Deuterium Isotope Effects on Acid-Base Equilibrium of Organic Compounds

Deuterium isotope effects on acid–base equilibrium have been investigated using a combined path integral and free-energy perturbation simulation method. To understand the origin of the linear free-energy relationship of ΔpKa=pKaD2O−pKaH2O versus pKaH2O, we examined two theoretical models for computing the deuterium isotope effects. In Model 1, only the intrinsic isotope exchange effect of the acid itself in water was included by replacing the titratable protons with deuterons. Here, the dominant contribution is due to the difference in zero-point energy between the two isotopologues. In Model 2, the medium isotope effects are considered, in which the free energy change as a result of replacing H2O by D2O in solute–solvent hydrogen-bonding complexes is determined. Although the average ΔpKa change from Model 1 was found to be in reasonable agreement with the experimental average result, the pKaH2O dependence of the solvent isotope effects is absent. A linear free-energy relationship is obtained by including the medium effect in Model 2, and the main factor is due to solvent isotope effects in the anion–water complexes. The present study highlights the significant roles of both the intrinsic isotope exchange effect and the medium solvent isotope effect.

Study of the surface properties of glauconite

The acid-base properties of the glauconite surface has been studied by potentiometric titration. Using a surface complexation model with a constant exchange capacity, it was shown that positively charged surface centers and exchange centers dominate in the acidic pH region, and negatively charged centers dominate in the neutral and alkaline regions. The corresponding constants of acid-base equilibrium have been calculated. The data obtained were used to study the sorption of cadmium and lead on glauconite.

The pH-dependent Swelling of Weak Polyelectrolyte Hydrogels Modeled at Different Levels of Resolution

The swelling of polyelectrolyte hydrogels has been often explained using simple models derived from the Flory-Rehner model. While these models qualitatively predict the experimentally observed trends, they also introduce strong approximations and neglect some important contributions. Consequently, they sometimes incorrectly ascribe the observed trends to contributions which are of minor importance under the given conditions. In this work, we investigate the swelling properties of weak (pH-responsive) polyelectrolyte gels at various pH and salt concentrations, using a hierarchy of models, gradually introducing various approximations. For the first time, we introduce a three-dimensional particle-based model which accounts for the topology of the hydrogel network, for electrostatic interactions between gel segments and small ions and for acid-base equilibrium coupled to the Donnan partitioning of small ions. This model is the most accurate one, therefore, we use it as a reference when assessing the effect of various approximations. As the first approximation, we introduce the affine deformation, which allows us to replace the network of many chains by a single chain, while retaining the particle-based representation. In the next step, we use the mean-field approximation to replace particles by density fields, combining the Poisson-Boltzmann equation with elastic stretching of the chain. Finally, we introduce an ideal gel model by neglecting the electrostatics while retaining all other features of the previous model. Comparing predictions from all four models allows us to understand which contributions dominate at high or low pH or salt concentrations. We observe that the field-based models overestimate the ionization degree of the gel because they underestimate the electrostatic interactions. Nevertheless, a cancellation of effects on the electrostatic interactions and Donnan partitioning causes that both particle-based and field-based models consistently predict the swelling of the gels as a function of pH and salt concentration. Thus, we can conclude that any of the employed models can rationalize the known experimental trends in gel swelling, however, only the particle-based models fully account for the true effects causing these trends. The full understanding of differences between various models is important when interpreting experimental results in the framework of existing theories and for ascribing the observed trends to particular contributions, such as the Donnan partitioning of ions, osmotic pressure or electrostatic interactions.

Effect of Humidity on the Reactive Uptake of Ammonia and Dimethylamine by Nitrogen-Containing Secondary Organic Aerosol

This study investigated the uptake of ammonia (NH3) by secondary organic aerosol (SOA) particles generated via limonene photooxidation or ozonolysis as well as the uptake of dimethylamine (DMA) by limonene ozonolysis, α-cedrene photooxidation, or toluene photooxidation SOA in an environmental chamber between 0–50% relative humidity. In addition to the acid-base equilibrium uptake, NH3 and DMA can react with SOA carbonyl compounds converting them into nitrogen-containing organic compounds (NOCs). The effective reactive uptake coefficients for the formation of NOCs from ammonia were measured on the order of 10−5. The observed DMA reactive uptake coefficients ranged from 10−5 to 10−4. Typically, the reactive uptake coefficient decreased with increasing relative humidity. This is consistent with NOC formation by a condensation reaction between NH3 or DMA with SOA, which produces water as a product. Ammonia is more abundant in the atmosphere than amines. However, the larger observed reactive uptake coefficient suggests that amine uptake may also be a potential source of organic nitrogen in particulate matter.

