Estimating the Bias of Model Compounds for the Determination of Species-Specific Protonation Constants.

The previously unknown extent of the goodness of using model compounds for the microspeciation of polyprotic systems was studied. Mirror-symmetric dibasic compounds and their monosubstituted derivatives were investigated to quantify how the derivatives are appropriate models of the minor microspecies to be mimicked in various microspeciation systems. The results were analyzed using statistical methods. It was found that the respective O-methyl and S-methyl derivatives of phenols and thiols as well as the methyl esters of carboxylic acids are sufficiently good derivatives for microspeciation. It was also found that the methyl esters are superior to the carboxylic amides for modeling the -COOH moiety.

Effect of Substitution Degree and Homogeneity on Cyclodextrin-Ligand Complex Stability: Comparison of Fenbufen and Fenoprofen Using CD and NMR Spectroscopy.

The stability of host-guest complexes of two NSAID drugs with similar physicochemical properties, fenbufen and fenoprofen, was investigated by comparing induced circular dichroism and 1H nuclear magnetic resonance methods using eight cyclodextrins of different degrees of substitution and isomeric purity as guest compounds. These cyclodextrins include native ß-cyclodextrin (BCyD), 2,6-dimethyl-ß-cyclodextrin 50 (DIMEB50), 80 (DIMEB80) and 95% (DIMEB95) isomerically pure versions, low-methylated CRYSMEB, randomly methylated ß-cyclodextrin (RAMEB) and 4.5 and 6.3 average substitution grade hydroxypropyl-ß-cyclodextrin (HPBCyD). The stability constants obtained by the two methods show good agreement in most cases. For fenbufen complexes, there is a clear trend that the stability constant increases with the degree of substitution while isomer purity has a smaller effect on the magnitude of stability constants. A significant difference was found in the case of DIMEB50 when compared to DIMEB80/DIMEB95, while the latter two are similar. In the fenbufen-fenoprofen comparison, fenbufen, with its linear axis, gives a more stable complex, while fenoprofen shows lower constants and poorly defined trends.

Properties of Selenolate-Diselenide Redox Equilibria in View of Their Thiolate-Disulfide Counterparts.

Selenium, the multifaceted redox agent, is characterized in terms of oxidation states, with emphasis on selenol and diselenide in proteinogenic compounds. Selenocysteine, selenocystine, selenocysteamine, and selenocystamine are depicted in view of their co-dependent, interfering acid-base, and redox properties. The pH-dependent, apparent (conditional), and pH-independent, highly specific, microscopic forms of the redox equilibrium constants are described. Experimental techniques and evaluation methods for the determination of the equilibrium and redox parameters are discussed, with a focus on nuclear magnetic resonance spectroscopy, which is the prime technique to observe selenium properties in organic compounds. The correlation between redox, acid-base, and NMR parameters is shown in diagrams and tables. The fairly accessible NMR and acid-base parameters are discussed to assess the predictive power of these methods to estimate the site-specific redox properties of selenium-containing moieties in large molecules.

Enantioselective Human Serum Albumin Binding of Apremilast: Liquid Chromatographic, Fluorescence and Molecular Docking Study.

The interaction between human serum albumin (HSA) and apremilast (APR), a novel antipsoriatic drug, was characterized by multimodal analytical techniques including high-performance liquid chromatography (HPLC), fluorescence spectroscopy and molecular docking for the first time. Using an HSA chiral stationary phase, the APR enantiomers were well separated, indicating enantioselective binding between the protein and the analytes. The influence of chromatographic parameters-type and concentration of the organic modifier, buffer type, pH, ionic strength of the mobile phase, flow rate and column temperature-on the chromatographic responses (retention factor and selectivity) was analyzed in detail. The results revealed that the eutomer S-APR bound to the protein to a greater extent than the antipode. The classical van 't Hoff method was applied for thermodynamic analysis, which indicated that the enantioseparation was enthalpy-controlled. The stability constants of the protein-enantiomer complexes, determined by fluorescence spectroscopy, were in accordance with the elution order observed in HPLC (KR-APR-HSA = 6.45 × 103 M-1, KS-APR-HSA = 1.04 × 104 M-1), showing that, indeed, the later-eluting S-APR displayed a stronger binding with HSA. Molecular docking was applied to study and analyze the interactions between HSA and the APR enantiomers at the atomic level. It was revealed that the most favored APR binding occurred at the border between domains I and II of HSA, and secondary interactions were responsible for the different binding strengths of the enantiomers.

