyroid hormones in the rat; amelioration of electroshock-induced amnesia; potent and long-lasting antiepileptic action (on intra-amygdaloid injection); affect the glutamatergic system in schizophrenic disorders. Roles for γ-GT in non-neural systems have also been reported, e.g., effects on the metamorphosis of amphibians; on plasma rennin regulation; on radiation protection; on uric acid levels; on human antibody-dependent cell-mediated cytotoxicity (ADCC) and many more.

Pentacyclic triterpenoids from Embelia schimperi.

Five oleanane-type pentacyclic triterpenoids were isolated by chromatographic separation of a chloroform extract of the stem bark of Embelia schimperi. Three of these compounds have a methyleneoxy bridge. Two compounds, embelinone and schimperinone, are reported here for the first time from a natural source (they have been synthesized previously during chemical transformations). Their structures were determined by spectroscopic techniques, among which 2-D NMR was useful for complete characterization. Three of the triterpenoids exhibited mild antibacterial properties against the gram-positive bacterial strain Rhodococcus sp.