Scanning Microcalorimetry Research Articles

Derivatives of Pyrimidine Nucleosides Affect Artificial Membranes Enriched with Mycobacterial Lipids.

The mechanisms of action of pyrimidine nucleoside derivatives on model lipid membranes of various compositions were studied. A systematic analysis of the tested agents' effects on the membrane physicochemical properties was performed. Differential scanning microcalorimetry data indicated that the ability of nucleoside derivatives to disorder membrane lipids depended on the types of nucleoside bases and membrane-forming lipids. The 5'-norcarbocyclic uracil derivatives were found to be ineffective, while N4-alkylcytidines demonstrated the most pronounced effects, significantly decreasing the dipalmitoylphosphocholine melting temperature and cooperativity of phase transition. The elongation of hydrophobic acyl radicals potentiated the disordering action of N4-alkylcytidines, while an increase in hydrophilicity due to replacing deoxyribose with ribose inhibited this effect. The ability of compounds to form transmembrane pores was also tested. It was found that 5-alkyluridines produced single, ion-permeable pores in phosphatidylglycerol membranes, and that methoxy-mycolic acid and trehalose monooleate potentiated the pore-forming activity of alkyloxymethyldeoxyuridines. The results obtained open up perspectives for the development of innovative highly selective anti-tuberculosis agents, which may be characterized by a low risk of developing drug resistance due to the direct action on the membranes of the pathogen.

The interaction of plant flavones with amphotericin B: Consequences for its pore-forming ability

The growth of antibiotic resistance to antifungal drugs contributes to the search for new ways to enhance their effectiveness and reduce toxicity. The undeniable advantage of polyene macrolide antibiotic amphotericin B (AmB) which ensures low pathogen resistance is its mechanism of action related to the formation of transmembrane pores in target lipid membranes. Here, we investigated the effects of plant flavones, chrysin, wogonin, baicalein, apigenin, scutellarein, luteolin, morin and fisetin on the pore-forming activity of AmB in the sterol-enriched membranes by electrophysiological assays. Сhrysin, wogonin, baicalein, apigenin, scutellarein, and luteolin were shown to decrease the AmB pore-forming activity in the bilayers composed of palmitoyloleylphosphocholine independently of their sterol composition. Morin and fisetin led to the increase and decrease in the AmB pore-forming activity in the ergosterol- and cholesterol-containing bilayers respectively. Differential scanning microcalorimetry of the gel-to-liquid crystalline phase transition of membrane forming lipids, molecular dynamics simulations, and absorbance spectroscopy revealed the possibility of direct interactions between AmB and some flavones in the water and/or in the lipid bilayer. The influence of these interactions on the antibiotic partitioning between aqueous solution and membrane and/or its transition between different states in the bilayer was discussed.

Thermodynamic Study of Alkylsilane and Alkylsiloxane-Based Ionic Liquids.

The thermodynamic properties of ionic liquids (ILs) bearing alkylsilane and alkylsiloxane chains, as well as their carbon-based analogs, were investigated. Effects such as the replacement of carbon atoms by silicon atoms, the introduction of a siloxane linkage, and the length of the alkylsilane chain were explored. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the thermal and phase behavior (glass transition temperature, melting point, enthalpy and entropy of fusion, and thermal stability). Heat capacity was obtained by high-precision drop calorimetry and differential scanning microcalorimetry. The volatility and cohesive energy of these ILs were investigated via the Knudsen effusion method coupled with a quartz crystal microbalance (KEQCM). Gas phase energetics and structure were also studied to obtain the gas phase heat capacity as well as the energy profile associated with the rotation of the IL side chain. The computational study suggested the existence of an intramolecular interaction in the alkylsiloxane-based IL. The obtained glass transition temperatures seem to follow the trend of chain flexibility. An increase of the alkylsilane chain leads to a seemingly linear increase in molar heat capacity. A regular increment of 30 J·K-1·mol-1 in the molar heat capacity was found for the replacement of carbon by silicon in the IL alkyl chain. The alkylsilane series was revealed to be slightly more volatile than its carbon-based analogs. A further increase in volatility was found for the alkylsiloxane-based IL, which is likely related to the decrease of the cohesive energy due to the existence of an intramolecular interaction between the siloxane linkage and the imidazolium headgroup. The use of Si in the IL structure is a suitable way to significantly reduce the IL's viscosity while preserving its large liquid range (low melting point and high thermal stability) and low volatilities.

