Cationic Analogues Research Articles

Poly(ethylene oxide)‐block‐poly[2‐(dimethylamino)ethyl methacrylate] as Strengthening Agent in Paper: Dynamic Mechanical Characterization

AbstractTo enhance adhesion properties of PEO on wood fibers, block polymers of PEO and 2‐(dimethylamino)ethyl methacrylate were synthesized. The polymers were further modified to obtain strongly cationic species. The resulting polymers were used as additives in paper sheets. Papers were studied by DMA in a controlled‐humidity chamber. Addition of the PEO block co‐polymers enhanced paper strength. The strength of the paper sheets was highest when polymer with molecular weight of 400 kg · mol−1 was used as an additive. Highly cationic block co‐polymers increased moduli of paper sheets more than their weakly cationic analogs, which indicated strong interaction with fiber surfaces. Strength of the paper sheets decreased both with increased temperature and humidity.magnified image

Cationic, Neutral, and Anionic PNP PdIIand PtIIComplexes: Dearomatization by Deprotonation and Double-Deprotonation of Pincer Systems

A series of cationic, neutral, and anionic Pd(II) and Pt(II) PNP (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) complexes were synthesized. The neutral, dearomatized complexes [(PNP*)MX] (PNP* = deprotonated PNP; M = Pd, Pt; X = Cl, Me) were prepared by deprotonation of the PNP methylene group of the corresponding cationic complexes [(PNP)MX][Cl] with 1 equiv of base (KN(SiMe(3))(2) or (t)BuOK), while the anionic complexes [(PNP**)MX](-)Y(+) (PNP** = double-deprotonated PNP; Y = Li, K) were prepared by deprotonation of the two methylene groups of the corresponding cationic complexes with either 2 equiv of KN(SiMe(3))(2) or an excess of MeLi. While the reaction of [(PNP)PtCl][Cl] with an excess of MeLi led only to the anionic complex without chloride substitution, reaction of [(PNP)PdCl][Cl] with an excess of MeLi led to the methylated anionic complex [(PNP**)PdMe](-)Li(+). NMR studies, X-ray structures, and density functional theory (DFT) calculations reveal that the neutral complexes have a "broken" aromatic system with alternating single and double bonds, and the deprotonated arm is bound to the ring by an exocyclic C=C double bond. The anionic complexes are best described as a pi system comprising the ring carbons conjugated with the exocyclic double bonds of the deprotonated "arms". The neutral complexes are reversibly protonated to their cationic analogues by water or methanol. The thermodynamic parameters DeltaH, DeltaS, and DeltaG for the reversible protonation of the neutral complexes by methanol were obtained.

New catanionic surfactants based on 1-alkyl-3-methylimidazolium alkylsulfonates, [CnH2n+1mim][CmH2m+1SO3]: mesomorphism and aggregation

Anionic and cationic alkyl-chain effects on the self-aggregation of both neat and aqueous solutions of 1-alkyl-3-methylimidazolium alkylsulfonate salts ([C(n)H(2n+1)mim][C(m)H(2m+1)SO(3)]; n = 8, 10 or 12; m = 1 and n = 4 or 8; m = 4 or 8) have been investigated. Some of these salts constitute a novel family of pure catanionic surfactants in aqueous solution. Examples of this class of materials are rare; they are distinct from both mixed cationic-anionic surfactants (obtained by mixing two salts) and gemini surfactants (with two or more amphiphilic groups bound by a covalent linker). Fluorescence spectroscopy and interfacial tension measurements have been used to determine critical micelle concentrations (CMCs), surface activity, and to compare the effects of the alkyl-substitution patterns in both the cation and anion on the surfactant properties of these salts. With relatively small methylsulfonate anions (n = 8, 10 and 12, m = 1), the salts behave as conventional single chain cationic surfactants, showing a decrease of the CMC upon increase of the alkyl chain length (n) in the cation. When the amphiphilic character is present in both the cation and anion (n = 4 and 8, m = 4 and 8), novel catanionic surfactants with CMC values lower than those of the corresponding cationic analogues, and which exhibited an unanticipated enhanced reduction of surface tension, were obtained. In addition, the thermotropic phase behaviour of [C(8)H(18)mim][C(8)H(18)SO(3)] (n = m = 8) was investigated using variable temperature X-ray scattering, polarising optical microscopy and differential scanning calorimetry; formation of a smectic liquid crystalline phase with a broad temperature range was observed.