The Effects of Cigarette Filter Ventilation on Delivery and Retention of Organic Acids

Summary A method for simultaneous identification and quantitative determination of 30 organic acids was established. The smoke yields and filter retentions of organic acids and routine smoke components, total particulate matter (TPM), nicotine-free dry particulate matter (NFDPM), nicotine and carbon monoxide (CO) at different filter ventilation levels were determined under both International Organization for Standardization (ISO) and Health Canadian Intense (HCI) smoking regimes. As a result of smoke dilution during filter ventilation, the yields of all organic acids were reduced in mainstream cigarette smoke. The spatial distribution pattern of the concentration of each organic acid in the filter was investigated at different ventilation levels and their filter retention determined. On one hand, the concentration of organic acids with a lower boiling point (BP) and lower molecular weight (MW) was relatively higher at the smoking end and the periphery part of the filter and spatial concentration distributions within the filter were significantly affected by smoke diffusion. On the other hand, those acids with high BPs and high MW were mainly distributed at the tobacco rod end and central part of the filter and spatial concentration distributions were only slightly influenced by their smoke diffusion within the filter whilst air compression around the filter vents also led to less change. This way, different acids in mainstream cigarette smoke were reduced to different extents which can also influence the acid-base equilibrium and sensory quality of the smoke. Compared with ISO smoking regime, the vent blocking and more intense smoking HCI regime led to different extents of yield increase for each of the studied acids. The effect of filter ventilation in the HCI smoking regime was not investigated, as the HCI smoking regime requires blocked ventilation holes. [Contrib. Tob. Nicotine Res. 30 (2021) 199–211]

Comparison between Crystalloid Versus Colloid in Septic Shock

Background Fluid resuscitation is a critical component to the emergency department (ED) management of patients with sepsis and septic shock. Fluids are administered to patients with sepsis in order to augment cardiac output and improve tissue perfusion and oxygenation. Recent evidence has suggested that the composition of fluids used in sepsis resuscitation may affect patient-centered outcomes. Objective To systematically review the effects of colloids compared with crystalloids in fluid resuscitation for septic shock. Data Sources Medline databases (PubMed, Medscape, ScienceDirect. EMF-Portal) and all materials available in the Internet till 2017. Data Extraction If the studies did not fulfill the inclusion criteria, they were excluded. Study quality assessment included whether ethical approval was gained, eligibility criteria specified, appropriate controls, and adequate information and defined assessment measures. Conclusion In contrast to 0.9% NS, balanced crystalloid solutions contain significantly lower concentrations of chloride. Instead of large amounts of chloride, balanced solutions contain organic anions (i.e., lactate, gluconate, citrate, acetate) which act as physiologic buffers and are rapidly converted to bicarbonate upon administration. In addition, balanced solutions have less of an adverse effect on acid-base equilibrium than 0.9% NS. Balanced solutions also contain varying amounts of cations (i.e., potassium, calcium). Many critically ill patients across the world receive HES solutions for resuscitation. HES solutions are defined by their molecular weight and degree of hydroxyethylation. Recent evidence has demonstrated significant harm with the use of HES solutions. The administration of albumin in the critical patient is not associated to demonstrated adverse effects, though it should be reserved for specific patient groups in which it has been shown to offer benefit.

