Low pKa Values Research Articles

A combination of distillation-precipitation polymerization and click chemistry: fabrication of boronic acid functionalized Fe3O4 hybrid composites for enrichment of glycoproteins.

Biomedical sciences, and in particular biomarker research, demand efficient glycoprotein enrichment platforms. In this paper, a facile and efficient approach combining distillation-precipitation polymerization (DPP) and click chemistry was developed to synthesize boronic acid ligand-modified magnetic nanoparticles for the enrichment of glycoproteins. Due to the relatively large amount of benzyl chloride groups introduced by DPP on the magnetic core, which easily can be transferred into azide groups, the alkyne-phenylboronic acid ligands were immobilized onto the surface of Fe3O4 with high efficiency via the Cu(i)-catalyzed azide-alkyne cycloaddition (CuAAC) 'click' reaction. The morphology, structure and composition of the resulting core-shell Fe3O4@poly(4-vinylbenylchloride)@amidophenylboronic acid (Fe3O4@pVBC@APBA) nanocomposites were characterized by transmission electron microscopy, X-ray powder diffraction, vibrating sample magnetometry, Fourier transform infrared spectroscopy, thermogravimetric analysis and X-ray photoelectron spectrometry. The Fe3O4@pVBC@APBA microspheres held a ∼50 nm polymeric shell, and exhibited high magnetic response to an external magnetic field. The binding results demonstrated that Fe3O4@pVBC@APBA possessed high adsorption capacity and remarkable selectivity to glycoproteins. Moreover, the glycoproteins in the egg white sample could be enriched under physiological conditions (pH 7.4) as well, due to the lower pKa value of the alkyne-phenylboronic acid ligand. The high stability and selectivity of Fe3O4@pVBC@APBA for the glycoproteins were retained over several separation cycles. This boronate affinity material has potential applications in biomedical and biotechnological fields including drug delivery and biosensing.

Computed CH Acidity of Biaryl Compounds and Their Deprotonative Metalation by Using a Mixed Lithium/Zinc‐TMP Base

With the aim of synthesizing biaryl compounds, several aromatic iodides were prepared by the deprotonative metalation of methoxybenzenes, 3-substituted naphthalenes, isoquinoline, and methoxypyridines by using a mixed lithium/zinc-TMP (TMP=2,2,6,6-tetramethylpiperidino) base and subsequent iodolysis. The halides thus obtained, as well as commercial compounds, were cross-coupled under palladium catalysis (e.g., Suzuki coupling with 2,4-dimethoxy-5-pyrimidylboronic acid) to afford various representative biaryl compounds. Deprotometalation of the latter compounds was performed by using the lithium/zinc-TMP base and evaluated by subsequent iodolysis. The outcome of these reactions has been discussed in light of the CH acidities of these substrates, as determined in THF solution by using the DFT B3LYP method. Except for in the presence of decidedly lower pKa values, the regioselectivities of the deprotometalation reactions tend to be governed by nearby coordinating atoms rather than by site acidities. In particular, azine and diazine nitrogen atoms have been shown to be efficient in inducing the reactions with the lithium/zinc-TMP base at adjacent sites (e.g., by using 1-(2-methoxyphenyl)isoquinoline, 4-(2,5-dimethoxyphenyl)-3-methoxypyridine, or 5-(2,5-dimethoxyphenyl)-2,4-dimethoxypyrimidine as the substrate), a behavior that has already been observed upon treatment with lithium amides under kinetic conditions. Finally, the iodinated biaryl derivatives were involved in palladium-catalyzed reactions.

Design and Synthesis of Amphiphilic Xanthone-Based, Membrane-Targeting Antimicrobials with Improved Membrane Selectivity

This work describes how to tune the amphiphilic conformation of α-mangostin, a natural compound that contains a hydrophobic xanthone scaffold, to improve its antimicrobial activity and selectivity for Gram-positive bacteria. A series of xanthone derivatives was obtained by cationic modification of the free C3 and C6 hydroxyl groups of α-mangostin with amine groups of different pKa values. Modified structures using moieties with high pKa values, such as AM-0016 (3b), exhibited potent antimicrobial properties against Gram-positive bacteria. Compound 3b also killed bacteria rapidly without inducing drug resistance and was nontoxic when applied topically. Biophysical studies and molecular dynamics simulations revealed that 3b targets the bacterial inner membrane, forming an amphiphilic conformation at the hydrophobic-water interface. In contrast, moieties with low pKa values reduced the antimicrobial activity of the parent compound when conjugated to the xanthone scaffold. This strategy provides a new way to improve "hits" for the development of membrane-active antibiotics that target drug-resistant pathogens.