Design of stable circular permutants of the GroEL chaperone apical domain

Enhancing protein stability holds paramount significance in biotechnology, therapeutics, and the food industry. Circular permutations offer a distinctive avenue for manipulating protein stability while keeping intra-protein interactions intact. Amidst the creation of circular permutants, determining the optimal placement of the new N- and C-termini stands as a pivotal, albeit largely unexplored, endeavor. In this study, we employed PONDR-FIT’s predictions of disorder propensity to guide the design of circular permutants for the GroEL apical domain (residues 191–345). Our underlying hypothesis posited that a higher predicted disorder value would correspond to reduced stability in the circular permutants, owing to the increased likelihood of fluctuations in the novel N- and C-termini. To substantiate this hypothesis, we engineered six circular permutants, positioning glycines within the loops as locations for the new N- and C-termini. We demonstrated the validity of our hypothesis along the set of the designed circular permutants, as supported by measurements of melting temperatures by circular dichroism and differential scanning microcalorimetry. Consequently, we propose a novel computational methodology that rationalizes the design of circular permutants with projected stability.8ghBPrif1HpKfHNvAL1G2xVideo

Cationic Proteins Rich in Lysine Residue Trigger Formation of Non-bilayer Lipid Phases in Model and Biological Membranes: Biophysical Methods of Study.

Cationic membrane-active toxins are the most abundant group of proteins in the venom of snakes and insects. Cationic proteins such as cobra venom cytotoxin and bee venom melittin are known for their pharmacological reactions including anticancer and antimicrobial effects which arise from the toxin-induced alteration in the dynamics and structure of plasma membranes and membranes of organelles. It has been established that these cationic toxins trigger the formation of non-bilayer lipid phase transitions in artificial and native mitochondrial membranes. Remarkably, the toxin-induced formation of non-bilayer lipid phase increases at certain conditions mitochondrial ATP synthase activity. This observation opens an intriguing avenue for using cationic toxins in the development of novel drugs for the treatment of cellular energy deficiency caused by aging and diseases. This observation also warrants a thorough investigation of the molecular mechanism(s) of lipid phase polymorphisms triggered by cationic proteins. This article presents a review on the application of powerful biophysical methods such as resonance spectroscopy (31P-, 1H-, 2H-nuclear magnetic resonance, and electron paramagnetic resonance), luminescence, and differential scanning microcalorimetry in studies of non-bilayer lipid phase transitions triggered by cationic proteins in artificial and biological membranes. A phenomenon of the triggered by cationic proteins the non-bilayer lipid phase transitions occurring within 10-2-10-11s is discussed in the context of potential pharmacological applications of cationic proteins. Next to the ATP dimer is an inverted micelle made of cardiolipin that serves as a vehicle for the transport of H+ ions from the intra-crista space to the matrix. It is proposed that such inverted micelles are triggered by the high density of H+ ions and the cationic proteins rich in lysine residue which compete with the conserved lysine residues of the ATP synthase rotor for binding to cardiolipin in the inner mitochondrial membrane and perturb the bilayer lipid packing of cristae. Phospholipids with a blue polar head represent cardiolipin and those with a red polar head represent other phospholipids found in the crista membrane.

Trametes versicolor laccase activity modulated by the interaction with gold nanoparticles