Understanding the pH Dependence of Supersaturation State-A Case Study of Telmisartan.

Creating supersaturating drug delivery systems to overcome the poor aqueous solubility of active ingredients became a frequent choice for formulation scientists. Supersaturation as a solution phenomenon is, however, still challenging to understand, and therefore many recent publications focus on this topic. This work aimed to investigate and better understand the pH dependence of supersaturation of telmisartan (TEL) at a molecular level and find a connection between the physicochemical properties of the active pharmaceutical ingredient (API) and the ability to form supersaturated solutions of the API. Therefore, the main focus of the work was the pH-dependent thermodynamic and kinetic solubility of the model API, TEL. Based on kinetic solubility results, TEL was observed to form a supersaturated solution only in the pH range 3-8. The experimental thermodynamic solubility-pH profile shows a slight deviation from the theoretical Henderson-Hasselbalch curve, which indicates the presence of zwitterionic aggregates in the solution. Based on pKa values and the refined solubility constants and distribution of macrospecies, the pH range where high supersaturation-capacity is observed is the same where the zwitterionic form of TEL is present. The existence of zwitterionic aggregation was confirmed experimentally in the pH range of 3 to 8 by mass spectrometry.

Close correlation between thiolate basicity and certain NMR parameters in cysteine and cystine microspecies.

The imbalance between prooxidants and antioxidants in biological systems, known as oxidative stress, can lead to a disruption of redox signaling by the reactive oxygen/nitrogen species and is related to severe diseases. The most vulnerable moiety targeted by oxidant species in the redox signaling pathways is the thiol (SH) group in the cysteine residues, especially in its deprotonated (S-) form. Cysteine, along with its oxidized, disulfide-containing form, cystine, constitute one of the most abundant low molecular weight biological redox couples, providing a significant contribution to the redox homeostasis in living systems. In this work, NMR spectra from cysteine, cystine, and cysteine-containing small peptides were thoroughly studied at the submolecular level, and through the chemical shift data set of their certain atoms it is possible to estimate either thiolate basicity or the also related standard redox potential. Regression analysis demonstrated a strong linear relationship for chemical shift vs thiolate logK of the cysteine microspecies data. The αCH 13C chemical shift is the most promising estimator of the acid-base and redox character.

Selenate-An internal chemical shift standard for aqueous 77 Se NMR spectroscopy.

The 77 Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D2 O:H2 O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for 77 Se NMR measurements; the chemical shift of this reference in a solution containing 5 V/V% D2 O at 25°C and 0.15 mol·dm-3 ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl3 and 1046.40 ppm relative to the 1 H signal of 0.03 V/V% TMS in CDCl3 . In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for 77 Se NMR measurements for the first time in the literature, and with the aforementioned results all previous 77 Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.

Solution Structure and Acid-Base Properties of Reduced α-Conotoxin MI.

The reduced derivative of α-conotoxin MI, a 14 amino acid peptide is characterized by NMR-pH titrations and molecular dynamics simulations to determine the protonation constants of the nine basic moieties, including four cysteine thiolates, and the charge-dependent structural properties. The peptide conformation at various protonation states was determined. The results show that the disulfide motifs in the native globular α-conotoxin MI occur between those cysteine moieties that exhibit the most similar thiolate basicities. Since the basicity of thiolates correlates to its redox potential, this phenomenon can be explained by the higher reactivity of the two thiolates with higher basicities. The folding of the oxidized peptide is further facilitated by the loop-like structure of the reduced form, which brings the thiolate groups into sufficient proximity. The 9 group-specific protonation constants and the related, charge-dependent, species-specific peptide structures are presented.

Investigating thermal stability based on the structural changes of lactase enzyme by several orthogonal methods.

