Strong Hydrogen Bonds Research Articles

Physicochemical and Antimicrobial Properties of Lactic Acid-Based Natural Deep Eutectic Solvents as a Function of Water Content

The interest in natural deep eutectic solvents (NADESs) in green technology as an alternative to organic solvents has grown over the past decades. In this work, for the first time, the effect of dilution with water on the physicochemical and antimicrobial properties of lactic acid-based NADES with choline chloride (NADES1), sorbitol (NADES2), and glucose (NADES3) was systematically studied. According to FTIR data, after the dilution of NADESs with water, the strong hydrogen bonds weakened, however, were not destroyed after dilution of up to 40% water. The dilution of NADES with water resulted in a linear decrease in density and refractive index and in a linear increase in pH. The equations for the prediction of NADES density, pH, and refractive index as a function of water content were calculated. The viscosity decreased by half after adding approximately 10% water. The initial viscosity of NADES2 and NADES3 was significantly different. However, after adding 20% of the water, the viscosity was almost the same. The most pronounced decrease in surface tension (by 46.7%) was found for NADES1. The water activity was decreased in the following order: NADES3 > NADES1 > NADES2. The dilution of NADES with water caused a gradual increase in water activity. NADES1 showed the lowest minimal inhibitory concentrations (MIC) (7.8, 3.9, and 0.98 mg/mL) and minimum bactericidal concentrations (MBC) (15.6, 7.8, and 1.95 mg/mL) for Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus, respectively. The antimicrobial activity was decreased by 2–8 times after the addition of 40% water. The water activity for all tested NADES together with low pH could explain the antimicrobial effect. The revealed regularity can be useful for the prediction of NADES properties and for the selection of green solvents on a laboratory and industrial scale.

1H and 13C NMR and FTIR Spectroscopic Analysis of Formic Acid Dissociation Dynamics in Water.

The formation and transport of ionic charges in formic acid-water (HCOOH-H2O) mixtures with initial water mole fractions ranging from XH2Oi = 0 to 1 were investigated using 13C and 1H NMR, FTIR spectroscopy, viscosity, conductivity, and pH measurements. The maximum molar concentration of ions (H3O+ and HCOO-), along with the relative differences between theoretical and experimental densities, spin-lattice relaxation times (T1), activation energies (Ea), viscosity (η), and conductivity (σ), were identified within the range of XH2Oi ≈ 0.5-0.7. These results indicate that pure formic acid (FA) solutions predominantly consist of cyclic dimers at room temperature. As the water mole fraction increases up to 0.6, a structural shift occurs from cyclic dimers to a mixture of linear and cyclic dimers, driven by the formation of strong hydrogen bonds. Beyond a water mole fraction of 0.6, the structure transitions to linear dimers, with FA molecules behaving as free entities in the water. Furthermore, the acidity was found to increase approximately 2-fold with every 0.1 increment in water mole fraction. These findings are critical for understanding the kinetics of formic acid anions in body fluids, the structure of the hydrogen bonding network, and ionization energies.

Local Electronic Correlation in Multicomponent Møller-Plesset Perturbation Theory.

We present in this contribution the first application of local correlation in the context of multicomponent methods. Multicomponent approaches allow for the targeted simulation of electrons together with other Fermions (most commonly protons) as quantum particles. These methods have become increasingly popular over the last years, particularly for the description of nuclear quantum effects (in strong hydrogen bonds, proton tunneling, and many more). However, most implementations are still based on canonical formulations of wave function theory, which we know for decades to be computationally inefficient for capturing dynamical correlation effects. Local correlation approaches, particularly with the use of pair natural orbitals (PNOs), enable asymptotically linear scaling of computational costs with very little impact on the overall accuracy. In this context, the efficient use of density fitting approximations in the integral calculation proves essential. We start by discussing our implementation of density-fitted NEO-MP2 and NEO-PNO-LMP2, upgrading the electronic correlation treatment up to PNO local coupled cluster level of theory. Several challenging examples are provided to benchmark the method in terms of accuracy as well as computational cost scaling. Following appropriate protocols, anharmonic corrections to localized X-H stretches can be applied routinely with little computational overhead.

Systematic Computational Approaches on Biosorption of Fluoride on Chitin: Crossover from Conventional to Short and Strong Hydrogen Bonds

Systematic Computational Approaches on Biosorption of Fluoride on Chitin: Crossover from Conventional to Short and Strong Hydrogen Bonds

Molecular diffusion in aqueous methanol solutions: The combined influence of hydrogen bonding and hydrophobic ends.

