Guanidinium Cations Research Articles

Synthesis and characterization of a new family of energetic salts based on guanidinium cation containing furoxanyl functionality

A new family of energetic salts based on a guanidinium cation containing furoxanyl functionality was synthesized and well characterized.

Proton transport behaviour and molecular dynamics in the guanidinium triflate solid and its mixtures with triflic acid

Knowledge of the proton transport behaviour in electrolyte materials is crucial for designing and developing novel solid electrolytes for electrochemical device applications such as fuel cells or batteries. In the present work, high proton conductivity (approximately 10−3 S cm−1) was observed in the triflic acid (HTf) containing guanidinium triflate (GTf) composites. The proton transport mechanism in the composite was elucidated by comparing the diffusion coefficients obtained from NMR and conductivity measurements. Several orders of magnitude enhancement of conductivity is observed upon addition of HTf to the organic solid, and this appears to follow percolation behaviour with a percolation threshold of approximately 2% HTf. The data support a structural diffusion (or Grotthuss) mechanism of proton transport with a calculated Haven ratio significantly less than unity. 13C SUPER and 14N overtone NMR experiments were used to study the mobility and symmetry of the triflate anion and guanidinium cation respectively at a molecular level. The former experiment shows that the CF3 group in the anion displays fast and isotropic motion at room temperature. In contrast to the high mobility of the anion group, the 14N overtone experiments indicate that the guanidinium cation is static in both the pure and the acid-containing GTf samples at room temperature. It is anticipated that these solid-state NMR techniques may be also applied to other organic solid state electrolyte materials to achieve a better understanding of their transport mechanisms and molecular dynamics.

Crystallisation-Driven Self-Sorting of H-Bonded Guanidinium or Sodium Sulfonate Constitutional Molecular Frameworks

Four single crystal structures were obtained in order to better understand the difference between the crystallisation-driven formation of solid state organo-sulfonate guanidinium superstructures and their sodium salt analogues. 5-Formyl-2-furfuryl sulfonate and 4,4′-diamino-2,2′-biphenyl disulfonate were chosen as anions due to functional and geometrical considerations together with guanidinium, aminoguanidinium, and sodium cations. 5-Formyl-2-fufuryl sulfonate reacts with aminoguanidinium cation in water leading to an imino-zwitterionic compound, able to form on its own a guanidinium-sulfonate superstructure in crystalline state.

A novel ionic liquid for Li ion batteries – uniting the advantages of guanidinium and piperidinium cations

We report on the synthesis and the properties of N,N,N′,N′-tetramethyl-N′′,N′′-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)imide (PipGuan-TFSI). The cation of this novel ionic liquid combines guanidinium and piperidinium structural elements. We tested it for its viscosity, ion conductivity, and also for its thermal and electrochemical stability. Furthermore, a 0.5 M solution of lithium TFSI in PipGuan-TFSI was tested as an electrolyte for Li-ion batteries. These experiments included cycles of Li deposition/dissolution on stainless steel and (de)intercalation into/from LiFePO4 electrodes. The tests involving LiFePO4 cathodes were performed at various C-rates and temperatures for a better quantitative comparison to other electrolyte systems. We discuss in how far PipGuan-TFSI successfully combines the advantages of guanidinium and piperidinium ionic liquids for battery electrolyte applications.

Cell-penetrating poly(disulfide)s: focus on substrate-initiated co-polymerization

Outperforming cell-penetrating peptides, cell-penetrating poly(disulfide)s are attracting increasing attention. Recently we have shown that cell-penetrating poly(disulfide)s can be grown directly on substrates of free choice before delivery and depolymerized right afterwards. These unique characteristics are compatible with the general, non-toxic, traceless yet covalent delivery of substrates in an unmodified form. The objective of this study was to elaborate on substrate-initiated co-polymerization. The original propagators contain a strained disulfide for ring-opening disulfide-exchange polymerization and a guanidinium cation to assure cell-penetrating activity. Here, we report individually optimized conditions to polymerize these original propagators together with several other propagators. The nature of these new propagators significantly affected polymerization efficiency and conditions as well as size, polydispersity and transport activity of the final co-polymers. According to gel permeation chromatography, the length of co-polymers increases with hydrophobicity, bulk and valency of the co-propagators, whereas ion pairing with boronates gives shorter co-polymers and branching increases polydispersity. The activity of co-polymers increases with length, π-acidity, superhydrophobicity and boronate counterions. Hydrophobicity, π-basicity, bulk and branching appear less important for activity in fluorogenic vesicles. The here reported design, synthesis and evaluation of substrate-initiated co-polymers will be essential to find the best cell-penetrating poly(disulfide)s.