6- and 14-Fluoro farnesyl diphosphate: mechanistic probes for the reaction catalysed by aristolochene synthase

The catalytic mechanism of the enzyme aristolochene synthase from Penicillium roqueforti (PR-AS) has been probed with the farnesyl diphosphate analogues 6- and 14-fluoro farnesyl diphosphate (1a and 1c). Incubation of these analogues with PR-AS followed by analysis of the reaction products by GC-MS and NMR spectroscopy indicated that these synthetic FPP analogues were converted to the fluorinated germacrene A analogues 3b and 3c, respectively. In both cases the position of the fluorine atom prevented the formation of the eudesmane cation analogues 4b and 4c. These results highlight that germacrene A is an on-path reaction intermediate during PR-AS catalysis and shed light on the mechanism by which germacrene A is converted to eudesmane cation. They support the proposal that the role of PR-AS in the cyclisation is essentially passive in that it harnesses the inherent chemical reactivity present in the substrate by promoting the initial ionisation of farnesyl diphosphate and by acting as a productive template to steer the reaction through an effective series of cyclisations and rearrangements to (+)-aristolochene (7a).

Stable Near-Infrared Anionic Polymethine Dyes: Structure, Photophysical, and Redox Properties

The concept of cyanine has been successfully extended to an anionic heptamethine dye featuring tricyanofuran (TCF) moieties in terms of structure, reactivity, and photophysical properties. Importantly, absorption and emission are red-shifted compared to its classical cationic analog without any cost in terms of thermal stability. In addition to its "cyanine" behavior, this molecule exhibits further redox properties: oxidation and reduction led to the reversible formation of radical species whose absorption is in marked contrast with that of cyanines.

Ion channels of various types induced in lipid membranes by gramicidin a derivatives carrying a cationic sequence at their C-termini

The channel-forming activity of gramicidin A derivatives carrying positively charged amino acid sequences at their C-termini was studied on planar bilayer lipid membranes and liposomes. We showed previously that, at low concentrations, these peptides form classical cation-selective pores typical of gramicidin A, whereas, at high concentrations, they form large nonselective pores. The ability of the peptides to form nonselective pores, which was determined by the efflux of carboxyfluorescein, an organic dye, from liposomes, decreased substantially as the length of the gramicidin fragment in the series of cationic analogues was truncated. CD spectra showed that large pores are formed by peptides having both beta6.3 single-stranded and beta5.6 double-stranded helical conformations of the gramicidin fragment, with the C-terminal cationic sequence being extended. The dimerization of the peptides by the oxidation of the terminal cysteine promoted the formation of nonselective pores. It was shown that nonselective pores are not formed in membranes of erythrocytes, which may indicate a dependence of the channel-forming ability on the membrane type. The results may be of interest for the directed synthesis of peptides with antibacterial activity.

Synthesis and Properties of a Cationic Bidentate Lewis Acid

As part of our efforts to increase the fluoride affinity of bidentate Lewis acids, we have set out to determine if the F(-) anion chelation occurring in such systems can be complemented by favorable Coulombic attractions. To explore this idea, the neutral B/Hg bidentate Lewis acid 1-{Mes(2)B}-8-{(2,6-Me(2)-4-Me(2)NC(6)H(2))Hg}C(10)H(6) (3) and its cationic analogue [1-{Mes(2)B}-8-{(2,6-Me2-4-Me(3)NC(6)H(2))Hg}C(10)H(6)]+ ([4]+) have been synthesized and studied. Compound 3 as well as the triflate salt of [4]+ react with [S(NMe(2))3][Me(3)SiF(2)] to afford the corresponding fluoride complexes [3-micro(2)-F]- and [4-micro(2)-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic centers in both [3-micro(2)-F]- and [4-micro(2)-F]. UV-vis titration experiments carried out in tetrahydrofuran/water (9/1, v/v) mixtures indicate that the fluoride binding constants of 3 and [4]+ are clearly differentiated and are equal to 1.3 (+/-0.1) x 10(2) M(-1) and 6.2 (+/-0.2) x 10(4) M(-1), respectively. The enhanced fluoride binding constant of [4]+, when compared to 3, confirms that the chelate effect occurring in these types of fluoride receptors can be combined with favorable Coulombic attractions to strengthen the host-guest interaction. Cation [4]+ remains highly selective for F- over other environmentally abundant anions including Cl-, Br-, NO(3)(-), H(2)PO(4)(-), and HSO(4)(-) and shows only a weak response to OAc(-). Finally, the addition of an aqueous solution of Al3+ to a solution containing [4-micro(2)-F] leads to complete regeneration of [4]+, showing that F(-) binding is reversible.