Although the nonmonotonic variation in the diffusion coefficients of alcohol and water with changing alcohol concentrations in aqueous solutions has been reported for many years, the underlying physical mechanisms remain unclear. Using molecular dynamics simulations, we investigated the molecular diffusion mechanisms in aqueous methanol solutions. Our findings reveal that the molecular diffusion is co-influenced by hydrogen bonding and the hydrophobic ends of methanol molecules. A stronger hydrogen bond (HB) network and a higher concentration of hydrophobic ends of methanol molecules both enhance molecular correlations, thereby slowing molecular diffusion in the solution. As methanol concentration increases, the HB network weakens, facilitating molecular diffusion. However, the increased concentration of hydrophobic ends counteracts this effect. Consequently, the diffusion coefficients of water and methanol molecules exhibit nonmonotonic changes. Previous studies have only focused on the role of HB networks. For the first time, we have identified the impact of the hydrophobic ends of alcohol on molecular diffusion in aqueous alcohol solutions. Our research contributes to a better understanding and manipulation of the properties of aqueous alcohol solutions and even liquids with complex compositions.

Tetrazaisoindigos:Serpentine Syntheses and Their Expected and Unexpected Photophysical and Electronic Properties.

Isoindigo, an electron-withdrawing building block for polymeric field-effect transistors, has long been considered to be non-fluorescent. Moreover, using electron-deficient heterocycle to replace the phenyl ring in the isoindigo core for better electron transport behaviour is synthetically challenging. Here we report the syntheses of a series of tetrazaisoindigos, including pyrazinoisoindigo (PyrII), pyrimidoisoindigo (PymII) and their hybrid (PyrPymII), and the investigation on their photophysical and electric properties. Proper flanking groups need to be chosen to stabilize these highly electron-deficient bislactams. Both PyrII and PymII derivatives show lower LUMO energy levels than that of naphthalene bisimide (NDI). Interestingly, PyrII is instinctively unstable and can be easily reduced, while both PymII derivatives are stable. More surprisingly, PymII derivatives are highly fluorescent and their photoluminescence quantum yields are around 40 %, 133 times higher than that of reported isoindigo derivatives. UV-vis spectroscopic results and theoretical calculations show that strong intramolecular hydrogen-bond exists in PymII, which prohibits it from non-radiative decay and accounts for its fluorescent behaviour. PymII derivatives are n-type semiconductors, while Ph-PyrII and the hybrid show balanced ambipolar charge transport behaviour, all among the best isoindigo derivatives. Our study not only discloses the structure-property relationship of tetrazaisoindigos, but also provides novel electron-deficient monomers for conjugated polymers.

Matching combination of amorphous ionic hydrogel with elastic fabric enables integrated properties for wearable sensing

The combination of hydrogels and fabrics opens up significant opportunities for flexible materials, particularly in biomedicine and wearable technology. However, the weak interface caused by insufficient interactions between the different phases and the substantial modulus mismatch limit their broader application. In this research, flexible and amorphous polyvinyl alcohol (PVA) hydrogels were integrated with polar and elastic fabrics to create multifunctional composites via in-situ freeze-thawing technology. First, by incorporating antifreeze inorganic salts into the precursor solution, we effectively suppressed ice crystal growth during the cooling process, promoting a uniform and loosely aligned PVA chain structure. This led to the formation of an amorphous crosslinked network while simultaneously releasing free hydroxyl groups. These hydroxyl groups facilitate the formation of robust interfaces within the composite. In addition, an elastic fabric composed of a polyester-polyurethane fiber blend was selected. The polyester fibers, rich in carbonyl groups along their polymer chains, form strong hydrogen bonds with the free hydroxyl groups from the PVA hydrogel, creating a highly resilient interface. The polyurethane fibers contribute to a lower Young's modulus, as well as excellent elasticity and ductility, reducing the fabric's inherent stiffness and mitigating fiber pull-out failure. Further, the incorporation of CaCl2 not only created an environment rich in free ions, but also provided the amorphous structure with increasing spacing between adjacent polymer chains, facilitating the transportation of ions. Therefore, the composite exhibits adept sensing capabilities for intricate human body movements, fostering auspicious prospects in smart sensor applications.