Theoretical study on the chemical fixation of carbon dioxide with propylene oxide catalyzed by ammonium and guanidinium salts

The cycloaddition of carbon dioxide onto propylene oxide catalysed by ammonium and guanidinium salts has been investigated using density functional theory (DFT) in order to understand the catalytic mechanism and the role of the cation and the anion. Two different possible pathways were considered, but it has been found that the one whereby the activation of the epoxide by the onium salt occurs before the addition of CO2 was consistent with the experimental findings. The rate-determining step was found to be the ring opening of the epoxide that results from the nucleophilic attack of the anion of the catalyst on the non-substituted carbon of the epoxide ring. Moreover, we found that in order to have an efficient catalyst, it is necessary to have an anion with an ambivalent nature (a high nucleophilicity and good leaving group ability). Finally, it turns out that for a given anion, the guanidinium salt is more efficient than the ammonium salt in opening the epoxide ring, thanks to the ability of the guanidinium cation to form hydrogen bonds with the oxygen of the epoxide.

Syntheses and characterization of liposome-incorporated adamantyl aminoguanidines

A series of mono and bis-aminoguanidinium adamantane derivatives has been synthesized and incorporated into liposomes. They combine two biomedically significant molecules, the adamantane moiety and the guanidinium group. The adamantane moiety possesses the membrane compatible features while the cationic guanidinium subunit was recognized as a favourable structural feature for binding to complementary molecules comprising phosphate groups. The liposome formulations of adamantyl aminoguanidines were characterized and it was shown that the entrapment efficiency of the examined compounds is significant. In addition, it was demonstrated that liposomes with incorporated adamantyl aminoguanidines effectively recognized the complementary liposomes via the phosphate group. These results indicate that adamantane derivatives bearing guanidinium groups might be versatile tools for biomedical application, from studies of molecular recognition processes to usage in drug formulation and cell targeting.

CATALYTIC MECHANISM OF ALL-TRANS-RETINOIC ACID 4-HYDROXYLATION MEDIATED BY CYTOCHROME P450 2C8: HOW DOES ARGININE 241 AFFECT THEC–HBOND ACTIVATION?

Experiments revealed that cytochrome P450 2C8 enzyme (CYP2C8) has two distinct substrate binding sites to the physiologically important molecules, retinoic acids, and the main difference between these two binding sites is whether there is a salt bridge interaction between the anionic carboxylate tail of retinoic acids and the surrounding protein environment. However, the influence of such salt bridge interaction toward catalysis is still elusive. In the present paper, density functional theory (DFT) calculations were employed to research the reaction mechanism of all-trans-retinoic acid (atRA) 4-hydroxylation mediated by CYP2C8. Our DFT calculations revealed that such salt bridge interaction has obvious effects on the reaction mechanism of atRA 4-hydroxylation. In the binding site containing a salt bridge interaction between the anionic carboxylate tail of atRA and the cationic guanidine group of Arg241, C – H bond activation proceeds via a normal hydrogen atom transfer (HAT) mechanism; in the other site without this salt bridge interaction, however, C – H bond activation is achieved via a stepwise electron transfer and hydrogen atom transfer, thus, a novel ET/HAT mechanism. These findings enrich the mechanism patterns of C – H bond activation catalyzed by metalloenzymes and their biomimetics. Meanwhile, the self-interaction error (SIE) problem encountered during our calculations in vacuum was affected and removed by the inclusion of an external electric field in the calculations.