Reduction of Quinones by NADH Catalyzed by Organoiridium Complexes

Reduction of Quinones by NADH Catalyzed by Organoiridium Complexes

Arylamine‐Catalyzed Enamine Formation: Cooperative Catalysis with Arylamines and Acids

The explosive growth of organocatalysis has had a huge impact on asymmetric catalysis in the past decade. Transition-metal catalysis, on the other hand, has been established for a long time as one of the most powerful methods in organic synthesis. Aminocatalysis is a major field in organocatalysis. The combination of organocatalysis with the more traditional metal Lewis acid catalysis has emerged, aiming to achieve organic transformations that cannot be accomplished by organocatalysis or metal catalysis independently. Although it promises huge potential, this research area has grown only slowly. The major challenge lies in the incompatibility of the catalysts, in particular, the combination of enamine catalysis with harder metal Lewis acid is very difficult. The circumvention of this problem would represent an important breakthrough, given the huge number of substrates that can be activated by the large variety of metal Lewis acids. Herein, we present the solution to this longstanding problem by using arylamines as the catalysts in enamine catalysis. Very importantly, we demonstrate that arylamines can serve as efficient amine catalysts in direct asymmetric aldol reactions. Furthermore, we have developed a highly chemoand enantioselective three-component azaDiels–Alder reaction by combining arylamines with metal Lewis acids. The combination of enamine catalysis with metal Lewis acid catalysis was first reported by Ibrahem and Codava in 2006. Since then, considerable progress has been made in this area, leading to a series of exciting discoveries. However, these combinations were limited to soft metals, such as Cu, Ag, Au, Ir, and Pd or Pd, activating either p-allyl electrophiles or alkynes (Scheme 1,A and B). Combining enamine catalysis with harder metal Lewis acid (Scheme 1, C) turned out to be very challenging because of acid–base self-quenching reactions, which render the catalysts inactive. In asymmetric aminocatalysis involving either an enamine or an iminium intermediate, a chiral aliphatic secondary or primary amine serves as the catalyst. Aliphatic amines are hard bases, and thus likely to be compatible with softer metals based on the soft/hard approach, but less likely to be compatible with harder metals. We hoped to find an amine catalyst that is compatible with a large variety of metal Lewis acids to significantly extend the scope of enamine/ metal Lewis acid catalysis, and to facilitate the development of a new research area of iminium/metal Lewis acid catalysis. We considered to use arylamines, such as aniline, because they have a much lower pKa value (4–6) than aliphatic amines (9–11), and should be much softer because of the delocalization of the lone pair to the aromatic p system. It appeared to us that arylamines are ideal candidates for combination with harder metal Lewis acids. Despite their ubiquity in organic chemistry, arylamines have never been used in enamine catalysis. This may be mainly due to the general understanding that the nucleophilicity of arylamines is much lower compared to aliphatic amines. However, List and co-workers suggested the formation of enamine intermediates from arylamines as a step in organocatalytic cascade reactions. In a recent report, Gong and co-workers also suggested that an achiral arylamine played a crucial role in controlling the stereochemistry of a Friedl nder condensation by forming an enamine intermediate. We speculate that arylamines might be suitable to serve as an efficient amine catalyst in enamine catalysis in conjunction with a stronger metal Lewis acid. The lower nucleophilicity of enamines can be compensated by the following factors: 1) facilitated formation of enamine in the presence of a metal Lewis acid; 2) higher activation of the electrophiles by a metal Lewis acid. The asymmetric aza-Diels–Alder reaction (ADAR) is the most convenient and powerful method to form nitrogencontaining heterocycles, which are one of the most important structural motifs in natural products, pharmaceuticals, and biosystems. While the recent progress on normal-electrondemand ADARs based on dienamines and imine dienophiles Scheme 1. Combination of enamine catalysis with metal catalysis.