In this study, the impact of gold nanoparticles (AuNPs) on the structure and activity of laccase from Trametes versicolor (Lc) was described. Fluorescence experiments revealed that AuNPs efficiently quench Lc's tryptophan fluorescence by a static and dynamic process. By using differential scanning microcalorimetry and circular dichroism spectroscopy, it was determined how the concentration of nanoparticles and the composition of the medium affected the secondary structure of Lc. The data revealed that upon binding with AuNPs, conformational changes take place mainly in presence of high amounts of nanoparticles. The complex kinetic analysis unveiled the Lc activity enhancement at low concentrations of AuNPs as opposed to the concentrated regime, where it can be reduced by up to 55%. The Michaelis-Menten tests highlighted that the activity of the biocatalyst is closely related to the concentration of AuNPs, while the Selwyn analysis demonstrated that even in a concentrated regime of Lc it is not deactivated regardless of the amount of AuNPs added. The thermal parameters improved by twofold in the presence of low AuNPs concentration, whereas the activation energy increased with AuNPs content, implying that not all collisions are beneficial to the enzyme structure. The effect of AuNPs on the decomposition of a recalcitrant dye (naphthol green B, NG) by Lc was also evaluated, and the Michaelis-Menten model revealed that only the high AuNPs content influenced negatively the Lc activity. The isothermal titration calorimetry revealed that hydrogen bonds are the main intermolecular forces between Lc and AuNPs, while electrostatic interactions are responsible for NG adsorption to AuNPs. The results of the docking analysis show the binding of NG near the copper T1 site of Lc with hydrogen bonds, electrostatic and hydrophobic interactions. The findings of this work provide important knowledge for laccase-based bio-nanoconjugates and their use in the field of environmental remediation.

Triggering the Amphotericin B Pore-Forming Activity by Phytochemicals.

The macrolide polyene antibiotic amphotericin B (AmB), remains a valuable drug to treat systemic mycoses due to its wide antifungal activity and low probability of developing resistance. The high toxicity of AmB, expressed in nephropathy and hemolysis, could be partially resolved by lowering therapeutic AmB concentration while maintaining efficacy. This work discusses the possibility of using plant polyphenols and alkaloids to enhance the pore-forming and consequently antifungal activity of AmB. We demonstrated that phloretin, phlorizin, naringenin, taxifolin, quercetin, biochanin A, genistein, resveratrol, and quinine led to an increase in the integral AmB-induced transmembrane current in the bilayers composed of palmitoyloleoylphosphocholine and ergosterol, while catechin, colchicine, and dihydrocapsaicin did not practically change the AmB activity. Cardamonin, 4'-hydroxychalcone, licochalcone A, butein, curcumin, and piperine inhibited AmB-induced transmembrane current. Absorbance spectroscopy revealed no changes in AmB membrane concentration with phloretin addition. A possible explanation of the potentiation is related to the phytochemical-produced changes in the elastic membrane properties and the decrease in the energy of formation of the lipid mouth of AmB pores, which is partially confirmed by differential scanning microcalorimetry. The possibility of AmB interaction with cholesterol in the mammalian cell membranes instead of ergosterol in fungal membranes, determines its high toxicity. The replacement of ergosterol with cholesterol in the membrane lipid composition led to a complete loss or a significant decrease in the potentiating effects of tested phytochemicals, indicating low potential toxicity of these compounds and high therapeutic potential of their combinations with the antibiotic. The discovered combinations of AmB with plant molecules that enhance its pore-forming ability in ergosterol-enriched membranes, seem to be promising for further drug development in terms of the toxicity decrease and efficacy improvement.

Release of Arabidopsis seed dormancy by cold atmospheric plasma relies on cytoplasmic glass transition

When mature Arabidopsis thaliana seeds are dormant, their germination is prevented in apparently favorable conditions. This primary dormancy can be released during seed dry storage through a process called after-ripening whose duration can last several months. To reduce this delay, cold atmospheric plasmas (CAPs) can be used as sources of reactive oxygen species (ROS) capable of inducing heterogeneous chemical reactions. While CAP are known to stimulate the germination of various seed species, the relationship between CAP treatments and the amorphous solid state of dry seeds remains unexplored. Here, we demonstrate that seed dormancy can be alleviated using a cold plasma of ambient air and that this alleviation can be amplified for seeds with high water-content (typically 30%DW) or seeds heated at 60 °C during plasma treatment. Differential scanning micro-calorimetry shows that these characteristics control the glassy/rubbery state of the seed cytoplasm. This technique indicates also that a glass transition to the rubbery state strengthens the CAP effects to alleviate seed dormancy. We propose that lower cytoplasmic viscosity can promote the oxidative signaling induced by CAP which, in turn, improves the germination process.