Excited-State Dynamics of Charged Dyes at Alkane/Water Interfaces in the Presence of Salts and Ionic Surfactants

The excited-state dynamics of the cationic dye malachite green (MG) and of the dianionic dye eosin B at the dodecane/water interface has been investigated using femtosecond time-resolved surface second harmonic generation (TR-SSHG). By using different probe wavelengths, the contributions of monomeric and aggregated MG to the signal could be spectroscopically distinguished. The effect of the addition of a small amount of surfactants was found to strongly depend on the relative charges of surfactant and dye. For surfactant/dye pairs with opposite charges, the TR-SSHG signal is dominated by the contribution from aggregates, whereas for pairs with the same charges, the signal intensity becomes vanishingly small. These effects are explained in terms of electrostatic interactions between surfactants and dyes that favor either attraction of the dye toward the interface or its repulsion toward the bulk. As a very similar behavior is observed with MG upon addition of NaSCN, we conclude that, in this case, this effect reflects the affinity of SCN¯ for the interface. On the other hand, the guanidinium cation was found to have a different effect than that of a positively charged surfactant on the SSHG signal of MG, indicating this cation does not accumulate in the interfacial region.

Radiation Stability of Cations in Ionic Liquids. 3. Guanidinium Cations

Due to their superb structural versatility, guanidinium cations find increasing use as constituent ions in room temperature ionic liquids (ILs). This versatility allows fine-tuning of hydrophobicity, which is an important concern for the use of ILs as diluents for metal ion separations. However, the presence of six C-N bonds in such cations poses a question, whether the guanidinium based ILs can be considered as diluents for nuclear separations, given that the radiation emitted by the decaying radionuclides can break these relatively weak bonds over the use cycle of the solvent. As nothing is presently known about the radiolytic stability of the guanidinium cations, we addressed this question using 2-dialkylamino-1,3-dimethylimidazolidine based cations (R = Et, Pr, and Bu) as a representative model for the entire class of such cations, and assessed their stability in 2.5 MeV electron beam radiolysis. Electron paramagnetic resonance spectroscopy, nuclear magnetic resonance spectroscopy, and mass spectrometry have been used to establish chemical mechanisms for radiation damage in guanidinium cations. Our conclusion is that radiation stability of these cations is not significantly different from that of more familiar aliphatic and aromatic IL cations. In fact, these cations yield exceptionally stable radicals, and fragmentation occurs only in their radiolytically generated excited states. The predominant chemical pathway for the cation decomposition is the elimination of their aliphatic arms, with radiolytic yields of 0.65 to 1.06 to 1.46 per 100 eV from R = Et to R = Bu, respectively. The total loss of the parent cation was estimated as 2.62, 1.65, and 1.98 species per 100 eV. While this attrition is not negligible, it is comparable to other organic cations that have fewer fissile C-N bonds. Many of the products are either modified guanidinium ions or protonated bases that are not expected to interfere with radionuclide separations.

Synthesis of guanidinium‐based anion exchange membranes and their stability assessment

Alkaline fuel cells potentially offer improved conversion efficiency and the prospect of using non‐noble metal catalysts; however, low conductivity and fast degradation of anion exchange membranes (AEMs) prevent their widespread application. In this work, a series of novel composite AEMs were synthesized by incorporating guanidinium‐based polymers into a porous polytetrafluoroethylene (PTFE) film. The guanidinium‐based polymers were polymerized using a condensation process between a guanidinium salt and two different diamines so that the guanidinium cations were tethered to the polymer backbone to enhance both conductivity and durability. In addition, polymer crosslinking was conducted to further reinforce the mechanical strength of the membranes and interlock the guanidinium moieties to the porous PTFE. It was found that the ionic conductivity of the synthesized membrane reached up to approximately 80 mS cm−1 at 20°C in deionized water. These membranes also exhibited superior stability compared to commercial quaternary ammonium AEMs after being exposed in 5 M KOH solution at 55°C for 50 h. Copyright © 2013 John Wiley & Sons, Ltd.

A facile strategy for the synthesis of guanidinium-functionalized polymer as alkaline anion exchange membrane with improved alkaline stability

A facile strategy for the synthesis of guanidinium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with improved alkaline stability was developed by the reaction of bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with 2-benzyl-1,1,3,3-tetramethylguanidine (BTMG). The chemical structure of guanidinium-functionalized polymers was confirmed by 1H NMR and FT-IR spectroscopy. A crosslinked membrane of the polymer which have an ionic exchange capacity (0.80mmol/g) and low swelling ratio (<15%) was fabricated. The ionic conductivity of the cross-linked membrane was higher than 1.0×10−2Scm−1 at room temperature and 5.1×10−2Scm−1 at 70°C. The microstructure of hydrophobic/hydrophilic phase separation of the film was observed by transmission electron microscopy (TEM). Finally, the alkaline stabilities of model guanidiniums and the cross-linked membrane were evaluated respectively. The results showed that the bulky guanidinium cation exhibited higher alkaline stability due to the steric hindrance of the substituents, and the cross-linked membrane remain the ionic conductivity of 4.2×10−2Scm−1 at 70°C after being immersed in 1M NaOH/D2O solution at 60°C for 2 days. The mechanical properties of the membranes were also measured before and after 2 days of stability testing. All results suggest that this alkaline anion exchange membrane (AAEM) functionalized by bulky guanidinium groups could be considered as a promising candidate for polymer electrolyte membrane in alkaline fuel cells.