Self-assembled ILs-PVA micelle nanostructure impart the pervaporation membrane with high ethanol dehydration performance

Achieving strong interaction with the targeted composition and constructing abundant transport channels is crucial to obtain the pervaporation (PV) membrane with high selectivity and flux. Here, three ionic liquids (ILs) were screened out based on their relative selectivity and capacity targeting for bioethanol dehydration by COSMO-RS. The interaction energies analysis between ILs, EtOH, and H2O suggests that the ILs can form strong hydrogen bonds with water and disrupt the hydrogen bond in the EtOH–H2O azeotropic mixture, which is beneficial for improving the selectivity. Furthermore, driven by the multiple hydrogen bonds, electrostatic interactions, and van der Waals forces, ILs could self-assemble with polyvinyl alcohol (PVA) to fabricate the PV membrane with well-ordered micelle nanostructure, as its structure was revealed by MD simulations. The formation of the ILs-PVA micelle dramatically influenced membrane surface morphology, roughness, and water contact angle, providing an extra transport channel for the membrane. The optimal membrane (at the cmc point) exhibited a superior ethanol dehydration separation factor of 1627, along with a flux of 684 g/m2h at 50 °C. It can be expected that this novel self-assembled ILs-PVA micelle nanostructure strategy will find promising applications in other azeotropic mixture separation processes, like ethanol-ethyl acetate, water-butanol, etc.

Hydronium-Crosslinked Inorganic Hydrogel Comprised of 1D Lepidocrocite Titanate Nanofilaments.

When a few drops of acid (hydrochloric, acrylic, propionic, acetic, or formic) are added to a colloid comprised of 1D lepidocrocite titanate nanofilaments (1DLs)-2 × 2 TiO6 octahedra in cross-section-a hydrogel forms, in many cases, within seconds. The 1DL synthesis process requires the reaction between titanium diboride with tetramethylammonium (TMA+), hydroxide. Using quantitative nuclear magnetic resonance (qNMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), the mass percent of TMA+ after synthesis is determined to be ≈ 13.1±0.1%. The TMA+ is completely removed from the gels after 2 water soak cycles, resulting in the first completely inorganic, TiO2-based hydrogels. Ion exchanging the TMA+ with hydronium results in gels with relatively strong hydrogen bonds. The hydrogels' compression strengths increased linearly with 1DL colloid concentration. At a 1DL concentration of 45gL-1, the compressive strength, at 80% deformation when acrylic acid is used, is ≈325kPa. The strengths are ≈ 50% greater after the TMA+ is removed. The removal of all residual organic components in the hydrogels, including TMA+, is confirmed by qNMR, Fourier-transformed infrared spectroscopy (FTIR), and TGA/DSC. The 1DL phase is retained after gelation, TMA+ removal, and 80% compression.

Investigation of linear and microscopic nonlinear optical responses of 1-indanone compounds in different environments based on polarity models

Molecular spectroscopic and nonlinear features can indicate positive changes by solvent molecules. In this work, DFT and spectroscopic techniques were used to study the polarity effects of different solvent environments. Polarity-based models were used for studying solvent induced interactions on the optical features of new groups of biomolecules. Despite the significant contribution of general effects on the molecular absorption spectra, there is considerable competition between general and specific environmental effects on the molecular emission properties. Under this condition, strong hydrogen bonds tend to increase molecular nonlinear responses. The same results were observed for the low order (first and second order) nonlinearity of biomolecules. Therefore, the studies on the environment effects on the biomolecules’ first order nonlinearity can give valuable information about higher-order optical responses. Moreover, 1-Indanone compounds with high nonlinearity can be considered as an effective element in designing optical devices.

Molecular dynamics investigation of epoxy resin adsorption mechanisms on clay surfaces and the mechanical properties of epoxy resin-clay

The strength of natural clay can be improved with epoxy resins. However, nanoscale curing mechanisms remain poorly understood, which is essential for enhancing stability. In this study, molecular dynamics simulation was employed to calculate the quantity of interface hydrogen bonds, adsorption energy, radius of gyration, and mechanical properties of clay cured by diglycidyl ether of bisphenol-A epoxy resin (DGEBA), diglycidyl ether 4,4’-dihydroxy diphenyl sulfone (DGEDDS), and Aliphatic epoxidation of olefin resin (AEOR). Adsorption behavior and mechanical properties of the clay cured by three epoxy resins were investigated: (1) The chain structure of AEOR led to 18.2% more hydrogen bonds than DGEBA and 59.1% more than DGEDDS. (2) The simulated adsorption energies for DGEBA, DGEDDS, and AEOR with kaolinite were 92.59, 98.25, and 116.87 kcal·mol−1, respectively. (3) The bulk and shear modulus of kaolinite increased by 4.93% and 4.80% when using AEOR. The interface stability and mechanical properties of kaolinite were also improved through strong hydrogen bonds and high adsorption energy. (4) The improvement in Young’s modulus of kaolinite was most significant with AEOR, followed by DGEDDS. AEOR excelled in the Z direction, while DGEDDS excelled in the X and Y directions. This research provided a theoretical foundation to effectively improve the properties of clay using epoxy resins.