Neutral and Cationic Cyclopentadienyliron Macromolecules Containing Azo Dyes

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Use of novel cationic bile salts in cholesterol crystallization and solubilization in vitro

Unnatural bile salts have been synthesized with a cationic group at the side chain of natural bile acids. These cationic bile salts aggregate in water and aqueous salt solutions in a manner similar to their natural counterparts. The critical micellar concentrations of the cationic bile salts were measured using a fluorescence method. Cationic bile salts aggregated at a concentration lower than natural deoxycholic acid. Since dihydroxy bile salt micelles are well known for cholesterol dissolution/removal, the dissolution in the cationic micelles has been evaluated. The cationic analogs dissolve approximately 70 mg/dL of cholesterol, which is comparable to taurochenodeoxycholate micelle under identical bile salt concentrations. Cholesterol dissolution in cationic bile salt micelle enhanced upon adding various amounts of PC. Cholesterol crystallization was studied in model bile at various cationic bile salt concentrations. The addition of 5, 15 and 30 mM of the cationic bile salts attenuated the crystallization process, without influencing the crystal observation time or decreasing the final amount of crystals formed. All these effects were comparable to those observed with cholic acid. These findings suggest that cationic bile salts have physico-chemical properties analogous to those of natural anionic bile salts, and thus may have therapeutic potential.

Pharmacokinetics of a New, Orally Available Ceftriaxone Formulation in Physical Complexation with a Cationic Analogue of Bile Acid in Rats

Oral administration of ceftriaxone associated with a bile acid-based new oral carrier, cholylethylenediamine, in 50% propylene glycol to rats at doses of 25 and 50 mg/kg of body weight resulted in a significant increase in intestinal absorption, as evidenced by 55% improvement in the bioavailability, whereas ceftriaxone alone showed a bioavailability of less than 1%.

Inhibition of Growth and Telomerase Activity by Novel Cationic Ceramide Analogs With High Solubility in Human Head and Neck Squamous Cell Carcinoma Cells

Inhibition of Growth and Telomerase Activity by Novel Cationic Ceramide Analogs With High Solubility in Human Head and Neck Squamous Cell Carcinoma Cells

Hydrogen-Substituted Osmium Silylene Complexes: Effect of Charge Localization on Catalytic Hydrosilation

Addition of bulky primary silanes to the osmium benzyl compound, Cp*(iPr3P)OsCH2Ph, afforded two neutral hydrogen-substituted silylene complexes via activation of two Si-H bonds. These species have been structurally characterized, and their reactivity has been examined experimentally and computationally. Comparison of these neutral silylene complexes with their cationic analogue highlights the dramatic influence of charge distribution on the reaction chemistry of metal silylene complexes.

Cationic Analog of Deoxycholate as an Oral Delivery Carrier for Ceftriaxone

A great number of currently available drugs and those being developed that fall under the class III of the BCS (biopharmaceutical classification system) possess high therapeutic potential but cannot be delivered by peroral absorption. In this study, we synthesized a cationic analog of deoxycholic acid and evaluated its efficacy as a potential oral drug carrier for anionic BCS class III drugs using ceftriaxone as a model drug. Cationic deoxycholylethylamine (DCEA), one of cationic analogs of deoxycholic acid, was synthesized and made into physical complexes at varied molar ratios with an anionic ceftriaxone. These drug complexes were formulated with propylene glycol and their oral bioavailabilities were evaluated. The results of the partition coefficient study revealed a highly water soluble ceftriaxone that became more hydrophobic as the molar ratio of the carrier in the complex increased. When the ceftriaxone/DCEA formulation was administered into a nonclosed segment of duodenum of rats, C(max) (the maximum drug concentration in plasma) and AUC (area under the curve) were significantly increased and its bioavailability was increased up to 70%. Therefore, the new cationic carriers proposed in this study could improve the absorption of BCS class III drugs in the intestine with maintaining their full biological activity.

Dihydrogen Complexes of Electrophilic Metal Centers: Observation of Cr(CO)5(H2), W(CO)5(H2) and [Re(CO)5(H2)]+

Photolysis of dichloromethane solutions of M(CO)6 (M = Cr, W) at low temperature in the presence of hydrogen gas affords W(CO)5(H2) (1) and Cr(CO)5(H2) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T1 values for the hydride resonances and a large HD coupling of 35.3 Hz (W) and 35.8 Hz (Cr) in the HD derivatives. A cationic analogue, [Re(CO)5(H2)]+ (3), was prepared by reaction of Re(CO)5Cl with [Et3Si][B(C6F5)4] in fluorobenzene under hydrogen. Complex 3-d1 exhibits JHD = 33.9 Hz. Complex 3 is strongly acidic, with complete deprotonation by diethyl ether; complexes 1 and 2 are moderately acidic. Deprotonation of 1 is complete in the presence of one equivalent of triethylamine.