Role of a Guanidinium Cation–Phosphodianion Pair in Stabilizing the Vinyl Carbanion Intermediate of Orotidine 5′-Phosphate Decarboxylase-Catalyzed Reactions

The side chain cation of Arg235 provides a 5.6 and 2.6 kcal/mol stabilization of the transition states for orotidine 5'-monophosphate (OMP) decarboxylase (OMPDC) from Saccharomyces cerevisiae catalyzed reactions of OMP and 5-fluoroorotidine 5'-monophosphate (FOMP), respectively, a 7.2 kcal/mol stabilization of the vinyl carbanion-like transition state for enzyme-catalyzed exchange of the C-6 proton of 5-fluorouridine 5'-monophosphate (FUMP), but no stabilization of the transition states for enzyme-catalyzed decarboxylation of truncated substrates 1-(β-d-erythrofuranosyl)orotic acid and 1-(β-d-erythrofuranosyl) 5-fluorouracil. These observations show that the transition state stabilization results from formation of a protein cation-phosphodianion pair, and that there is no detectable stabilization from an interaction between the side chain and the pyrimidine ring of substrate. The 5.6 kcal/mol side chain interaction with the transition state for the decarboxylation reaction is 50% of the total 11.2 kcal/mol transition state stabilization by interactions with the phosphodianion of OMP, whereas the 7.2 kcal/mol side chain interaction with the transition state for the deuterium exchange reaction is a larger 78% of the total 9.2 kcal/mol transition state stabilization by interactions with the phosphodianion of FUMP. The effect of the R235A mutation on the enzyme-catalyzed deuterium exchange is expressed predominantly as a change in the turnover number kex, whereas the effect on the enzyme-catalyzed decarboxylation of OMP is expressed predominantly as a change in the Michaelis constant Km. These results are rationalized by a mechanism in which the binding of OMP, compared with that for FUMP, provides a larger driving force for conversion of OMPDC from an inactive open conformation to a productive, active, closed conformation.

Non-Covalent Interactions: Complexes of Guanidinium with DNA and RNA Nucleobases

Considering that guanidine-based derivatives are good DNA minor groove binders, we have theoretically studied, using the Polarizable Continuum model mimicking water solvation, the complexes formed by the biologically relevant guanidinium cation and the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine, and uracil). The interactions established within these complexes both by hydrogen bonds and by cation-π interactions have been analyzed by means of the Atoms in Molecules and Natural Bond Orbital approaches. Moreover, maps of electron density difference have been produced to understand the cation-π complexes. Finally, the NICS and three-dimensional NICS maps of the cation-π complexes have been studied to understand the effect of the guanidinium cation on the aromaticity of the nucleobases.

Liquid–Vapor Interfacial Properties of Aqueous Solutions of Guanidinium and Methyl Guanidinium Chloride: Influence of Molecular Orientation on Interface Fluctuations

The guanidinium cation (C(NH2)3(+)) is a highly stable cation in aqueous solution due to its efficient solvation by water molecules and resonance stabilization of the charge. Its salts increase the solubility of nonpolar molecules ("salting-in") and decrease the ordering of water. It is one of the strongest denaturants used in biophysical studies of protein folding. We investigate the behavior of guanidinium and its derivative, methyl guanidinium (an amino acid analogue) at the air-water surface, using atomistic molecular dynamics (MD) simulations and calculation of potentials of mean force. Methyl guanidinium cation is less excluded from the air-water surface than guanidinium cation, but both cations show orientational dependence of surface affinity. Parallel orientations of the guanidinium ring (relative to the Gibbs dividing surface) show pronounced free energy minima in the interfacial region, while ring orientations perpendicular to the GDS exhibit no discernible surface stability. Calculations of surface fluctuations demonstrate that, near the air-water surface, the parallel-oriented cations generate significantly greater interfacial fluctuations compared to other orientations, which induces more long-ranged perturbations and solvent density redistribution. Our results suggest a strong correlation with induced interfacial fluctuations and ion surface stability. These results have implications for interpreting molecular-level, mechanistic action of this osmolyte's interaction with hydrophobic interfaces as they impact protein denaturation (solubilization).