Identification and anticoagulant mechanisms of novel factor XIa inhibitory peptides by virtual screening of a in silico generated deep-sea peptide database

The objective of this study was to identify novel anticoagulant peptides from the deep-sea using multiple in silico methods, and to investigate their inhibitory activity and molecular mechanisms. A deep-sea peptide database was firstly constructed by performing virtual proteolysis on protein sequences from animals inhabiting deep-sea hydrothermal vents and cold seeps. Candidate anticoagulant peptides were identified through molecular docking and binding free energy screening against FXIa as the target. Two novel anticoagulant peptides, PRNIF (IC50 = 0.67 mM) and GNDRCL (IC50 = 1.52 mM), were identified, and their anticoagulant activities were verified in vitro. PRNIF was demonstrated to be a noncompetitive inhibitor of FXIa, and caused significant prolongation of the thrombin time (TT) and the activated partial thromboplastin time (APTT), whereas GNDRCL markedly prolonged only APTT. Molecular dynamics simulations demonstrated considerable conformational shifts of both anticoagulant peptides when bound to the active sites of FXIa. The lowest energy binding poses of the FXIa-peptide complexes for PRNIF and GNDRCL exhibited comparable numbers of hydrogen bonds and binding free energies. However, occupancy analysis revealed completely distinct stability characteristics of the hydrogen bond interactions. The conserved residue Asp569 in the S1 pocket of FXIa formed strong and stable hydrogen bonds as well as a salt bridge with the arginine residues of PRNIF, which were not observed in the FXIa-GNDRCL complex. To our knowledge, PRNIF represented the first FXIa inhibitory peptide derived from the deep-sea, which may contribute to the development and utilization of deep-sea peptides resources. Two deep-sea peptides may potentially serve as an alternative food-derived ingredient that could be utilized for thrombosis prevention.

Charge-Transfer Interactions Between Antibiotics and Small Organic Acids: Spectroscopic Characterization and Computational Investigation

Six new charge-transfer complexes using ofloxacin (OFL) and sulfamethazine (SMR) as electron donors and coumaric acid (COA), cinnamic acid (CNA), and salicylic acid (SAA) as acceptors via equimolar mixture have been synthesized. The experiment used UV-vis spectroscopy to determine the formation of the complex in methanol through the presence of a new broad absorption band with a maximum wavelength in the 200-400 nm range. The molecular composition of the charge-transfer complexes was determined by the spectrophotometric titration method and found to be 1:1 (donor: acceptor). These complexes have been characterized by infrared (FTIR) and scanning electron microscopy (SEM). In the FTIR spectra, the CT complexes showed a wavelength shift compared to the reactants. The complexes exhibited various morphologies by SEM, including spherical particles, short rods, and flattened shapes. Additionally, quantum chemical calculations at the DFT/B3LYP level of theory investigated the complexes' steady-state structures, energies, and charge densities. The intermolecular binding energies was negative, indicating that the reactions of the six complexes proceeded spontaneously. There was strong van der Waals forces and hydrogen bonds between the donor and acceptor, which contributed to the complexes' strong molecular stability. The C-N and N-H groups in the donor molecule, and the -COOH group in the acceptor molecule, played key roles in the complexation process. DFT calculation results were appropriate to support our experimental results. This study highlights the molecular mechanisms of donor and acceptor action in charge-transfer interactions, providing a theoretical basis for the synthesis of antibiotic complexes and the removal of antibiotics.