Development of poly((3-acrylamidophenyl)boronic acid-co-N,N-methylenebisacrylamide) monolithic capillary for the selective capture of cis-diol biomolecules

A new hydrophilic boronate affinity monolithic capillary was synthesized for the selective capture of cis-diol biomolecules including glycoproteins and nucleosides, using (3-acrylamidophenyl)boronic acid (APBA) and N,N-methylenebisacrylamide (MBAA) as the functional monomer and cross-linker, respectively. A comparison with other compositionally similar monolithic capillaries was carried out in terms of reversed-phase retention, selectivity, binding capacity and binding pH. The poly(APBA-co-MBAA) monolithic capillary exhibited notably different properties. As both the functional monomer and the cross-linker are hydrophilic, the monolith capillary showed suppressed reversed-phase retention and as a result excellent selectivity toward both nucleosides and glycoproteins was observed. It exhibited the highest binding capacity as compared with other compositionally similar monolithic capillaries. Besides, because of the lower pKa value of APBA, the monolithic capillary was able to selectively capture nucleosides at near-physiological pH.

Oligomerization and its Effect on Function in 7-Transmembrane Proteins

Proteorhodopsin (PR) is a solar-powered membrane protein–a proton pump from marine bacteria that has significant structural and dynamical commonalities to seven-transmembrane (7TM) mammalian proteins, including the G-protein Coupled Receptors (GPCRs). Furthermore, PR associates with itself in the membrane to form oligomers similarly to GPCRs, and these different structural forms are thought to play a large role in function. Unfortunately, studying the structure and function of membrane proteins in oligomeric assemblies is very challenging at the molecular level due to the oligomers' large size, disordered nature, and ability to resist crystallization–making PR a facile model system for capturing elusive details of dynamics and function. Past work has revealed PR's hexameric interface in dodecylmaltoside (DDM) micelles, and here we focus on understanding the effect of oligomerization on function. We seek to probe oligomeric interfaces as we vary surfactant environment and oligomeric state. using fast protein liquid chromatography (FPLC) and optical absorption experiments we show that the hexameric state of PR in DDM surfactant has a lower pKa value for the proton accepting residue than both the monomeric and dimeric protein, suggesting that the hexamer is more optimized for proton transport. Time-resolved electron paramagnetic resonance (EPR) and optical absorption experiments show that the hexameric state of PR has much slower photocycle dynamics than the monomeric state. Our work shows that varying surfactant environment appears to have less of an effect on kinetics and function than does oligomerization. Protein-protein interactions therefore have a central role in tuning protein function, a result that lends insight into the functional mechanisms of more complex mammalian membrane proteins.

Maximum residue level validation of triclabendazole marker residues in bovine liver, muscle and milk matrices by ultra high pressure liquid chromatography tandem mass spectrometry

Triclabendazole is the only anthelmintic drug, which is active against immature, mature and adult stages of fluke. The objective of this work was to develop an analytical method to quantify and confirm the presence of triclabendazole residues around the MRL. In this work, a new analytical method was developed, which extended dynamic range to 1–100 and 5–1000μgkg−1 for milk and tissue, respectively. This was achieved using a mobile phase containing trifluoroacetic acid (pKa of 0.3), which resulted in the formation of the protonated pseudomolecular ions, [M+H]+, of triclabendazole metabolites. Insufficient ionisation of common mobile phase additives due to low pKa values (<2) was identified as the cause of poor linearity. The new mobile phase conditions allowed the analysis of triclabendazole residues in liver, muscle and milk encompassing their EU maximum residue levels (MRL) (250, 225 and 10μgkg−1 respectively). Triclabendazole residues were extracted using a modified QuEChERS method and analysed by positive electrospray ionisation mass spectrometry with all analytes eluted by 2.23min. The method was validated at the MRL according to Commission Decision (CD) 2002/657/EC criteria. The decision limit (CCα) of the method was in the range of 250.8–287.2, 2554.9–290.8 and 10.9–12.1μgkg−1 for liver, muscle and milk, respectively. The performance of the method was successfully verified for triclabendazole in muscle by participating in a proficiency study, the method was also applied to incurred liver, muscle and milk samples.

