Chemical reactivity of lumazine derivatives. Acid–base and redox reactions of a number of lumazines, deazalumazines, and isoalloxazines. An unusual effect of methyl substitution

1981 ◽  
Vol 59 (18) ◽  
pp. 2755-2765 ◽  
Author(s):  
Ross Stewart ◽  
R. Srinivasan ◽  
Stewart J. Gumbley
Keyword(s):  
Chemical Reactivity ◽  
Chemical Properties ◽  
Methyl Groups ◽  
Polarographic Reduction ◽  
Structural Variations ◽  
Methyl Group ◽  
Methyl Substitution ◽  
General Base ◽  
Conjugate Acid ◽  
Resonance Stabilization

Three lumazines, seven deazalumazines, and two isoalloxazines have been studied and the following chemical properties measured; equilibrium acidity (pKa), equilibrium basicity [Formula: see text] polarographic reduction potential [Formula: see text] and, where applicable, the rates of the general acid and general base catalyzed dissociation of the protons in the C-5 and C-7 methyl groups [Formula: see text] The latter quantities were determined by means of isotope exchange or iodination. Structural variations in the substrate produce compatible results in pKa and [Formula: see text] and, to lesser extents, in [Formula: see text] and in the catalytic rate data. Anomalies in the rate constants appear when a methyl group is present at C-6; it facilitates ionization of the C-5 and C-7 methyl groups, reversing the quite large effect of more distant methyl. Buttressing effects appear to be absent. The effects of methyl substitution are attributed, in part, to (a) resonance stabilization by 5- and 7-methyl groups in the reactant molecules (neutral substrate and conjugate acid), and (b) relief of steric strain in going from essentially aromatic reactants to nonplanar intermediates. The significance of the 6-methyl group in the lumazine–flavin conversion is noted.

General-base-catalyzed Proton Exchange in Glycocyamidiniom Ions: A Comparison of Activation by Protonation and by Alkylation

1975 ◽  
Vol 53 (19) ◽  
pp. 2906-2910 ◽  
Author(s):  
Ross Stewart ◽  
R. Srinivasan
Keyword(s):  
Exchange Rate ◽  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Proton Magnetic Resonance Spectroscopy ◽  
Proton Exchange ◽  
Resonance Spectroscopy ◽  
Methyl Groups ◽  
General Base ◽  
Satisfactory Model ◽  
Conjugate Acid

The rates of exchange of methylene protons in seven glycocyamidinium ions in formate and chloroacetate buffers were measured in D2O by means of proton magnetic resonance spectroscopy. When protonation and methylation of glycocyamidines occur at the same site the methyated products react faster than the protonated products by factors of 1.8–1.9. Replacing ring protons in glycocyamidinium ions by methyl groups causes small increases in the méthylène exchange rate whereas corresponding substitution at the exocyclic nitrogen causes small rate decreases. It is concluded that in this reaction a quaternary ion ZR+ can serve as a satisfactory model for the analogous conjugate acid ZH+.

Acid and Base Catalysis in the Oxidation of 6,7,8-Trimethyllumazine. II. Kinetics and Mechanism

1974 ◽  
Vol 52 (23) ◽  
pp. 3884-3894 ◽  
Author(s):  
Ross Stewart ◽  
Kiyotaka Oyama
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Base Catalysis ◽  
Methyl Group ◽  
General Base ◽  
Kinetic Isotope ◽  
Conjugate Acid ◽  
Acid And Base ◽  
Rate Plot ◽  
Kinetics Of

The kinetics of the oxidation of 6,7,8-trimethyllumazine, 1, by permanganate in aqueous solution is examined. The pH–rate plot shows a minimum at pH 3.5 and there is catalysis by buffers. Above pH 3.5 the 7-keto compound, 5, is produced by a general base catalyzed route the rate of which is identical to that of the reaction that causes exchange at the 7-methyl group. The kinetic isotope effect for oxidation of the 7-CD3 compound is 6.89 at 31.4°.Below pH 3.5 scission of the pyrazine ring occurs, there is a small isotope effect, and the rate is somewhat faster than that observed for exchange of the 7-methyl group. There appear to be three routes leading to product; one, a permanganate-independent reaction catalyzed by general acids, a second which is also permanganate independent and involves hydration of the substrate, and a third which is permanganate dependent and involves attack by permanganate on the conjugate acid of the substrate.The utility of oxidation as a means of locating hydration sites in heterocyclic compounds is discussed and some comments are made on the mechanism of the enzymic oxidation of the ribityl analog of 1.

