Guanidinium Cations Research Articles

A New Organic–Inorganic Hybrid Oxyfluorotitanate [Hgua]2·(Ti5O5F12) as a Transparent UV Filter

A new generation UV absorber is obtained by microwave-heating-assisted hydrothermal synthesis: [Hgua](2)·(Ti(5)O(5)F(12)). The structure of this hybrid titanium(IV) oxyfluoride is ab initio determined from powder X-ray data by combining a direct space method, Rietveld refinement [orthorhombic, Cmm2, a = 22.410(1) Å, b = 11.191(1) Å, c = 3.802(1) Å], and density functional theory geometry optimization. The three-dimensional network is built up from infinite inorganic layers (∞)(Ti(5)O(5)F(12)) separated by guanidinium cations. The theoretical optical gap (3.2 eV) estimated from density of state calculations is in good agreement with the experimental gap (3.3 eV) obtained by UV-vis diffuse reflectivity. The optical absorption is mainly due to O(2p) → Ti(3d) and F(2p) → Ti(3d) transitions at higher energies. The refraction index is low in the visible range (n ≈ 1.9) compared to that of TiO(2) and, consequently, [Hgua](2)·(Ti(5)O(5)F(12)) shows a good transparency adapted to UV shielding. Under UV irradiation at 254 nm for 40 h, the white microcrystalline powder turns to light purple-gray. This color change is caused by the reduction of Ti(IV) to Ti(III), confirmed by magnetic measurements.

Model for the Exceptional Reactivity of Peroxiredoxins 2 and 3 with Hydrogen Peroxide: A KINETIC AND COMPUTATIONAL STUDY

Peroxiredoxins (Prx) are thiol peroxidases that exhibit exceptionally high reactivity toward peroxides, but the chemical basis for this is not well understood. We present strong experimental evidence that two highly conserved arginine residues play a vital role in this activity of human Prx2 and Prx3. Point mutation of either ArgI or ArgII (in Prx3 Arg-123 and Arg-146, which are ∼3-4 Å or ∼6-7 Å away from the active site peroxidative cysteine (C(p)), respectively) in each case resulted in a 5 orders of magnitude loss in reactivity. A further 2 orders of magnitude decrease in the second-order rate constant was observed for the double arginine mutants of both isoforms, suggesting a cooperative function for these residues. Detailed ab initio theoretical calculations carried out with the high level G4 procedure suggest strong catalytic effects of H-bond-donating functional groups to the C(p) sulfur and the reactive and leaving oxygens of the peroxide in a cooperative manner. Using a guanidinium cation in the calculations to mimic the functional group of arginine, we were able to locate two transition structures that indicate rate enhancements consistent with our experimentally observed rate constants. Our results provide strong evidence for a vital role of ArgI in activating the peroxide that also involves H-bonding to ArgII. This mechanism could explain the exceptional reactivity of peroxiredoxins toward H(2)O(2) and may have wider implications for protein thiol reactivity toward peroxides.

Capillary electrophoresis analysis of inorganic cations in post-blast residue extracts applying a guanidinium-based electrolyte and bilayer-coated capillaries

A new CE method was developed for the identification and quantitation of inorganic cations in post-blast residues. The simultaneous analysis in 20 min total runtime of eight cations in post-blast residues (ammonium, potassium, monomethylammonium, calcium, sodium, magnesium, strontium), plus lithium cation as the internal reference, was carried out with a BGE involving a non-CMR (carcinogenic, mutagenic, and harmful to reproduction) chromophore (guanidinium cation) and a double-layer modified capillary (hexadimethrine bromide/polyvinylsulfonate). A study of UV detection conditions using guanidinium ion as the probe led us to set the analysis and reference wavelengths and their associated bandwidths as well as the probe concentration fixed at 15 mM. The successive multiple ionic-polymer layer approach limited the cation adsorption on capillary wall and improved the EOF stability. These caused a significant improvement in method repeatability. Intermediate precisions were 2.4% for corrected areas and 1.3% for normalized migration times. Limits of detection close to 1 mg/L for all cations were obtained. The matrix effects were studied with chemometric approach for different matrices representative of those collected after explosion. Tests with blank matrix extracts of soil, cloth, and with simulated matrix extract containing 800 mg/L Ca²⁺ and 500 mg/L Fe²⁺ were carried out and no significant matrix effects were observed. Finally, analyses of real residues collected after cash dispenser and homemade firework explosions demonstrate excellent correlation between the CE results and those obtained with the ion chromatography method used routinely.