Discrimination of fluorescence light-up effects induced by pH and metal ion chelation on a spirocyclic derivative of rhodamine B

In the present work we describe the structure and the spectroscopic characterization of a spirocyclic derivative of a rhodamine B ligand whose properties allow discrimination of light-up effects induced by metal ion chelation and variation of pH. Distinction of the two effects is important for the use of this type of ligand to detect and monitor metal ions in aqueous solutions. The synthesis of the ligand was performed in two steps, which involve the reaction of rhodamine B with hydrazine hydrate to form rhodamine B hydrazide followed by condensation with 2-pyridinecarboxaldehyde and was successfully optimized using a solvent free approach under microwave irradiation. The ligand was obtained in the expected spirolactam form and was characterized in the solid state by EA, MS and single-crystal X-ray diffraction. The ligand was characterized in solution by NMR and absorption and fluorescence spectroscopies and its properties were found to be sensitive to pH and concentration of iron(III). The study of the fluorescence properties at variable pH shows that the compound is fluorescent in the range 2 < pH < 4 with maximum intensity at pH 3 and allowed the determination of two pK(a) values (pK(a1) = 2.98, pK(a2) = 2.89) and establishment of the corresponding distribution diagram. The very low pK(a) values guarantee that above pH equal to 4 the ligand is mostly present in the fully non-protonated and non-fluorescent form L. The study of the interaction of the ligand with iron(iii) was performed in DMSO and DMSO-H(2)O to exclude the influence of pH and due to the low solubility of the compound. The results indicate that the presence of iron(III) triggers the opening of the spirolactam form of the ligand and the maximum intensity obtained at a metal : ligand ratio of 1 : 2 is consistent with the formation of an iron(III) complex with the tridentate ligand.

The -Cys-X1-X2-Cys- Motif of Reduced Glutaredoxins Adopts a Consensus Structure That Explains the Low pKa of Its Catalytic Cysteine

The -Cys-X1-X2-Cys- active site motif is central to the function of enzymes of the thioredoxin superfamily, including glutaredoxins. Their chemistry depends on the lowered pK(a) of the N-terminal thiolate cysteine of the -Cys-X1-X2-Cys- sequence; therefore its structure, dynamics, and electrostatics matter. Much information about the glutaredoxin structures was obtained by nuclear magnetic resonance (NMR), yet these various NMR structures produced heterogeneous and discordant views of the -Cys-X1-X2-Cys- motifs. This study addresses these inconsistencies by a computational and experimental investigation of three diverse reduced -Cys-X1-X2-Cys- motifs, from human glutaredoxin 1 (hGrx1), Escherichia coli glutaredoxin 2 (EcGrx2), and T4 virus glutaredoxin (T4Grx). The NMR models do not account for the low pK(a) of the N-terminal cysteine. However, extensive investigations of the NMR conformers by simulations yielded consensus structures for the -Cys-X1-X2-Cys- motif, with well-defined orientations for the cysteines. pK(a) calculations indicated that the consensus structure stabilizes the thiolate by local hydrogen bonds. The consensus structures of EcGrx2 and T4Grx formed the basis for predicting low pK(a) values for their N-terminal cysteines. Subsequent experimental titrations showed that these pK(a) values are <5, supporting the validity of the consensus structure. The simulations also revisited the conformational dynamics of side chains around the -Cys-X1-X2-Cys- motif, which allowed reconciliation of calculated and measured pK(a) values for important hGrx1 mutants. The conformational spread of these side chains, which differs between NMR and molecular dynamics models, is likely to be relevant to substrate recognition. The new structural models determined in this work should prove to be valuable in future molecular studies of the glutaredoxins.

Prodrugs of fumarate esters for the treatment of psoriasis and multiple sclerosis—a computational approach