Monolayer-Controlled Substrate Selectivity Using Noncovalent Enzyme−Nanoparticle Conjugates

Electrostatic interactions were used to noncovalently conjugate chymotrypsin to gold nanoparticles featuring hybrid tetraethylene(glycol)alkanethiol monolayers terminated with carboxylate groups. This conjugation process greatly alters the substrate selectivity of the adsorbed chymotrypsin, inhibiting the hydrolysis of anionic subtrates without affecting the hydrolysis rate of cationic analogues.

The rearrangement of dicarboranyl methyl cation: a possible synthetic strategy toward cationic closo-tricarbaboranes.

A quantitative study on cationic closo-tricarbaboranes proved their stability and a possible use for them as weakly coordinating ions due to the dispersal of positive charge throughout the cage. The current study explores computationally a synthetic strategy toward their realization in parallel with the benzyl cation-tropylium ion rearrangement. This study shows that cage expansion along with the incorporation of a carbon atom into the cage starting from the dicarboranyl methyl cation is in the realm of the possible. The rearrangements are found to have favorable energy barriers with one transition state. The geometry of the dicarboranyl methyl cations (benzyl cation analogues) with bent CH(2) groups favors the rearrangement into the tropylium analogues. Thus, the comparison of well-known benzyl ion-tropylium ion rearrangement with similar reactions among polyhedral boranes unravels the feasibility of cationic tricarboranes.

2′‐O‐[2‐[2‐(N,N‐Dimethylamino)ethoxy]ethyl] Modified Oligonucleotides: Symbiosis of Charge Interaction Factors and Stereoelectronic Effects.

Oligonucleotides with a novel, 2‘-O-[2-[2-(N,N-dimethylamino)ethoxy]ethyl] (2‘-O-DMAEOE) modification have been synthesized. This modification, a cationic analogue of the 2‘-O-(2-methoxyethyl) (2‘-O-MOE) modification, exhibits high binding affinity to target RNA (but not to DNA) and exceptional resistance to nuclease degradation. Analysis of the crystal structure of a self-complementary oligonucleotide containing a single 2‘-O-DMAEOE modification explains the importance of charge factors and gauche effects on the observed antisense properties. 2‘-O-DMAEOE modified oligonucleotides are ideal candidates for antisense drugs.

Structural Preorganization of Peptide Nucleic Acids: Chiral Cationic Analogues with Five‐ or Six‐Membered Ring Structures

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ab initio/GIAO-MP2-calculated structures and (11)B-(13)C NMR chemical shift relationship in hypercoordinate onium-carbonium dications and isoelectronic onium-boronium cations.

The boronium-carbonium ion continuum was extended to include hypercoordinated onium-carbonium dications and the isoelectronic onium-boronium cation analogs. Structures and (13)C and (11)B NMR chemical shifts of the onium-carbonium dications and the corresponding isoelectronic and isostructural onium-boronium cations were calculated with the ab initio/GIAO-MP2 method. The data show a good linear correlation between (11)B and (13)C NMR chemical shifts, indicating that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated onium ions and dications.

Structural Preorganization of Peptide Nucleic Acids: Chiral Cationic Analogues with Five- or Six-Membered Ring Structures

The advent of aminoethylglycyl peptide nucleic acids, aegPNAs, as strong and specific DNA/RNA binding agents has triggered much research activity directed towards the development of PNA-based antisense/antigene therapeutics. These efforts have mainly been directed towards further refinement of aegPNA properties such as water solubility, cellular uptake and discrimination between parallel and antiparallel binding modes. Introduction of chirality and also of positive/negative charges in the PNA backbone has met with some success in this direction. The conformational freedom in the nucleobase linker arm and in the backbone aminoethyl and glycyl segments in the aegPNA backbone were found to be causes of unfavourable entropic loss during complex formation. Suitable clamping in the aegPNA backbone may reduce entropic loss and help produce a conformation appropriate for maximum enthalpic benefits from nucleobase recognition. Introduction of constraint by means of five- or six-membered ring structures in the aegPNA and their contributions to maintaining the balance between rigidity and flexibility in the backbone have shown interesting effects on the overall stability of PNA-DNA/RNA complexes. This review presents an account of the literature in this direction. The significant promise of our approach, which makes use of the naturally occurring trans-4-hydroxy-L-proline to arrive at different chirally pure cyclic PNA analogues, is presented in this review, together with the DNA binding properties of the compounds. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)