Carboxylate Binding by Indole‐Based Guanidinium Receptors: Acylguanidinium Cations are Better than Aromatic Guanidinium Cations

AbstractThe synthesis of three new indole‐based guanidinium cations 3, 4, and 16, that feature two different types of anion binding sites, either an acylguanidium cation (3) or an aromatic guanidinium cation (4) or both (16), is presented. NMR binding studies with N‐acetylalaninecarboxylate as substrate in dimethylsulfoxide (DMSO) show that the acylated guanidinium cation is a significantly better anion binding site than the aromatic cation by at least one order of magnitude. Therefore, in dication 16, which possesses both binding sites, stepwise formation of the 1:1 and the 1:2 complex is observed with similar affinities for each binding site to those determined for the monocations 3 and 4. However, a more detailed analysis using isothermal titration calorimetry (ITC) studies revealed that this apparent similarity in affinity is due to completely different thermodynamic reasons. For example, whereas substrate binding by the acylguanidinium cation in 3 is controlled by enthalpy, complex formation by the same binding site in dication 16 is driven by entropy. Simply looking at association constants or ΔG values can, therefore, be misleading. A thorough understanding of molecular recognition events requires closer inspection of ΔH and ΔS.

Comprehensive screening of octopus amphiphiles as DNA activators in lipid bilayers: implications on transport, sensing and cellular uptake

Dynamic octopus amphiphiles contain one charged "head," here a guanidinium cation, together several hydrophobic "tails" (or "tentacles") that can be attached and exchanged in situ by reversible hydrazone formation. Quite surprisingly, their ability to activate DNA as transporters in lipid bilayer membranes was found to increase with the number of tails (up to four) as well as with their length (up to eight carbons). Both encouraged and puzzled by these results, we decided that a comprehensive screening of octopus amphiphiles with regard to number (from one to six) and length (from three to eighteen carbons) of their tails would be appropriate at this point. For this purpose, we here report the synthesis of cationic hexahydrazide peptide dendrons together with that of aldehydes with long, saturated, unsaturated and branched hydrophobic tails. Comprehensive screening of the completed collection of tails and heads reveals that the ability of octopus amphiphiles to activate DNA transporters shifts with increasing number of tails to decreasing length of the tails. Moreover, cis-alkenyl and branched alkyl tails are more active than their linear analogs, branched aromatic tails are best. These overall very meaningful trends for octopus amphiphiles will be of importance for sensing applications and fragrant cellular uptake.

Regulating low-dimensional magnetic behavior of organic radicals in crystalline hydrogen-bonded host frameworks