Protic Ionic Solids and Liquids Based on the Guanidinium Cation as Phase‐Change Energy‐Storage Materials

Energy storage worth its salt: A series of protic salts are synthesized and their thermal properties characterized; the guanidinium triflate and mesylate salts exhibit high heats of fusion (ΔHf), which indicates their promising role in energy storage applications, particularly in the temperature range of 100–200 °C.

Layered Dinitrostilbene-Based Molecular Solids with Tunable Micro/Nanostructures and the Reversible Fluorescent Response to Explosives

The ability to modulate and control the fluorescence properties of molecular solids at the micro/nanoscale is important to develop high-performance optoelectronic materials and sensors. Here we report the tunable one-photon and two-photon fluorescence as well as micro/nanostructures of dinitrostilbene-based (DNS) chromophore by the formation of layered multicomponent crystals with guanidinium cation (GD) through hydrogen-bonding assembly. The as-prepared GD2DNS bulk crystal shows a red-shift emission as well as enhanced photoluminescence quantum yield and fluorescence lifetime compared with those of the Na2DNS sample, which is related to the structural transfer of DNS from staggered arrangement to parallel fashion within the crystal. Periodic density functional theoretical calculations further show that the introduction of different cationic units can modify the frontier orbital distribution and electronic structure of DNS anions within the multicomponent crystals. Moreover, one-dimensional GD2DNS nanobel...

A Comparative Study on Insensitive Energetic Derivatives of 5‐(1,2,4‐Triazol‐C‐yl)‐tetrazoles and their 1‐Hydroxy‐tetrazole Analogues

AbstractThe synthesis and characterization of selected nitrogen‐rich salts based on 5‐(1,2,4‐triazol‐C‐yl)tetrazoles and their 1‐hydroxy‐tetrazole analogues is presented. The combination with guanidinium, triaminoguanidinium, and hydroxylammonium cations leads to enhanced performance and sensitivities. The main focus of this work is on the energetic properties of those ionic derivatives in comparison to the neutral compounds. Additionally, the positive influence of the introduction of N‐oxides in energetic materials is shown. Structural characterization was accomplished by means of Raman, IR, and multinuclear NMR spectroscopy. The standard enthalpies of formation were calculated for selected compounds at the CBS‐4M level of theory, the detonation parameters were calculated using the EXPLO5.05 program. Additionally, thermal stability was measured via DSC and sensitivities against impact, friction, and electrostatic discharge were determined.

Synthesis, structure, and absorption spectra of complex neptunium(V) chromate with the guanidinium cation, [C(NH2)3]3[NpO2(CrO4)2] · (H2O)

A new complex of Np(V) with the chromate ion and an organic outer-sphere guanidinium cation is isolated from an aqueous solution. Its composition and structure are determined by X-ray diffraction analysis. The structure of [C(NH2)3]3[NpO2(CrO4)2](H2O) (I) is based on anionic chains [CpO2(CrO4)2][n3nt- between which guanidinium cations and crystallization water molecules are localized. Coordination polyhedra of the Np atoms (pentagonal bipyramids) in the anionic chains are joined in pairs through common equatorial edges. Singularities of the electronic and IR absorption spectra of compound I are discussed.

Substrate-initiated synthesis of cell-penetrating poly(disulfide)s.

Lessons from surface-initiated polymerization are applied to grow cell-penetrating poly(disulfide)s directly on substrates of free choice. Reductive depolymerization after cellular uptake should then release the native substrates and minimize toxicity. In the presence of thiolated substrates, propagators containing a strained disulfide from asparagusic or, preferably, lipoic acid and a guanidinium cation polymerize into poly(disulfide)s in less than 5 min at room temperature at pH 7. Substrate-initiated polymerization of cationic poly(disulfide)s and their depolymerization with dithiothreitol causes the appearance and disappearance of transport activity in fluorogenic vesicles. The same process is further characterized by gel-permeation chromatography and fluorescence resonance energy transfer.