Cyclic lipopeptides as membrane fusion inhibitors against SARS-CoV-2: New tricks for old dogs

With the resurgence of the coronavirus pandemic, the repositioning of FDA-approved drugs against coronovirus and finding alternative strategies for antiviral therapy are both important. We previously identified the viral lipid envelope as a potential target for the prevention and treatment of SARS-CoV-2 infection with plant alkaloids (Shekunov et al., 2021). Here, we investigated the effects of eleven cyclic lipopeptides (CLPs), including well-known antifungal and antibacterial compounds, on the liposome fusion triggered by calcium, polyethylene glycol 8000, and a fragment of SARS-CoV-2 fusion peptide (816–827) by calcein release assays. Differential scanning microcalorimetry of the gel-to-liquid-crystalline and lamellar-to-inverted hexagonal phase transitions and confocal fluorescence microscopy demonstrated the relation of the fusion inhibitory effects of CLPs to alterations in lipid packing, membrane curvature stress and domain organization. The antiviral effects of CLPs were evaluated in an in vitro Vero-based cell model, and aculeacin A, anidulafugin, iturin A, and mycosubtilin attenuated the cytopathogenicity of SARS-CoV-2 without specific toxicity.

Exploring the role of preferential solvation in the stability of globular proteins through the study of ovalbumin interaction with organic additives

<abstract> <p>The impact of denaturing and stabilizing osmolytes on protein conformational dynamics has been extensively explored due to the significant contribution of protein solvation to the stability, function, malfunction and regulation of globular proteins. We studied the effect of two nonspecific organic molecules, urea, which is a conventional denaturant, and dimethyl sulfoxide (DMSO), which is a multilateral organic solvent, on the stability and conformational dynamics of a non-inhibitory serpin, ovalbumin (OVA). A differential scanning microcalorimetry (DSC) experimental series conducted in the phosphate buffer solutions containing 0–30% of additives revealed the destabilizing impact of both urea and DMSO in a mild acidic media, manifested in the gradual decrease of thermal unfolding enthalpy and transition temperature. These findings differ from the results observed in our study of the mild alkaline DMSO buffered solutions of OVA, where the moderate stabilization of OVA was observed in presence of 5–10% of DMSO. However, the overall OVA interaction patterns with urea and DMSO are consistent with our previous findings on the stability and conformational flexibility of another model globular protein, α-chymotrypsin, in similar medium conditions. The obtained results could be explained by preferential solvation patterns. Positive preferential solvation of protein by urea in urea/water mixtures mainly weakens the hydrophobic interactions of the protein globule and eventually leads to the disruption of the tertiary structure within the whole range of urea concentrations. Alternatively, under certain experimental conditions in DMSO/water mixtures, positive preferential solvation by water molecules can be observed. We assume that the switch to the positive preferential solvation by DMSO, which is shown to have a soft maximum around 20–30% DMSO, could be shifted towards lower additive concentrations due to the intrinsic capability of ovalbumin OVA to convert into a heat-stable, yet flexible set of conformations that have increased the surface hydrophobicity, characteristic to molten-globule-like states.</p> </abstract>

On the structure and stability of novel cationic DPPC liposomes doped with gemini surfactants

A novel formulation of cationic liposomes was studied by mixing dipalmitoylphosphatidylcholine (DPPC) with tetradecyltrimethylammonium bromide gemini surfactants with different alkane spacer groups lengths attached to their ammonium head-groups. The physicochemical characterization of the cationic liposomes was obtained by combining experimental results from differential scanning microcalorimetry (DSC) with molecular dynamic simulations, in order to understand their structural configuration. An adapted Ising model was used to interpret the results in terms of cooperativity of the phase transitions.The gemini surfactants partition into the lipid bilayer of DPPC liposomes, and the induced changes in colloidal stability and phase transition were analyzed in detail. The DPPC liposomes became positively charged upon gemini surfactant partition, showing increased colloidal stability. Our results show significant differences in structural configuration between gemini surfactants with short and long spacer lengths. While gemini with shorter spacers allocate within the lipid bilayer with both headgroups in the same layer, geminis with longer spacers unexpectedly intercalate in the lipid membrane in a particular zig-zag configuration, with each headgroup located at a different side of the bilayer, altering the coupling degree parameters of the membrane’s phase transition.The extraordinary increase of colloidal stability of DPPC liposomes with gemini surfactants at very low molar ratio and the possibility to tune the physicochemical properties of the membrane by control de spacer length of the geminis opens new possibilities for cationic liposomal formulations with potential applications in vaccines, drug/gene delivery or biosensing.