Experimental and Computational Insights into the Effects of -NH2 and -Cl Functional Groups on the Copper Corrosion Inhibition by Benzotriazole

The influences of -NH2 and -Cl groups on the copper corrosion inhibition in 0.5 M H2SO4 solution by benzotriazole were studied using experimental methods and density functional theory calculations. Mass loss tests, potentiodynamic polarization, and surface morphology analyses revealed notable differences: while unmodified benzotriazole showed an inhibition efficiency of 76.6%, the -Cl derivative increased this to 87.6%, whereas the -NH2 derivative dropped it to 61.0% at the concentration of 5 × 10−4 M. Density functional theory calculations indicated these differences are not due to electronic properties or inhibitor-copper interaction energies but rather to the inhibitor’s influence on the oxygen reduction reaction, especially O2 and H2O adsorption. The -NH2 group formed strong hydrogen bonds with O2 and H2O, reducing oxygen reduction inhibition, while the -Cl group repelled O2, resulting in enhanced inhibition. Free energy analysis of the oxygen reduction reaction supported these findings. These new insights into benzotriazole derivatives’ copper corrosion inhibition mechanisms offer valuable guidance for developing next-generation corrosion inhibitors.

Design, Synthesis, Biological and Computational Analysis of Isatin-based bis-Thiourea Analogues as Anti-diabetic and Anti-nematode Agents

A new series of isatin derivatives were synthesized, characterized by 1HNMR, 13CNMR and HREI-MS, and screened for α-glucosidase and alpha amylase inhibition. All the analogues were found to be dual inhibitors and showed good inhibitory potentials with IC50 values ranging from 5.28 ± 0.10 to 38.66 ± 0.30 µM (against alpha-amylase), and 5.45 ± 0.10 to 39.25 ± 0.50 µM (against alpha-glucosidase), as compared to the standard drug acarbose (IC50 = 11.12 ± 0.15 and 11.29 ± 0.07 µM, respectively). The most potent inhibitor among the series was analogue 24 (IC50 = 5.28 ± 0.10 for alpha-amylase and IC50 = 5.46 ± 0.10 µM for alpha-glucosidase), which has a nitro group attached at the meta-position of the phenyl ring A and the para position of phenyl ring B. Structure-activity relationship has been established mainly based on the position, nature and number of the substituent(s) attached to the phenyl ring. To investigate the binding interaction of the potent analog with the active site of an enzyme, molecular docking studies were carried out. To study the drug-likeness properties, the ADME study was also carried out. The most active compounds engage most of the amino acids composing the active site and display maximum interactions. These interactions majorly include formation of strong hydrogen bonds, which might be due to the presence of highly electronegative heteroatoms on aromatic rings. All the analogues were also tested for in vivo anti-nematodal activity against C. elegans to assess their cytotoxicity in comparison to the reference Levamisole. The cytotoxicity profile demonstrated that analogues 2, 7, 20 and 22 displayed minimum cytotoxicity at every concentration.

The Fate of the Formic Acid Proton on the Anatase TiO2(101) Surface

AbstractAs a prototype adsorption reaction of gas Brønsted acid on oxides, we study the adsorption of formic acid on anatase. We perform infrared spectroscopy measurements of adsorbed HCOOH and HCOOD on TiO2 nanopowders, from 13 K up to room temperature in an ultra‐high vacuum chamber. We assign the IR signals via computed spectra from nuclear quantum dynamics simulations using our divide‐and‐conquer semiclassical ab initio molecular dynamics method. The acid proton forms an extraordinarily short and strong hydrogen bond with the surface oxygen. The strength of this hydrogen bond, that compares to H bonds in ice at high pressures, is at the root of a substantial redshift with respect to the typical free OH stretching frequency, which eludes its straightforward detection.

Quantum Chemical Model Calculations of Adhesion and Dissociation between Epoxy Resin and Si-Containing Molecules.

There is no doubt that when solid surfaces are modified, the functional groups and atoms directly bonded to solid atoms play a major role in adsorption interactions with molecules or resins. In this study, the adhesion and dissociation between epoxy resin and molecules containing Si atoms were analyzed. The analysis, conducted in contact with the solid surface of silicon, utilized quantum chemical calculations based on a molecular model. We compared some Si-containing molecular models to test quantum chemical calculations that contribute to the study of adhesion and dissociation between epoxy resins and solid surfaces somehow other than simple potential energy curve calculations. The AFIR (artificial force induced reaction) method, implemented in the GRRM (global reaction route mapping) program, was employed to separate an epoxy resin model molecule and three types of silicon compounds (Si(CH3)2(OH)2, Si(CH3)4, and (CH3)2SiF2) in three directions, determining their minimum dissociation energy when changing the applied energy by 2.5 kJ/mol. In systems with weak hydrogen bonds, such as Si(CH3)4 or (CH3)2SiF2, the energy required for dissociation was not large; however, in systems with strong hydrogen bonds, such as Si(CH3)2(OH)2, dissociation was more difficult in the vertical direction. Although anisotropy due to hydroxyl groups was calculated in the horizontal direction, dissociation remained relatively easy.