Density functional theory (DFT) calculations at B3LYP/6-31 G (d,p) and B3LYP/6-311+G(d,p) levels for the substituted pyridine-catalyzed isomerization of monomethyl maleate revealed that isomerization proceeds via four steps, with the rate-limiting step being proton transfer from the substituted pyridinium ion to the C=C double bond in INT1. In addition, it was found that the isomerization rate (maleate to fumarate) is solvent dependent. Polar solvents, such as water, tend to accelerate the isomerization rate, whereas apolar solvents, such as chloroform, act to slow down the reaction. A linear correlation was obtained between the isomerization activation energy and the dielectric constant of the solvent. Furthermore, linearity was achieved when the activation energy was plotted against the pKa value of the catalyst. Substituted-pyridine derivatives with high pKa values were able to catalyze isomerization more efficiently than those with low pKa values. The calculated relative rates for prodrugs 1-6 were: 1 (406.7), 2 (7.6×10(6)), 3 (1.0), 4 (20.7), 5 (13.5) and 6 (2.2×10(3)). This result indicates that isomerizations of prodrugs 1 and 3-5 are expected to be slow and that of prodrugs 2 and 6 are expected to be relatively fast. Hence, prodrugs 2 and 3-5 have the potential to be utilized as prodrugs for the slow release of monomethylfumarate in the treatment of psoriasis and multiple sclerosis.

Discovery of benzothiazole guanidines as novel inhibitors of thrombin and trypsin IV

In a project to find novel neutral P1 fragments for the synthesis of thrombin inhibitors with improved pharmacokinetic properties, fragments containing a benzothiazole guanidine scaffold were identified as weak thrombin inhibitors. WaterLOGSY (Water-Ligand Observed via Gradient SpectroscopY) NMR was used to detect fragments binding to thrombin and these fragments were followed up by Biacore A100 affinity measurements and enzyme assays. A crystal structure of the most potent compound with thrombin was obtained and revealed an unexpected binding mode as well as the key interactions of the fragment with the protein. Based on these results, the structure-based design and synthesis of a small series of optimized novel substituted benzothiazole guanidines with comparatively low pKa values was accomplished. Testing of these compounds against human trypsin I and human trypsin IV revealed unexpected inhibitory activity and selectivity of some of the compounds, making them attractive starting points for selective trypsin inhibitors.

Exchange rates of creatine kinase metabolites: feasibility of imaging creatine by chemical exchange saturation transfer MRI

Creatine (Cr), phosphocreatine (PCr) and adenosine-5-triphosphate (ATP) are major metabolites of the enzyme creatine kinase (CK). The exchange rate of amine protons of CK metabolites at physiological conditions has been limited. In the current study, the exchange rate and logarithmic dissociation constant (pKa) of amine protons of CK metabolites were calculated. Further, the chemical exchange saturation transfer effect (CEST) of amine protons of CK metabolites with bulk water was explored. At physiological temperature and pH, the exchange rate of amine protons in Cr was found to be 7-8 times higher than PCr and ATP. A higher exchange rate in Cr was associated with lower pKa value, suggesting faster dissociation of its amine protons compared to PCr and ATP. CEST MR imaging of these metabolites in vitro in phantoms displayed predominant CEST contrast from Cr and negligible contribution from PCr and ATP with the saturation pulse parameters used in the current study. These results provide a new method to perform high-resolution proton imaging of Cr without contamination from PCr. Potential applications of these finding are discussed.

Modification of enzymatic activity in soils of contrasting pH contaminated with 2,4-dichlorophenol and 2,4,5-trichlorophenol

According to previous studies, acidic soils may receive larger quantities of 2,4-dichlorophenol (2,4-DCP) and of 2,4,5-trichlorophenol (2,4,5-TCP) than the concentrations indicated in the prevailing legislation for defining a soil as contaminated, without any important changes in their biochemical properties. In this study, we investigated whether neutral or slightly alkaline soils behave in the same way as acidic soils in response to contamination by these compounds. For this purpose, a large number of acidic soils (pH between 4.2 and 5.9) and calcareous soils (pH between 6.5 and 8.0) were contaminated in the laboratory with different doses of 2,4-DCP (up to 10,000 times the GRL) and of 2,4,5-TCP (up to 500 times the GRL). After an incubation period of three days, the activities of several enzymes (dehydrogenase, catalase, ß-glucosidase and phosphomonoesterase) were measured in the soils. The effects of 2,4,5-TCP were much greater than those of 2,4-DCP in both the acidic and calcareous soils, regardless of the dose applied. Phosphomonoesterase and ß-glucosidase activities were scarcely affected by either of the contaminants in any of the soils, whereas the catalase activity decreased slightly. The dehydrogenase and urease activities were strongly affected in all soils and in some cases even disappeared, particularly after the application of 2,4,5-TCP. Multiple regression analysis of the percentage reductions in dehydrogenase and urease activities in relation to contaminant dose and different soil properties indicated that the reduction in enzyme activity depended, in decreasing order, on the dose of contaminant applied, total carbon content and soil pH. We suggest that the processes that regulate the toxicity of these compounds in soils are their adsorption by soil organic matter and the dissociation of the non-adsorbed compound into phenolate ions (which are toxic to microorganisms). In fact, the chlorophenols scarcely affected the biochemical properties of the soils under study because of their high organic matter contents (A horizons with total carbon contents of up to 11%). Moreover, both chlorophenols had slightly stronger effects on the calcareous soils than on the acidic soils, probably because the dissociation process was favoured at higher pH. On the other hand, the 2,4,5-TCP had stronger effects on soil biochemical properties than 2,4-DCP, which may be explained by the lower pKa value of 2,4,5-TCP (6.9) than that of 2,4-DCP (7.9). The results show that the GRL values established by the legislation are not appropriate for either of these chlorophenol compounds.