Thermal stability of lactase (ß-galactosidase) enzyme has been studied by a variety of physico-chemical methods. ß-galactosidase is the main active ingredient of medications for lactose intolerance. It is typically produced industrially by the Aspergillus oryzae filamentous fungus. Lactase was used as a model to help understand thermal stability of enzyme-type biopharmaceuticals. Enzyme activity (hydrolyzation of lactose) of ß-galactosidase was determined after storing the solid enzyme substance at various temperatures. For a better understanding of the relationship between structure and activity changes we determined the mass and size of the molecules with gel electrophoresis and dynamic light scattering and detected aggregation processes. A bottom-up proteomic procedure was used to determine the primary amino acid sequence and to investigate changes in the N-glycosylation pattern of the protein. NMR and CD spectroscopic methods were used to observe changes in higher order structures and to reveal relationships between structural and functional changes.

Species-Specific, pH-Independent, Standard Redox Potential of Selenocysteine and Selenocysteamine.

Microscopic redox equilibrium constants and standard redox potential values were determined to quantify selenolate-diselenide equilibria of biological significance. The highly composite, codependent acid-base and redox equilibria of selenolates could so far be converted into pH-dependent, apparent parameters (equilibrium constants, redox potentials) only. In this work, the selenolate-diselenide redox equilibria of selenocysteamine and selenocysteine against dithiothreitol were analyzed by quantitative nuclear magnetic resonance (NMR) methods to characterize the interfering acid-base and redox equilibria. The directly obtained, pH-dependent, conditional redox equilibrium constants were then decomposed by our method into pH-independent, microscopic constants, which characterize the two-electron redox transitions of selenocysteamine and selenocysteine. The 12 different, species-specific parameter values show close correlation with the respective selenolate basicities, providing a tool to estimate otherwise inaccessible site-specific selenolate-diselenide redox potentials of related moieties in large peptides and proteins.

Characterization of the species-specific acid-base equilibria of adrenaline and noradrenaline.

Adrenaline, noradrenaline, the biogenic catecholamines of vital importance, and four closely related compounds were studied by 1H NMR-pH titrations, and the resulting acid-base properties are quantified in terms of three macroscopic and twelve microscopic protonation constants for both molecules. The species-specific basicities are interpreted by means of inductive and shielding effects by comparing the protonation constants of the catecholamines, including dopamine. The site-specific basicities determined this way could be key parameters for the interpretation of biochemical behavior.

Enantiomeric quality control of R-Tofisopam by HPLC using polysaccharide-type chiral stationary phases in polar organic mode.

A novel, fast and economic chiral HPLC method was developed and validated for the resolution of the four isomers of tofisopam. The separation capacity of eleven different chiral columns: six polysaccharide-type including three amylose-based (Chiralpak AD, Chiralpak AD-RH and Chiralpak AS) and three cellulose-based (Chiralcel OD, Chiralcel OJ and Lux Cellulose-4); three cyclodextrin- (Quest-BC, Quest-C2 and Quest-CM) and two macrocyclic glycopeptide antibiotic-type (Chirobiotic T and Chirobiotic TAG) were screened using polar organic or reversed-phase mode. Chiralpak AD, based on amylose tris(3,5-dimethylphenylcarbamate) as chiral selector with neat methanol was identified as the most promising system. In order to improve resolution, an orthogonal experimental design was employed, altering the concentration of 2-propanol, column temperature, and flow rate in a multivariate manner. Using the optimized method (85/15 v/v methanol/2-propanol, 40°C, flow rate: 0.7 mL/min) we were not only able to separate the four isomers but also detect 0.1% S-enantiomer as chiral impurity in R-tofisopam. This is important since the latter is under development as a single enantiomeric agent. Thermodynamic investigation revealed an unusual entropy and enthalpy-entropy co-driven controlled enantioseparation on Chiralcel OJ and on Chiralpak AD column, respectively. Our newly developed HPLC method was validated according to the ICH guidelines and its application was tested on a pharmaceutical formulation containing the racemic mixture of the drug. As a further novelty, a separate circular dichroism method was applied for the investigation of the interconversion kinetics of tofisopam conformers, which proved to be crucial for sample preparation and method validation.

Population, basicity and partition of short-lived conformers. Characterization of baclofen and pregabalin, the biaxial, doubly rotating drug molecules.