A series of S = ½ organic radicals—TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl), 4-methoxy-TEMPO, and 4-oxo-TEMPO—have been included in the cavities of porous lamellar host frameworks constructed from guanidinium (G) cations and organodisulfonate (DS) anions, which assemble into hydrogen-bonded guanidinium sulfonate (GS) sheets connected by organodisulfonate pillars that create cavities between the sheets. The pillars can project above or below the GS sheet in different configurations, affording various architectural framework isomers whose selectivity is determined by the relative sizes of the guests and the pillars. Each framework isomer is endowed with uniquely sized and shaped cavities, which influence the molecular arrangement of the radical guests included within the host lattice. Notably, TEMPO guests in the 1-D channels of simple brick G22BPDShost (BPDS = biphenyl-4,4′-disulfonate) arrange as a two-leg ladder (a chain of dimers), whereas TEMPO guests in the cavities of zigzag brick G22NDShost (NDS = naphthalene-2,6-disulfonate) form dimers that are distributed into a 2-D square-planar lattice. The high-temperature magnetic susceptibility, χmol, conformed with Curie–Weiss behavior, with negative Weiss constants, consistent with bulk antiferromagnetic ordering. The Neel temperatures (TN) for G22BPDS·2(TEMPO) and G22NDS·2(TEMPO) were found to be 3.83 K and 4.10 K, respectively, as indicated by the presence of a broad maximum in χmol near these temperatures. In contrast, TN for host-free crystalline TEMPO is lower at 2.3 K, suggesting that enforced ordering can alter the magnetic properties of the TEMPO guests. The low-temperature χmol data for G22BPDS·2(TEMPO) agreed well with the Troyer two-leg ladder spin model, with J/kB = −7.29 K along the rungs of the ladder (intra-dimer) and J/kB = −0.45 K along the legs of the ladder (inter-dimer). The χmol data for G22NDS·2(TEMPO) agreed best with Bleaney–Bowers behavior for isolated dimers with J/kB = −3.31 K for the encapsulated TEMPO dimer. These results demonstrate that magnetic behavior, including dimensionality, can be manipulated through judicious selection of host and magnetic guests.

Interactions of guanidinium with benzene-sulphonic, -phosphonic and -arsonic acids and several of their nitro-derivatives

The supramolecular interactions of the planar guanidinium cation ( C ( NH 2 ) 3 + ) with benzenesulphonate (1), -phosphonate (2) and -arsonate (3) anions, and several of their 3 - and 4 -nitro-substituted derivatives is reported. In all cases well-defined crystalline materials, containing hydrogen-bonded networks with quasi-hexagonal sheet lattices were formed. However, the unsubstituted sulphonate (1) formed a 1:1 guanidinium:sulphonate bilayer structure, whilst the unsubstituted phosphonate (2) and arsonate (3) formed 2:1 guanidinium:phosphonate/arsonate single-layer structures with water occluded within the crystal voids. The additional H-bonding interactions resulting in distortion of the crystal voids in (2)/ (3) as compared to the symmetrical hexagonal-form in (1). In the case of the nitro-substituted sulphonate derivatives, the 1:1 bilayer structure of the parent (1) was retained for the 3-nitrobenzenesulphonate (4), but transformed to a 1:1 single-layer system for the 4-nitrobenzenesulphonate (6). The reverse was observed for the nitrated phosphonic acids, whereby the 4-nitrobenzenephosphonate anion in (5) caused little disruption to the 2:1 single-layered structure and quasi-hexagonal sheet seen in (2), but the 3-nitrobenzenephosphonate caused a breakdown of the network forming a new, complex ribbon system (7). The greater complexity of the P/As (−2) structures compared to the S (−1) structure is attributed to the higher charge on the former anions providing additional opportunities for H-bonding. The observation of such interactions clearly indicates the likelihood of such species interacting with biologically-important arginine residues in vivo with concomitant unintended, but likely, toxic consequences.

Three-dimensional hydrogen-bonded structures in the guanidinium salts of the monocyclic dicarboxylic acidsrac-trans-cyclohexane-1,2-dicarboxylic acid (2:1, anhydrous), isophthalic acid (1:1, monohydrate) and terephthalic acid (2:1, trihydrate)