Investigating the self-assembly of pH-sensitive switchable diamine surfactant using sum frequency generation spectroscopy and molecular dynamics simulations.

The responses of the N-alkyl diamine groups to variations in pH affect their conformations and surface activities, making them relevant to applications relying on interfacial interactions, such as controlled emulsification and mineral flotation. An in-depth understanding of interfacial self-assembly is crucial. Herein, a molecular-level study was performed to investigate the adsorption and self-assembly of N-dodecylpropane-1,3-diamine (DPDA) at the air-water (A/W) interface using sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulations. The SFG spectra of DPDA, acquired under three pH conditions, suggest that the protonation of the DPDA diamine group influences the alkyl chain arrangement at a varying degree at the A/W interface. Analysis of the di-cationic DPDA SFG spectrum at a low pH showed fewer gauche defects at low concentration, as indicated by the relatively higher intensity ratio (ICH3SS/ICH2SS) of 18.1 ± 0.6. The density profiles from MD simulations at different surface areas per molecule and pH conditions, showing varying degrees of packing, support the observation of gauche defects in SFG. With MD simulation, the radial distribution factor for di-cationic species has the highest probability of forming hydrogen bonds compared to mono-cationic and non-ionic species. These g(r) probability results conform with observations obtained from SFG spectroscopy, where we observed a strong hydrogen bond interaction at low pH conditions with di-cationic species, forming tetrahedrally arranged water molecules at the A/W interface. Overall, comprehensive insights will facilitate the visualization of alkyl diamines and their potential derivatives at the A/W interface, enabling a better understanding of their behavior across various applications.

Photochemical and Collision-Induced Cross-Linking in Stereochemically Distinct Scaffolds of Peptides and Nitrile Imines in Gas-Phase Ions.

Intramolecular cross-linking between peptides and nitrile-imine intermediates was studied in stereochemically distinct conjugates in which the reacting components were mounted on cis-1,2-cyclohexane and trans-1,4-cyclohexane scaffolds that we call 1,2-s-peptides and 1,4-s-peptides, respectively. The nitrile-imine intermediates were generated by N2 loss from 2,5-diaryltetrazole tags upon UV-photodissociation at 213 and 250 nm or by collision-induced dissociation, and further interrogated by CID and UVPD-MS3. Peptide fragment ion series originating from linear structures and macrocyclic cross-links were distinguished and used to quantify the cross-linking yields. The yields in MS2 varied between 27% for AAAG conjugates to 78% for GAAAK conjugates, depending on the peptide sequence. The CID-MS3 yields were in a 57-97% range, depending on the peptide sequence. Structures of 1,2-s-peptide and 1,4-s-peptide ions as well as several of their nitrile-imine intermediates and cross-links were investigated by high-resolution cyclic ion mobility in combination with Born-Oppenheimer molecular dynamics and density functional theory calculations. Matches between the experimental and calculated collision cross sections and ion relative Gibbs energies were used to assign peptide structures. Peptide conjugates C-terminated with Gly and Lys residues underwent cross-linking by the carboxyl group, as established by MS3 sequencing and corroborated by carboxyl blocking experiments that lowered the cross-linking yields. Peptide conjugates C-terminated with Arg also cross-linked via the side-chain guanidine group. A notable feature of the 1,4-s-peptide ions was the participation of low-energy twist-boat cyclohexane conformers that was enforced by strong hydrogen bonds between the peptide and nitrile imine.

Keep Your TEMPO Up: Nitroxide Radicals as Sensors of Intermolecular Interactions.

This study examines experimental data on the influence of the surrounding medium and non-covalent interactions on the isotropic hyperfine coupling constant, Aiso(14N), of the stable nitroxide radical 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in solution. The data were used to identify a density functional theory functional/basis set combination that accurately reproduces the experimental Aiso(14N) values. The variations in Aiso(14N) due to external factors are two orders of magnitude greater than the accuracy of its experimental measurements, making Aiso(14N) a highly sensitive experimental probe for quantifying these effects. Additionally, it was found that the proton-accepting ability of the N-O• moiety in TEMPO resembles that of the P=O moiety, enabling the simultaneous formation of two equally strong hydrogen bonds.