Populations, protonation constants and octanol-water partition coefficients were determined and assigned specifically to fast interconverting individual conformers, exemplified in baclofen and pregabalin, the GABA-related drug molecules of biaxial, double rotations. Rotamer statuses along both axes in water and octanol were elucidated from 1H NMR vicinal coupling constants. Conformer abundances were obtained by the appropriate combination of the rotamer populations in the two adjacent moieties in the molecule. The bulky aromatic group in baclofen versus the aliphatic side chain of pregabalin explains why baclofen exists mainly in trans-trans conformeric form, throughout the pH range, unlike pregabalin that has no any highly dominant form. Characteristically enough, for pregabalin, the lipophilicity of the conformers is primarily influenced by the conformation state. Conformers in gauche state are of higher lipophilicity. The conformers of the two compounds were ranked by their membrane-influx and -outflow propensities.

Dopamine: Acid-base properties and membrane penetration capacity.

Dopamine and 4 related compounds were studied by 1H NMR-pH titrations and a case-tailored evaluation method. The resulting acid-base properties of dopamine are quantified in terms of 3 macroscopic and 12 microscopic protonation constants and the concomitant 3 interactivity parameters. The species- and site-specific basicities are interpreted by means of inductive and shielding effects through various intra- and intermolecular comparisons. The site-specific basicities determined this way are key parameters for the prediction of pharmacokinetic behavior and receptor-binding at the molecular level.

Left and right ventricular parameters corrected with threshold-based quantification method in a normal cohort analyzed by three independent observers with various training-degree.

While cardiac magnetic resonance (CMR) is the reference method to evaluate left and right ventricular functions, volumes and masses, there is no widely accepted method for the quantitative analysis of trabeculae and papillary muscles (TPM). The aim of this study was to investigate the effect of TPM quantification on left and right ventricular CMR values in a normal cohort and to investigate interobserver variability of threshold-based (TB) analysis by three independent observers with variant experience in CMR. At our clinic, 60 healthy volunteers (30 males, mean age 25.6 ± 4.7 years) underwent CMR scan performed on a 1.5T Philips Achieva MR machine. On short-axis cine images, endo- and epicardial contours were detected by three independent observers with variable experience in CMR (low- ca. 120, mid- > 800, high-experienced > 5000 original CMR cases). Using Conv and TB methods (Medis 7.6 QMass software Leiden, The Netherland), we measured LV and RV ejection fractions, end-diastolic, end-systolic, stroke volumes and masses. We used TB method for quantifying TPM in ventricles using epicardial contour layers. Interobserver variability was evaluated, and the observer's experience as an impact on variability of each investigated parameters was assessed. Comparing Conv and TB quantification methods' significant difference were detected for all LV and RV parameters in case of all observers (H, M and L p < 0.0001). The global intraclass correlation coefficient (G-ICC) representing interobserver agreement for all investigated parameters was lower with Conv method (G-ICCConv vs. G-ICCTB 0.86 vs. 0.92 p < 0.0001). The ICC of LV parameters was higher using TB quantification (LV-ICCConv vs. LV-ICCTB 0.92 vs. 0.96 p < 0.0001), and for the evaluation of RV values, the TB method also had significantly higher interobserver agreement (RV-ICCConv vs. RV-ICCTB 0.80 vs. 0.89 p < 0.0001). The TB algorithm could be a consistent method to assess LV and RV CMR values, and to measure trabeculae and papillary muscles quantitatively in various level of experience in CMR.

DNA binding of sunitinib: Spectroscopic evidence via circular dichroism and nuclear magnetic resonance.

Sunitinib is a non-selective tyrosine kinase inhibitor, but in its chemical structure there can be discovered certain features, which suggest the ability to bind to DNA. These elements are the planar aromatic system and the tertiary amine function, which is protonated at the pH of the organism. In this study, the binding of the drug sunitinib to DNA was investigated using circular dichroism (CD), 1H NMR and UV spectroscopies, along with CD melting. For these studies DNA was isolated from calf thymus (CT), salmon fish sperm (SS), and chicken erythrocyte (CE), however for our purposes an artificially constructed and highly purified plasmid DNA (pUC18) preparation proved to be the most suitable. DNA binding of the drug was confirmed by shifts in the characteristic CD bands of the DNA, the appearance of an induced CD (ICD) signal in the upper absorption region of sunitinib (300 nm-500 nm), and the evidence from CD melting studies and the NMR. Based on the CD and NMR measurements, it can be assumed that sunitinib has a multiple-step binding mechanism.