The structures of bis(guanidinium) rac-trans-cyclohexane-1,2-dicarboxylate, 2CH(6)N(3)(+)·C(8)H(10)O(4)(2-), (I), guanidinium 3-carboxybenzoate monohydrate, CH(6)N(3)(+)·C(8)H(5)O(4)(-)·H(2)O, (II), and bis(guanidinium) benzene-1,4-dicarboxylate trihydrate, 2CH(6)N(3)(+)·C(8)H(4)O(4)(2-)·3H(2)O, (III), all reveal three-dimensional hydrogen-bonded framework structures. In anhydrous (I), both guanidinium cations form classic cyclic R(2)(2)(8) N-H...O,O'(carboxylate) and asymmetric cyclic R(2)(1)(6) hydrogen-bonding interactions, while one cation forms an unusual enlarged cyclic interaction with O-atom acceptors of separate ortho-related carboxylate groups [graph set R(2)(2)(11)]. Cations and anions also associate across inversion centres, giving cyclic R(4)(2)(8) motifs. In the 1:1 guanidinium salt, (II), the cation forms two separate cyclic R(2)(1)(6) interactions, one with a carboxyl O-atom acceptor and the other with the solvent water molecule. The structure is unusual in that both carboxyl groups form short interanion O...H...O contacts, one across a crystallographic inversion centre [O...O = 2.483 (2) Å] and the other about a twofold axis of rotation [O...O = 2.462 (2) Å], representing shared sites on these elements for the single acid H atom. The water molecule links the cation-anion ribbon structures into a three-dimensional framework. In (III), the repeating molecular unit comprises a benzene-1,4-dicarboxylate dianion which lies across a crystallographic inversion centre, two guanidinium cations and two solvent water molecules (each set related by twofold rotational symmetry), and a single water molecule which lies on a twofold axis. Each guanidinium cation forms three types of cyclic interaction with the dianions: one R(2)(1)(6), the others R(3)(2)(8) and R(3)(3)(10) (both of these involving the water molecules), giving a three-dimensional structure through bridges down the b-cell direction. The water molecule at the general site also forms an unusual cyclic R(2)(2)(4) homodimeric association across an inversion centre [O...O = 2.875 (2) Å]. The work described here provides further examples of the common cyclic guanidinium-carboxylate hydrogen-bonding associations, as well as featuring other less common cyclic motifs.

Complex Nuclear Relaxation Processes in Guanidinium Compounds [C(NH2)3]3Sb2X9 (X = Br, Cl): Effects of Quadrupolar Interactions

Temperature dependences of proton spin–lattice relaxation times for two guanidinium compounds, [C(NH2)3]3Sb2Br9 and [C(NH2)3]3Sb2Cl9, are reanalyzed by taking into account all relaxation pathways available for protons. It turns out that the experimental data reflect two relaxation processes, which have to be described separately. The bi-exponentiality of relaxation results from the structure of these compounds, which contain two structurally and dynamically distinct guanidinium cations. Complete relaxation formulas accounting for effects of the quadrupolar interactions on the energy level structure of the entire system, as well as of the quadrupolar relaxation, are applied. This advanced description allows for a consistent interpretation of the relaxation data assuming the simplest motional model (one thermally activated correlation time for every guanidinium cation) while former attempts based on very simple relaxation formulas but including sophisticated motional models failed. The presented relaxation description and the analysis of the relaxation data can serve as an example of a complete treatment of relaxation results for complex molecular systems containing nuclei possessing quadrupolar moments.

High Conductivity of Ionic Liquids, Based on the Symmetrical Guanidinium Cation

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Guanidinium 3-nitrobenzoate

The title compound, CH6N3 +·C7H4NO4 −, an anhydrous guanidinium salt, shows a N—H⋯O hydrogen-bond network in which the guanidinium cation is involved in three cyclic R 2 1(6) hydrogen-bonding associations with separate carboxyl­ate O-atom acceptors. Further peripheral associations include a cyclic R 1 2(4) cation–anion inter­action, forming inter­linked undulating sheets in the three-dimensional structure.

Guanidinium 2-phenylacetate

In the structure of the title salt, CH6N3 +·C8H7O2 −, the guanidinium cation gives three cyclic hydrogen-bonding inter­actions with O-atom acceptors of three independent phenyl­acetate anions, one R 2 2(8) and two R 2 1(6), giving one-dimensional columnar structures which extend down the 42 axis in the tetra­gonal cell. Within these structures, there are solvent-accessible voids of volume 86.5 Å3.