Product-assisted Catalysis as the Basis of the Reaction Specificity of Threonine Synthase

Threonine synthase (TS), which is a pyridoxal 5'-phosphate (PLP)-dependent enzyme, catalyzes the elimination of the γ-phosphate group from O-phospho-L-homoserine (OPHS) and the subsequent addition of water at Cβ to form L-threonine. The catalytic course of TS is the most complex among the PLP enzymes, and it is an intriguing problem how the elementary steps are controlled in TS to carry out selective reactions. When L-vinylglycine was added to Thermus thermophilus HB8 TS in the presence of phosphate, L-threonine was formed with k(cat) and reaction specificity comparable with those when OPHS was used as the substrate. However, in the absence of phosphate or when sulfate was used in place of phosphate, only the side reaction product, α-ketobutyrate, was formed. Global analysis of the spectral changes in the reaction of TS with L-threonine showed that compared with the more acidic sulfate ion, the phosphate ion decreased the energy levels of the transition states of the addition of water at the Cβ of the PLP-α-aminocrotonate aldimine (AC) and the transaldimination to form L-threonine. The x-ray crystallographic analysis of TS complexed with an analog for AC gave a distinct electron density assigned to the phosphate ion derived from the solvent near the Cβ of the analog. These results indicated that the phosphate ion released from OPHS by γ-elimination acts as the base catalyst for the addition of water at Cβ of AC, thereby providing the basis of the reaction specificity. The phosphate ion is also considered to accelerate the protonation/deprotonation at Cγ.

Photosensitized degradation in water of the phenolic pesticides bromoxynil and dichlorophen in the presence of riboflavin, as a model of their natural photodecomposition in the environment

Within the context of environmentally friendly methods for the elimination of surface-water pollutants, the photodegradation of the phenolic pesticides bromoxynil (BXN) and dichlorophen (DCP) under simulated natural conditions has been studied. The work was done in the presence of the visible-light absorber photosensitizer riboflavin (Rf), usually present in trace quantities in natural waters. Under aerobic conditions, an efficient photooxidation of both pesticides was observed. The relatively intricate photochemical mechanism involves pesticide and oxygen consumption and, to a lesser extent, Rf degradation. The kinetic and mechanistic study supports that both H2O2 and singlet molecular oxygen, O2(1Δg), are involved in the process. Kinetic data for the O2(1Δg)-mediated oxidation indicate that BXN and DCP are photodegraded with this species faster than the parent compound phenol, very frequently employed as a model for aquatic contaminants, likely due to their lower pKa values. This observation allows the design of phenolic pesticides with different photodegradation rates under environmental conditions.

ChemInform Abstract: Solvatochromism and Prototropism in 2-(Aminophenyl)benzothiazoles.

The absorption and fluorescence spectra of 2-(2-aminophenyl)-, 2-(3-aminophenyl)-, and 2-(4-aminophenyl)benzothiazoles (o-APBT, m-APBT, and p-APBT, respectively) have been studied in different solvents and at various acid/base concentrations. The ultraviolet and fluorescence spectra and the low pKa value for the monocation–neutral equilibrium have indicated the presence of an intramolecular hydrogen bond (IHB) in o-APBT in the S0 state. Excited state intra-molecular proton transfer (ESIPT) in o-APBT, gives rise to a non-fluorescent phototautomer. Large solvatochromic shifts in polar solvents have indicated the increase in the dipolemoment of m-APBT in S1 state. It has been observed that in protic solvents, m-APBT forms a complex with solvent molecule in the S1 state. The equilibrium constants for different prototropic equilibria of the molecules have been determined in both S0 and S1 states. The absorption and fluorescence spectra of the prototropic species have been explained and compared with those of 2...