Bovine hemoglobin thermal stability in the presence of naringenin: Calorimetric, spectroscopic and molecular modeling studies

Calorimetric, spectroscopic and dynamic light scattering experiments, together with complementary computational investigations afforded a detailed analysis of the thermal stability and conformation of the bovine hemoglobin (BHb)–naringenin (NAR) complex. Differential scanning microcalorimetry (μDSC) revealed complex, multi-step aggregation processes above 330 K. There is a definite enhancement and alteration of the thermal signature of protein aggregation with increasing NAR concentration. Circular dichroism (CD) spectroscopy indicated minor conformational changes in the protein secondary structure upon ligand binding at 298 K. Temperature-dependent CD measurements evidenced steep variations of the α-helix (decrease) and unordered structures (increase), with only a slight increase of β-structure as a result of unfolding and aggregation. UV–Vis absorbance and DLS measurements pointed to large size aggregates formation in the presence of ligand in high concentration.Molecular docking data identified the binding site of NAR, located in the internal cavity of BHb. Molecular dynamics simulations indicated hydrogen bonds, electrostatic and hydrophobic interactions as main contributions in the binding process; the bound ligand produced no sizable structural alterations of the protein molecule. Therefore, the clearly evidenced (μDSC, CD, DLS) influence of the flavonoid on hemoglobin thermal aggregation is exerted as a free (not bound) component of the studied systems.

Interactions in dopamine and indole loaded thermosensitive hydrogels seen by high sensitivity microDSC. Implications for drug delivery

Controlled and targeted drug delivery systems consist of a carrier matrix and one or more active ingredients. One of the roles of the matrix is to regulate the release of the drug. Uptake, release, differential scanning microcalorimetry (DSC) and powder X-ray diffraction (XRD) techniques were used to reveal the interactions governing the release of dopamine and indole from a thermoresponsive model polymer gel. The dopamine can be completely recovered from the loaded polymer matrix. The self-assembling affinity of the dopamine molecules reduces their interaction with the polymer, and the water molecules can form a uniform protecting water sheath. Thus neither the kinetics nor the temperature of the phase transition of the carrier matrix are influenced by the dopamine. The DSC results imply the formation of polymer and dopamine-rich domains above the phase transition. Indole, on the other hand, readily substitutes for the water molecules through the interaction between the C=O sites of the polymer and the NH groups of the drug. The loss of the protecting hydrophobic water and the decelerated fluctuation of the indole decorated polymer chains result in a much slower phase transition and a depleted phase transition temperature. The interaction between the carrier matrix and the indole results in a uniform distribution of the drug and after drying the indole is found in amorphous form. Dopamine, on the contrary, forms crystalline regions.

Environmentally Friendly Method of Assembly of Cardanol and Cholesterol into Nanostructures Using a Continuous Flow Microfluidic Device

This study shows a viable and straightforward microfluidic method of assembly of cardanol (CA) and cholesterol (CH) into amphiphile nanostructures obtained through a hydrodynamic focusing approach according to which an alcoholic solution of CA and CH is constrained within a two-dimensional lamina shape by two lateral streams of borate buffer solution. The process is performed within glass-made cross-shaped micro-sized fluidic chips specially designed to achieve a laminar regime. CA, distilled from the cashew nut shell liquid, is demonstrated as a surface-active molecule in borate buffer basic medium and when mixed with CH it produces versatile nanovesicles through an in-batch solvent-free process. Compared to this conventional method, the microfluidic route allows operating under continuous flows, with a reduced amount of reagents and at lower experimental temperatures, ensuring no waste formation and the achievement of size-monodisperse amphiphile nanostructures that do not need further steps of purification. Electron microscopy analyses demonstrate that upon increasing CH in the lipid mixture, a switchover from spherical CA micelles to CA/CH mixed closed vesicles occurs. Differential scanning microcalorimetry confirms the formation of vesicular structures and evidences the primary role of CH, which increasingly lowers the temperature of transition depending on its concentration.

DMSO and TMAO-Differences in Interactions in Aqueous Solutions of the K-Peptide.