Guanidinium phenylarsonate–guanidine–water (1/1/2)

In the structure of the title compound, CH6N3 +·C6H6AsO3 −·CH5N3·2H2O, the phenyl­arsonate anion participates in two R 2 2(8) cyclic hydrogen-bonding inter­actions, one with a guanidinium cation, the other with a guanidine mol­ecule. The anions are also bridged by the water mol­ecules, one of which completes a cyclic R 5 3(9) hydrogen-bonding association with the guanidinum cation, conjoint with one of the three R 2 2(8) associations about that ion, as well as forming an R 2 1(6) cyclic association with the guanidine mol­ecule. The result is a three-dimensional framework structure.

Nitrocyanamide‐Based Ionic Liquids and Their Potential Applications as Hypergolic Fuels

Nitrocyanamide ionic liquids with substituted imidazolium, guanidinium, and tetrazolium cations have been synthesized and fully characterized. Aminoguanidinium nitrocyanamide (7) crystallizes in the triclinic system P1. The results obtained from theoretical calculations based on 7 are consistent with the single-crystal structure data. These ionic liquids exhibit desirable physicochemical properties, such as low melting points and good thermal stabilities. Furthermore, they are all impact insensitive materials. Their energetic performances, including heats of formation, detonation pressures, and detonation velocities, were studied by a combination of theoretical and empirical calculations. The ionic liquids 1-4 have large liquid ranges and low viscosities. They were shown to be promising candidates as hypergolic ionic liquids through the combustion tests with 100% HNO(3).

Contributions of Components in Guanidine Hydrochloride to Hemoglobin Unfolding Investigated by Protein Film Electrochemistry

The contribution of the chloride anion (Cl(-)) and guanidinium cation (Gdn(+)) to the denaturing efficiency of guanidine hydrochloride (GdnCl) upon the unfolding of hemoglobin (Hb) was investigated electrochemically. Hb was entrapped in a didodecyldimethylammonium bromide (DDAB)-film-modified glassy carbon electrode and unfolded by the components of GdnCl. The changes of the direct electrochemical behaviors of Hb, including peak current (I(p)), formal potential (E(o)'), and peak-to-peak separation (DeltaE), were utilized to characterize different unfolded states of Hb. UV-vis, circular dichroism, and fluorescence spectroscopy were also used to verify the structural information of Hb during the unfolding process. The results indicated that the denaturing efficiency of GdnCl was contributed to by Gdn(+) and Cl(-) in synchronization, and the portions of such contributions were concentration-dependent. In addition, the presence of Gdn(+), Cl(-), or GdnCl can enhance the reversibility of the redox reaction of Hb to the same degree. The method provides not only an easy way to better understand the conformational changes of Hb but also a strategy to control its conformation.

Guanidinium-Enhanced Production of Hydrogen on Nafion-Coated Dye/TiO2 under Visible Light

Guanidinium cations (abbreviated as G) enhanced the visible light-induced hydrogen production by three times in the dye(Ru(bpy)32+)-sensitized system using TiO2 nanoparticles coated with nafion polymer. The enhanced photoactivity in the presence of G adsorbed on the nafion coating was related to the retardation of the charge recombination between the electrons injected into the TiO2 conduction band (CB) and the oxidized dye molecules, which was confirmed by time-resolved diffuse reflectance spectroscopic measurements. It is proposed that G cations located near the TiO2 surface repel the oxidized dyes to retard the recombination with CB electrons. In addition, the dye luminescence in nafion was enhanced in the presence of G, which was ascribed to the retardation of self-quenching of the excited dyes. The dual roles of G in the nafion layer increase the photoelectron density in TiO2 CB and subsequently enhance the production of hydrogen.

