3,5-Dimethylpyrazole Complexes with Strong Carboxylic Acids

Ligands that accelerate nanoceria dissolution may greatly affect its fate and effects. This project assessed the carboxylic acid contribution to nanoceria dissolution in aqueous, acidic environments. Nanoceria has commercial and potential therapeutic and energy storage applications. It biotransforms in vivo. Citric acid stabilizes nanoceria during synthesis and in aqueous dispersions. In this study, citrate-stabilized nanoceria dispersions (∼4 nm average primary particle size) were loaded into dialysis cassettes whose membranes passed cerium salts but not nanoceria particles. The cassettes were immersed in iso-osmotic baths containing carboxylic acids at pH 4.5 and 37 °C, or other select agents. Cerium atom material balances were conducted for the cassette and bath by sampling of each chamber and cerium quantitation by ICP-MS. Samples were collected from the cassette for high-resolution transmission electron microscopy observation of nanoceria size. In carboxylic acid solutions, nanoceria dissolution increased bath cerium concentration to >96% of the cerium introduced as nanoceria into the cassette and decreased nanoceria primary particle size in the cassette. In solutions of citric, malic, and lactic acids and the ammonium ion ∼15 nm, ceria agglomerates persisted. In solutions of other carboxylic acids, some select nanoceria agglomerates grew to ∼1 micron. In carboxylic acid solutions, dissolution half-lives were 800–4000 h; in water and horseradish peroxidase they were ≥55,000 h. Extending these findings to in vivo and environmental systems, one expects acidic environments containing carboxylic acids to degrade nanoceria by dissolution; two examples would be phagolysosomes and in the plant rhizosphere.

Dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) is a recently synthesized energetic ionic salt (EISs) with some excellent properties including high energy content, high density, low impact sensitivity and low toxicity, and therefore is a promising alternative energetic material. In contrast to commonly applied energetic CHON materials, TKX-50 features strong intermolecular hydrogen bonds (HBs). However, the effects of these strong HBs on its important properties remain unclear. We report herein the two-sided effects of the strong HBs on the stability of TKX-50, through ab initio simulations on shear deformation and thermal decomposition. That is, on the one hand, the strong HBs in TKX-50 lead to a layer-like arrangement of the cations and dianions in the (010) planes with the adjacent layers connected only by two types of weakest HBs, which facilitates the conversion of external kinetic energy into interlayer sliding, and contributes to low impact sensitivity; on the other hand, the extensive HBs in TKX-50 act as a potential source to facilitate H-transfer (including proton transfer), which accelerates subsequent thermal decomposition, and thus deteriorates the thermal stability of TKX-50 relative to its notably low impact sensitivity. These two-sided effects of the strong HBs in a TKX-50 crystal should be useful to understand properties of other EISs, in which strong HBs are universal, and enrich the knowledge of the sensitivity mechanism of energetic materials.

Crystal structure, phase transitions and vibrational spectra of bis(betaine) nitrate

Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. However, structures with medium and strong hydrogen bonds are difficult to study due to their characteristically broad vibrational bands and large anharmonicity. This is further complicated by strong interactions between the high-frequency hydrogen-bonded vibrational modes, fingerprint modes, and low-frequency intradimer modes that modulate the hydrogen-bonding. Understanding these structures and their associated dynamics requires studying much of the vibrational spectrum. Here, mid-IR continuum spectroscopy of the cyclic 7-azaindole-acetic acid (7AI-AcOH) heterodimer reveals the vibrational relaxation dynamics and couplings of this complex hydrogen-bonded system. Within this dimer, the NH bond of 7AI exhibits a band at 3250 cm-1 caused by a medium strength hydrogen bond, while the strongly hydrogen-bonded OH modes of acetic acid exhibit a broad double-peaked vibrational feature spanning 1750 to 2750 cm-1. Transient IR and 2D IR experiments were performed using three excitation frequencies, centered on the high-frequency OH and NH modes, and probed with a mid-IR continuum to measure the spectral response from 1000 to 3500 cm-1. While the NH stretch is observed to relax in 300 fs, the strongly hydrogen-bonded OH modes relax within the time resolution of the experiment (sub-100 fs). The difference in the strength of the hydrogen bonds is also reflected in the coupling pattern in the fingerprint region observed with 2D IR spectroscopy. Here the NH is strongly coupled to fingerprint modes involving the 7AI monomer, while the OH vibrations are strongly coupled to vibrational modes across the entire dimer. Together, the results show strong coupling and rapid energy transfer across the hydrogen-bonded interface and through the structure of the 7-azaindole-acetic acid heterodimer, highlighting the need to study the full vibrational spectrum for strongly hydrogen-bonded systems.