XIII.—The Appearance in Cross-section of the Hairs of some Carnivores and Rodents

1944 ◽  
Vol 62 (1) ◽  
pp. 99-104 ◽  
Author(s):  
J. L. Stoves
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Chemical Reactivity ◽  
Chemical Properties ◽  
Morphological Differentiation ◽  
Taxonomic Significance ◽  
Structural Variations ◽  
Wide Range ◽  
Different Types ◽  
Whole Mounts

Examination of mammalian pelts reveals the presence of two types of hair—long, glossy fibres known as “guard”- or “top-hair,” and shorter, softer, more curly fibres termed ”ground”- or “under-wool.” Guard-hairs are of two kinds: primary fibres classified by Toldt (1910) as “Leithaare,” and shorter, less robust secondary fibres which Toldt described as “Grannenhaare.” The under-wool is analogous to Toldt's “Wohlhaare.” More recently Toldt (1935) has described the appearance of the various fibre types occurring in hair from a wide range of mammals, and gives an extensive bibliography on hair from many orders of mammalia, including the two at present under consideration. Glaister (1931) made a photomicrographic survey of whole mounts and transverse sections of fibres from a large number of mammals. This work, however, suffers from the disadvantage that the photomicrographs were taken at low magnifications. When photographed at magnifications of the order × 500, transverse sections of fibres readily demonstrate the distribution and structure of cuticle, cortex and medulla. Such information is valuable as a means of studying structural variations in hair from different types of mammals, and often possesses considerable taxonomic significance and practical value—Dearborn (1939) reports the use of cross-sections of mammalian guard-hairs as a help to identifying small hairs found in the stomachs and excreta of predators, while the importance of fibre histology in forensic science has been reviewed by Smith and Glaister (1939). Furthermore, microchemical tests carried out on wool fibres (Bekker and King, 1931) and on transverse sections of kolinsky and hare fibres (Stoves, 1943 a) have shown that cuticle, cortex, and medulla exhibit different chemical properties, so that morphological differentiation is accompanied by variation in chemical reactivity.

scholarly journals Synthesis, Characterization and Evaluation of Peptide Nanostructures for Biomedical Applications

Molecules ◽  
2021 ◽  
Vol 26 (15) ◽  
pp. 4587
Author(s):  
Fanny d’Orlyé ◽  
Laura Trapiella-Alfonso ◽  
Camille Lescot ◽  
Marie Pinvidic ◽  
Bich-Thuy Doan ◽  
...  
Keyword(s):  
Amino Acids ◽  
Biomedical Applications ◽  
Chemical Reactivity ◽  
Physical Stability ◽  
Chemical Properties ◽  
Building Blocks ◽  
Imaging Probes ◽  
Therapeutic Molecules

There is a challenging need for the development of new alternative nanostructures that can allow the coupling and/or encapsulation of therapeutic/diagnostic molecules while reducing their toxicity and improving their circulation and in-vivo targeting. Among the new materials using natural building blocks, peptides have attracted significant interest because of their simple structure, relative chemical and physical stability, diversity of sequences and forms, their easy functionalization with (bio)molecules and the possibility of synthesizing them in large quantities. A number of them have the ability to self-assemble into nanotubes, -spheres, -vesicles or -rods under mild conditions, which opens up new applications in biology and nanomedicine due to their intrinsic biocompatibility and biodegradability as well as their surface chemical reactivity via amino- and carboxyl groups. In order to obtain nanostructures suitable for biomedical applications, the structure, size, shape and surface chemistry of these nanoplatforms must be optimized. These properties depend directly on the nature and sequence of the amino acids that constitute them. It is therefore essential to control the order in which the amino acids are introduced during the synthesis of short peptide chains and to evaluate their in-vitro and in-vivo physico-chemical properties before testing them for biomedical applications. This review therefore focuses on the synthesis, functionalization and characterization of peptide sequences that can self-assemble to form nanostructures. The synthesis in batch or with new continuous flow and microflow techniques will be described and compared in terms of amino acids sequence, purification processes, functionalization or encapsulation of targeting ligands, imaging probes as well as therapeutic molecules. Their chemical and biological characterization will be presented to evaluate their purity, toxicity, biocompatibility and biodistribution, and some therapeutic properties in vitro and in vivo. Finally, their main applications in the biomedical field will be presented so as to highlight their importance and advantages over classical nanostructures.