Understanding the Functional Expression of Na+-Coupled SLC4 Transporters in the Renal and Nervous Systems: A Review

Acid-base homeostasis is crucial for numerous physiological processes. Na+/HCO3− cotransporters (NBCs) belong to the solute carrier 4 (SLC4) family, which regulates intracellular pH as well as HCO3− absorption and secretion. However, knowledge of the structural functions of these proteins remains limited. Electrogenic NBC (NBCe-1) is thought to be the primary factor promoting the precise acid–base equilibrium in distinct cell types for filtration and reabsorption, as well as the function of neurons and glia. NBC dysregulation is strongly linked to several diseases. As such, the need for special drugs that interfere with the transmission function of NBC is becoming increasingly urgent. In this review, we focus on the structural and functional characteristics of NBCe1, and discuss the roles of NBCe1 in the kidney, central nervous system (CNS), and related disorders, we also summarize the research on NBC inhibitors. NBCe1 and the related pathways should be further investigated, so that new medications may be developed to address the related conditions.

Synthesis, Structural and Physicochemical Characterization of a Titanium(IV) Compound with the Hydroxamate Ligand N,2-Dihydroxybenzamide

The siderophore organic ligand N,2-dihydroxybenzamide (H2dihybe) incorporates the hydroxamate group, in addition to the phenoxy group in the ortho-position and reveals a very rich coordination chemistry with potential applications in medicine, materials, and physical sciences. The reaction of H2dihybe with TiCl4 in methyl alcohol and KOH yielded the tetranuclear titanium oxo-cluster (TOC) [TiIV4(μ-O)2(HOCH3)4(μ-Hdihybe)4(Hdihybe)4]Cl4∙10H2O∙12CH3OH (1). The titanium compound was characterized by single-crystal X-ray structure analysis, ESI-MS, 13C, and 1H NMR spectroscopy, solid-state and solution UV–Vis, IR vibrational, and luminescence spectroscopies and molecular orbital calculations. The inorganic core Ti4(μ-O)2 of 1 constitutes a rare structural motif for discrete TiIV4 oxo-clusters. High-resolution ESI-MS studies of 1 in methyl alcohol revealed the presence of isotopic distribution patterns which can be attributed to the tetranuclear clusters containing the inorganic core {Ti4(μ-O)2}. Solid-state IR spectroscopy of 1 showed the presence of an intense band at ~800 cm−1 which is absent in the spectrum of the H2dihybe and was attributed to the high-energy ν(Ti2–μ-O) stretching mode. The ν(C=O) in 1 is red-shifted by ~10 cm−1, while the ν(N-O) is blue-shifted by ~20 cm−1 in comparison to H2dihybe. Density Functional Theory (DFT) calculations reveal that in the experimental and theoretically predicted IR absorbance spectra of the ligand and Ti-complex, the main bands observed in the experimental spectra are also present in the calculated spectra supporting the proposed structural model. 1H and 13C NMR solution (CD3OD) studies of 1 reveal that it retains its integrity in CD3OD. The observed NMR changes upon addition of base to a CD3OD solution of 1, are due to an acid–base equilibrium and not a change in the TiIV coordination environment while the decrease in the complex’s lability is due to the improved electron-donating properties which arise from the ligand deprotonation. Luminescence spectroscopic studies of 1 in solution reveal a dual narrow luminescence at different excitation wavelengths. The TOC 1 exhibits a band-gap of 1.98 eV which renders it a promising candidate for photocatalytic investigations.

Nuclear Singlet Relaxation by Chemical Exchange

The population imbalance between nuclear singlet states and triplet states of strongly coupled spin-1/2 pairs, also known as nuclear singlet order, is well protected against several common relaxation mechanisms. We study the nuclear singlet relaxation of 13C pairs in aqueous solutions of 1,2-13C2 squarate, over a range of pH values. The 13C singlet order is accessed by introducing 18O nuclei in order to break the chemical equivalence. The squarate dianion is in chemical equilibrium with hydrogen-squarate (SqH−) and squaric acid (SqH2) characterised by the dissociation constants pKa1 = 1:5 and pKa2 = 3:4. Surprisingly, we observe a striking increase in the singlet decay time constants TS when the pH of the solution exceeds ~ 10, which is far above the acid-base equilibrium points. We derive general rate expressions for chemical-exchange-induced nuclear singlet relaxation and provide a qualitative explanation of the TS behaviour of the squarate dianion. We identify a kinetic contribution to the singlet relaxation rate constant which depends explicitly on kinetic rate constants. Qualitative agreement is achieved between the theory and the experimental data. This study shows that infrequent chemical events may have a strong effect on the relaxation of nuclear singlet order.