Strong hydrogen bonds in 1:1 and 2:1 complexes of pyridine betaine with strong acids

The binding of galactose-specific lectins from Erythrina indica (EIL), Erythrina arborescens (EAL), Ricinus communis (agglutinin; RCA-I), Abrus precatorius (agglutinin; APA), and Bandeiraea simplicifolia (lectin I; BSL-I) to fluoro-, deoxy-, and thiogalactoses were studied in order to determine the strength of hydrogen bonds between the hydroxyl groups of galactose and the binding sites of the proteins. The results have allowed insight into the nature of the donor/acceptor groups in the lectins that are involved in hydrogen bonding with the sugar. The data indicate that the C-2 hydroxyl group of galactose is involved in weak interactions as a hydrogen-bond acceptor with uncharged groups of EIL and EAL. With RCA-I, the C-2 hydroxyl group forms two weak hydrogen bonds in the capacity of a hydrogen-bond acceptor and a donor. On the other hand, there is a strong hydrogen bond between the C-2 hydroxyl group of galactose, which acts as a donor, and a charged group on BSL-I. The C-2 hydroxyl group of the sugar is also a hydrogen-bond donor to APA. The lectins are involved in strong hydrogen bonds through charged groups with the C-3 and C-4 hydroxyl groups of galactose, with the latter serving as hydrogen-bond donors. The C-6 hydroxyl group of the sugar is weakly hydrogen bonded with neutral groups of EIL, EAL, and APA. With BSL-I, however, a strong hydrogen bond is formed at this position with a charged group of the lectin. The C-6 hydroxyl groups is a hydrogen-bond acceptor for EIL and EAL, a hydrogen-bond donor for APA and BSL-I, and appears not to be involved in binding to RCA-I. The data with the thiosugars indicate the involvement of the C-1 hydroxyl group of galactose in binding to EIL, EAL, and BSL-I, but not to RCA-I and APA. We have also performed a similar analysis of the binding data of fluoro- and deoxysugars to concanavalin A [Poretz, R. D. and Goldstein, I. J. (1970) Biochemistry 9, 2890-2896]. This has allowed comparison of the donor/acceptor properties and free energies of hydrogen bonding of the hydroxyl groups of methyl alpha-D-mannopyranoside to concanavalin A with the results in the present study. On the basis of this analysis, new assignments are suggested for amino acid residues of concanavalin A [corrected] that may be involved in hydrogen bonding to the sugar.

Electrochemistry and contact angle measurements on polymer films of ω-(3-pyrrolyl)-alkanoic acids in aqueous solution

(2,6-Dichloro-4-methoxyphenyl)-(2,4-dichlorophenyl)-methyl trichloroacetimidate can be efficiently activated by TMSOTf (10∼100 mol%) to react with alcohols and carboxylic acids. Under these conditions a wide variety of alcohols can be transformed into the corresponding ethers in excellent yields with a slight excess of the trichloroacetimidate. The resulting ethers are not susceptible to typical deprotection conditions for benzyl and 4-methoxybenzyl ether groups, however, they can be conveniently deprotected by treatment with 30∼50% trifluoroacetic acid in dichloromethane. Polymer-bound (2,6-dichloro-4-alkoxyphenyl)-(2,4-dichlorophenyl)methyl trichloroacetimidate is useful for immobilization of alcohols and carboxylic acids.