scholarly journals Topological invariants for the line graphs of some classes of graphs

2019 ◽  
Vol 17 (1) ◽  
pp. 1483-1490
Author(s):  
Xiaoqing Zhou ◽  
Mustafa Habib ◽  
Tariq Javeed Zia ◽  
Asim Naseem ◽  
Anila Hanif ◽  
...  
Keyword(s):  
Communication Theory ◽  
Chemical Reactivity ◽  
Line Graph ◽  
Chemical Properties ◽  
Topological Invariants ◽  
Line Graphs ◽  
Physical And Chemical Properties ◽  
Physical And Chemical ◽  
Ladder Graphs ◽  
And Chemical Properties

AbstractGraph theory plays important roles in the fields of electronic and electrical engineering. For example, it is critical in signal processing, networking, communication theory, and many other important topics. A topological index (TI) is a real number attached to graph networks and correlates the chemical networks with physical and chemical properties, as well as with chemical reactivity. In this paper, our aim is to compute degree-dependent TIs for the line graph of the Wheel and Ladder graphs. To perform these computations, we first computed M-polynomials and then from the M-polynomials we recovered nine degree-dependent TIs for the line graph of the Wheel and Ladder graphs.

Net charges and valence AO's in the methylamines; the hydrogen basis set and the properties of nitrogen

1985 ◽  
Vol 63 (7) ◽  
pp. 1487-1491 ◽  
Author(s):  
Giuseppe Del Re ◽  
Sándor Fliszár ◽  
Michel Comeau ◽  
Claude Mijoule
Keyword(s):  
Basis Set ◽  
Basis Sets ◽  
Methyl Groups ◽  
Extended Form ◽  
Amine Nitrogen ◽  
Methyl Group ◽  
Net Charges ◽  
Extended Basis

Net charges and valence AO's for ammonia, methylamine, dimethylamine, and trimethylamine were calculated using extended basis sets. Superposition effects, evaluated by replacing Pople's standard 6-31G* basis by an extended form in which the basis of the ammonia H atoms and of the methyl groups of trimethylamine are retained in the treatment of each molecule, indicate that the quality of the treatment of amine nitrogen atoms is strongly dependent on the number of methyl groups. A new, augmented basis is proposed for the hydrogens, which appears to be reasonably well balanced: comparison with familiar (e.g., 6-31G*) calculations illustrates in what manner the treatment of nitrogen is worsened when even just one methyl group is replaced by hydrogen unless the impoverishment of the basis is suitably taken care of.

The Rates and Products of Addition of 4-Chlorobenzenesulfenyl Chloride to a Series of Side Chain Methyl Substituted Styrenes

1974 ◽  
Vol 52 (9) ◽  
pp. 1807-1812 ◽  
Author(s):  
George H. Schmid ◽  
Dennis G. Garratt
Keyword(s):  
Steric Hindrance ◽  
Side Chain ◽  
Methyl Groups ◽  
Methyl Group

The rates of addition and the product compositions have been determined for the addition of 4-chlorobenzenesulfenyl chloride to a series of seven side chain methyl substituted styrenes in 1,1,2,2-tetrachloroethane at 25°. Unlike the addition to the corresponding series of methylated ethylenes, the effect of the methyl groups is not cumulative. The effect of the methyl groups depends upon whether or not the β-methyl group is cis to the phenyl. When it is cis, the rate of addition is decreased compared to styrene and substitution of additional methyl groups has only a small effect on the rate of addition. In compounds lacking a cis-β-methyl group the rate of addition more closely resembles that for addition to the methylated ethylenes. Steric hindrance between the cis-methyl and phenyl groups is believed to be the cause of this difference in behavior between the ethylene and styrene series.

scholarly journals Comparative effects of trifluoromethyl- and methyl-group substitutions in proline

2018 ◽  
Vol 42 (16) ◽  
pp. 13461-13470 ◽  
Author(s):  
Vladimir Kubyshkin ◽  
Stanislav Pridma ◽  
Nediljko Budisa
Keyword(s):  
Methyl Group ◽  
Methyl Substitution

What is the outcome of trifluoromethyl-/methyl-substitution in each position of the proline ring? Look inside to find out.

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