Water-Induced Relaxation of a Degenerate Vibration of Guanidinium Using 2D IR Echo Spectroscopy

The nearly degenerate asymmetric stretch vibrations near 1600 cm(-1) of the guanidinium cation in D-glycerol/D(2)O mixtures having different viscosity were studied by 2D IR photon echo spectroscopy. The polarization-dependent photon echo signal shows two separate frequency distributions in the 2D spectrum in D(2)O, even though only one band is evident from inspection of the linear FTIR spectrum. The split components are more clearly seen at higher viscosity where the distortion of the molecule from 3-fold symmetry is even more evident. The interactions with solvent induce energy transfer between the degenerate component modes on the time scale of 0.5 ps. The energy transfer between modes is directly observed in 2D IR and distinguished by the waiting time dependence of the cross peaks from the transfers between configurations of the distorted ion and solvent. The 2D IR analysis carried out for various polarization conditions gave frequency-frequency auto- and cross-correlation functions for the degenerate components which derive from the solvent induced wagging of the -ND(2) groups of the guanidinium ion.

Novel Layered Hybrid Fluoroaluminate in the Composition Space Diagram of the Al(OH)3-HguaCl-HFaq-EtOH System

A composition space diagram is used to establish the stability regions of hybrid fluoroaluminates synthesized by solvothermal reactions in the Al(OH)(3)-HguaCl-HF(aq)-EtOH system. The syntheses are conducted using microwave heating at 190 degrees C for 1 h. The structures of two new guanidinium fluoroaluminates, (H(3)O)(2) x [Hgua](16) x (Al(4)F(18))(3) x H(2)O and [Hgua](2) x (Al(5)F(17)), are reported. (H(3)O)(2) x [Hgua](16) x (Al(4)F(18))(3) x H(2)O, determined from single-crystal X-ray data, consists of discrete (Al(4)F(18))(6-) polyanions, while [Hgua](2) x (Al(5)F(17)), solved from powder X-ray diffraction data in direct space, contains unprecedented Al(5)F(17) layers constructed from the intergrowth of HTB and perovskite columns. A disorder of H(2)O and H(3)O(+) is evidenced in (H(3)O)(2) x [Hgua](16) x (Al(4)F(18))(3) x H(2)O together with a disorder of guanidinium cations in the interlayer space of [Hgua](2) x (Al(5)F(17)).

Guanidinium chlorochromate

In the title compound, guanidinium chloridotrioxidochrom­ate(VI), (CH6N3)[CrClO3], both the cation and anion are generated by crystallographic mirror symmetry, with one O and one N atom and the Cr, Cl and C atoms lying on the mirror plane. The bond lengths in the guanidinium cation are inter­mediate between normal C—N and C=N bond lengths, indicating significant delocalization in this species. In the crystal structure, inter­molecular N—H⋯Cl inter­actions generate R 2 1(6) ring motifs. These ring motifs are further inter­connected by inter­molecular N—H⋯O hydrogen bonds into infinite chains along [010].

Activation of R235A Mutant Orotidine 5′-Monophosphate Decarboxylase by the Guanidinium Cation: Effective Molarity of the Cationic Side Chain of Arg-235

The R235A mutation at yeast orotidine 5'-monophosphate decarboxylase (OMPDC) results in a 1300-fold increase in K(m) and a 14-fold decrease in k(cat) for decarboxylation of orotidine 5'-monophosphate, corresponding to a 5.8 kcal/mol destabilization of the transition state. There is strong activation of this mutant enzyme by added guanidinium cation (Gua(+)): 1 M Gua(+) stabilizes the transition state by ca. 3 kcal/mol. This stabilization is due to the binding of Gua(+) to the binary E(mut) x OMP complex, with a K(d) of 50 mM, to form the 9-fold more reactive ternary E(mut) x OMP x Gua(+) complex. The "effective molarity" of the cationic side chain of Arg-235 at the wild-type enzyme is calculated to be 160 M.