The distribution ratio of uranium between tri-n-butyl phosphate and water decreased with the addition of water-insoluble carboxylic acids such as benzoic, lauric, oleic and stearic acids at various concentrations of nitric acid. It is suggested that the decrease of uranium distribution ratio may be due to hydrogen bond formation between tri-n-butyl phosphate and carboxylic acid. It was indicated from infrared spectrometry that a strong hydrogen bond is formed between tri-n-butyl phosphate and carboxylic acid. In other cases, water-soluble carboxylic acids such as acetic, adipic and oxalic acids had no effect at all on the distribution ratio of uranium at various concentrations of nitric acid.

Interaction of para-substituted meso-tetraphenylporphyrins and meso-tetra(n-propyl)porphyrin with weak and strong carboxylic acids: A UV–Vis spectroscopic study

Toughening diene elastomers by strong hydrogen bond interactions

Non-ideal adsorption of divalent cations on a Langmuir monolayer: A theoretical model for predicting the composition of the resulting Langmuir–Blodgett films

A metal‐free and catalytic method for the synthesis of 2‐substituted benzoxazoles and benzothiazoles from N‐protected amino acids and carboxylic acids is described. The synthesis of benzoxazole on a solid support is demonstrated by the synthesis of benzoxazole attached to a human islet amyloid polypeptide, hIAPP22–27, fragment and benzoxazole derivatives of the side chain of aspartic acid on a modified fragment of Aβ18−21 (amyloid β) peptide. The reactions proceeded through a two‐step mechanism. The coupling step is realized with a relatively new reagent, ethyl 2‐cyano‐2‐(2‐nitrobenzenesulfonyloxyimino)acetate (ortho‐NosylOXY), and the cyclization step takes place thanks to an inexpensive organocatalyst, para‐toluenesulfonic acid, under microwave irradiation. To the best of our knowledge, this is the first report of the complete synthesis of benzoxazole derivatives of a polypeptide on a resin solid support.

The secondary relaxation of poly(2-vinylpyridine) (PVPy) in a complex and blends with poly(4-vinylphenol) (PVPh) was studied by broadband dielectric spectroscopy. The polymer complex can be obtained by mixing PVPh and PVPy in methyl ethyl ketone as a result of the very strong hydrogen bonding between two polymers, and it has a glass transition temperature higher than that of either component. The secondary relaxation of PVPy is clearly suppressed in the complex, and it is found that the reduction of its dielectric relaxation strength is approximately proportional to the intermolecular hydrogen bonding fraction. In addition, the relaxation time of unassociated PVPy side groups is also slowed down by about 1 decade compared to that of the neat PVPy. Homogeneous PVPh/PVPy blends can be achieved if the polymers are mixed in N,N-dimethylformamide. However, the degree of intercomponent hydrogen bonding is much lower in the blends, and therefore the suppression of the PVPy secondary relaxation is not as signific...

Many germ line antibodies have asparagine residues at specific sites to achieve specific antigen recognition. To study the role of asparagine residues in the stabilization of antigen-antibody complexes, we examined the interaction between hen egg white lysozyme (HEL) and the corresponding HyHEL-10 variable domain fragment (Fv). We introduced Ala and Asp substitutions into the Fv side chains of L-Asn-31, L-Asn-32, and L-Asn-92, which interact directly with residues in HEL via hydrogen bonding in the wild-type Fv-HEL complex, and we investigated the interactions between these mutant antibodies and HEL. Isothermal titration calorimetric analysis showed that all the mutations decreased the negative enthalpy change and decreased the association constants of the interaction. Structural analyses showed that the effects of the mutations on the structure of the complex could be compensated for by conformational changes and/or by gains in other interactions. Consequently, the contribution of two hydrogen bonds was minor, and their abolition by mutation resulted in only a slight decrease in the affinity of the antibody for its antigen. By comparison, the other two hydrogen bonds buried at the interfacial area had large enthalpic advantage, despite entropic loss that was perhaps due to stiffening of the interface by the bonds, and were crucial to the strength of the interaction. Deletion of these strong hydrogen bonds could not be compensated for by other structural changes. Our results suggest that asparagine can provide the two functional groups for strong hydrogen bond formation, and their contribution to the antigen-antibody interaction can be attributed to their limited flexibility and accessibility at the complex interface.