Bioinformatic Analysis, Molecular Modeling of Role of Lys65 Residue in Catalytic Triad of D-aminopeptidase from Ochrobactrum anthropi

A bioinformatic and phylogenetic study has been performed on a family of penicillin-binding proteins including D-aminopeptidases, D-amino acid amidases, DD-carboxypeptidases, and β -lactamases. Significant homology between D-aminopeptidase from Ochrobactrum anthropi and other members of the family has been shown and a number of conserved residues identified as S62, K65, Y153, N155, H287, and G289. Three of those (Ser62, Lys65, and Tyr153) form a catalytic triangle - the proton relay system that activates the generalized nucleophile in the course of catalysis. Molecular modeling has indicated the conserved residue Lys65 to have an unusually low pKa value, which has been confirmed experimentally by a study of the pH-profile of D-aminopeptidase catalytic activity. The resulting data have been used to elucidate the role of Lys65 in the catalytic mechanism of D-aminopeptidase as a general base for proton transfer from catalytic Ser62 to Tyr153, and vice versa, during the formation and hydrolysis of the acyl - enzyme intermediate.

Energy Levels and Redox Properties of Aqueous Mn2+/3+ from Photoemission Spectroscopy and Density Functional Molecular Dynamics Simulation

Energy-resolved photoemission spectroscopy and density functional molecular dynamics simulations are combined to construct an energy level diagram for the Mn(2+/3+) redox reaction in aqueous solution. Two peaks centered at 8.88 and 10.26 eV electron binding energies can be assigned to the Mn2+ hexa-aquo complex with a peak area ratio of 2:2.83. Using the notation of crystal field theory, the peak at lower energies can be interpreted as arising from ionization from the e(g) levels (highest occupied molecular orbital, HOMO), and the peak at higher energies are from ionization of the t(2g) levels. The difference corresponds to the average crystal field splitting, 1.38 eV. From the position of the HOMO level and the absolute redox potential, an experimental value for the reorganization free energy of the aqueous Mn3+ hexa-aquo complex is estimated to be 2.98 eV. Density functional molecular dynamics simulations can reproduce the experimental vertical ionization energy, redox free energy, and reorganization free energies fairly well, provided that the absolute potential shift in periodic boundary conditions, finite size effects, and inaccuracies of the exchange correlation functional are taken into account. Most strikingly, in the simulations, we observe spontaneous and reversible deprotonation of the aqueous Mn3+ hexa-aquo complex to form MnOH(H2O)5(2+) + H+, in line with the low experimental pKa value of this ion. The interconversion between protonation states leads to interesting redox phenomena for aqueous Mn3+, culminating in a bimodal thermal distribution of the electron affinity.

Chitosan-4-mercaptobenzoic acid: synthesis and characterization of a novel thiolated chitosan

Thiomers are thiolated polymers with excellent mucoadhesive properties. The aim of this study was to synthesize a novel thiolated chitosan with higher reactivity in order to further improve mucoadhesion. For this purpose, 4-mercaptobenzoic acid was chosen to be covalently attached to chitosan. The aromatic structure of the ligand should exhibit a higher reactivity due to a comparatively low pKa value of the thiol group, which was determined to be 6.8. Mucoadhesion, in situ gelation, biocompatibility and toxicity were evaluated. Chitosan-4-mercaptobenzoic acid was proved to be 60-fold more mucoadhesive compared to unmodified chitosan. After 24 hours, the viscosity of the novel thiomer solution increased 2974-fold compared to unmodified chitosan and after 48 hours 4487-fold. Chitosan-4-mercaptobenzoic acid tablets disintegrated twice as slowly as unmodified chitosan. Furthermore, the novel thiomer proved to be biodegradable and to be non-toxic. Due to the above mentioned properties, chitosan-4-mercaptobenzoic acid is a promising excipient for oral formulations or in situ gelling formulations.