Species-Specific Standard Redox Potential of Thiol-Disulfide Systems: A Key Parameter to Develop Agents against Oxidative Stress.

Microscopic standard redox potential, a new physico-chemical parameter was introduced and determined to quantify thiol-disulfide equilibria of biological significance. The highly composite, codependent acid-base and redox equilibria of thiols could so far be converted into pH-dependent, apparent redox potentials (E'°) only. Since the formation of stable metal-thiolate complexes precludes the direct thiol-disulfide redox potential measurements by usual electrochemical techniques, an indirect method had to be elaborated. In this work, the species-specific, pH-independent standard redox potentials of glutathione were determined primarily by comparing it to 1-methylnicotinamide, the simplest NAD+ analogue. Secondarily, the species-specific standard redox potentials of the two-electron redox transitions of cysteamine, cysteine, homocysteine, penicillamine, and ovothiol were determined using their microscopic redox equilibrium constants with glutathione. The 30 different, microscopic standard redox potential values show close correlation with the respective thiolate basicities and provide sound means for the development of potent agents against oxidative stress.

Skeletal muscle ATP synthesis and cellular H(+) handling measured by localized (31)P-MRS during exercise and recovery.

(31)P magnetic resonance spectroscopy (MRS) is widely used for non-invasive investigation of muscle metabolism dynamics. This study aims to extend knowledge on parameters derived from these measurements in detail and comprehensiveness: proton (H(+)) efflux, buffer capacity and the contributions of glycolytic (L) and oxidative (Q) rates to ATP synthesis were calculated from the evolutions of phosphocreatine (PCr) and pH. Data are reported for two muscles in the human calf, for each subject and over a wide range of exercise intensities. 22 subjects performed plantar flexions in a 7T MR-scanner, leading to PCr changes ranging from barely noticeable to almost complete depletion, depending on exercise protocol and muscle studied by localized MRS. Cytosolic buffer capacity was quantified for the first time non-invasively and individually, as was proton efflux evolution in early recovery. Acidification started once PCr depletion reached 60-75%. Initial and end-exercise L correlated with end-exercise levels of PCr and approximately linear with pH. Q calculated directly from PCr and pH derivatives was plausible, requiring fewer assumptions than the commonly used ADP-model. In conclusion, the evolution of parameters describing cellular energy metabolism was measured over a wide range of exercise intensities, revealing a relatively complete picture of muscle metabolism.

Physicochemical Profiling of α-Lipoic Acid and Related Compounds.

Lipoic acid, the biomolecule of vital importance following glycolysis, shows diversity in its thiol/disulfide equilibria and also in its eight different protonation forms of the reduced molecule. In this paper, lipoic acid, lipoamide, and their dihydro derivatives were studied to quantify their solubility, acid-base, and lipophilicity properties at a submolecular level. The acid-base properties are characterized in terms of six macroscopic, 12 microscopic protonation constants, and three interactivity parameters. The species-specific basicities, the pH-dependent distribution of the microspecies, and lipophilicity parameters are interpreted by various intramolecular effects, and contribute to understanding the antioxidant, chelate-forming, and enzyme cofactor behavior of the molecules observed.

Species-Specific Thiol-Disulfide Equilibrium Constant: A Tool To Characterize Redox Transitions of Biological Importance.

Microscopic redox equilibrium constants, a new species-specific type of physicochemical parameters, were introduced and determined to quantify thiol-disulfide equilibria of biological significance. The thiol-disulfide redox equilibria of glutathione with cysteamine, cysteine, and homocysteine were approached from both sides, and the equilibrium mixtures were analyzed by quantitative NMR methods to characterize the highly composite, co-dependent acid-base and redox equilibria. The directly obtained, pH-dependent, conditional constants were then decomposed by a new evaluation method, resulting in pH-independent, microscopic redox equilibrium constants for the first time. The 80 different, microscopic redox equilibrium constant values show close correlation with the respective thiolate basicities and provide sound means for the development of potent agents against oxidative stress.