Interactions between a solvent and their co-solute molecules in solutions of peptides are crucial for their stability and structure. The K-peptide is a synthetic fragment of a larger hen egg white lysozyme protein that is believed to be able to aggregate into amyloid structures. In this study, a complex experimental and theoretical approach is applied to study systems comprising the peptide, water, and two co-solutes: trimethylamide N-oxide (TMAO) or dimethyl sulfoxide (DMSO). Information about their interactions in solutions and on the stability of the K-peptide was obtained by FTIR spectroscopy and differential scanning microcalorimetry. The IR spectra of various osmolyte–water–model-peptide complexes were simulated with the DFT method (B3LYP/6-311++G(d,p)). The FTIR results indicate that both solutes are neutral for the K-peptide in solution. Both co-solutes affect the peptide to different degrees, as seen in the shape of its amide I band, and have different influences on its thermal stability. DFT calculations helped simplify the experimental data for easier interpretation.

Model Systems for Evidencing the Mediator Role of Riboflavin in the UVA Cross-Linking Treatment of Keratoconus.

Riboflavin under UVA radiation generates reactive oxygen species (ROS) that can induce various changes in biological systems. Under controlled conditions, these processes can be used in some treatments for ocular or dermal diseases. For instance, corneal cross-linking (CXL) treatment of keratoconus involves UVA irradiation combined with riboflavin aiming to induce the formation of new collagen fibrils in cornea. To reduce the damaging effect of ROS formed in the presence of riboflavin and UVA, the CXL treatment is performed with the addition of polysaccharides (dextran). Hyaluronic acid is a polysaccharide that can be found in the aqueous layer of the tear film. In many cases, keratoconus patients also present dry eye syndrome that can be reduced by the application of topical solutions containing hyaluronic acid. This study presents physico-chemical evidence on the effect of riboflavin on collagen fibril formation revealed by the following methods: differential scanning microcalorimetry, rheology, and STEM images. The collagen used was extracted from calf skin that contains type I collagen similar to that found in the eye. Spin trapping experiments on collagen/hyaluronic acid/riboflavin solutions evidenced the formation of ROS species by electron paramagnetic resonance measurements.

Rational design of nanocarriers based on gellan gum/retrograded starch exploiting polyelectrolyte complexation and ionic cross-linking processes: A potential technological platform for oral delivery of bevacizumab

The structural fragility of monoclonal antibodies (mAbs), such as bevacizumab (BVZ), is a critical parameter for oral administration and can limit the use of several technologies to produce oral nanocarriers for these biomolecules. Nanoparticles (NPs) based on gellan gum (GG) and retrograded starch (RS) were rationally designed through polyelectrolyte complexation, and ionic cross-linking was exploited as an additional technological strategy to modulate the properties of nanocarriers. According to static light scattering analysis, the molecular weights of GG and RS were approximately 158 kDa and 1803 kDa, respectively. The influence of pH on the zeta potential (ZP) of polymers allowed the selection of pH 6.2 as the most suitable pH for the complexation of these polyelectrolytes. Non-cross-linked NPs were prepared at different polymer:drug ratios and the effects of formulation variables (polyelectrolyte ratio, drug and cross-linker concentrations, and polymer:drug ratio) on the formation and properties (size, ZP, and PDI) of cross-linked NPs were evaluated using a 33 full-factorial design. The average size of non-cross-linked and cross-linked NPs ranged from 260.1 - 299.6 nm to 265.7–629.9 nm, respectively. NPs-negative ZP (>- 20 mV) and high association efficiency (AE%) (>56.16%) were achieved. Cross-linking significantly increased the BVZ AE% (85%–100%). Analyses by attenuated total reflectance-Fourier transform infrared, fluorescence, circular dichroism, and differential scanning microcalorimetry techniques demonstrated that polyelectrolyte complexation and ionic cross-linking did not denature the secondary and tertiary structures of BVZ. The results revealed the suitability of the technological approaches used to produce BVZ-loaded nanocarriers with tailored properties, representing a potential platform for the oral delivery of BVZ.

Is the Membrane Lipid Matrix a Key Target for Action of Pharmacologically Active Plant Saponins?

This study was focused on the molecular mechanisms of action of saponins and related compounds (sapogenins and alkaloids) on model lipid membranes. Steroids and triterpenes were tested. A systematic analysis of the effects of these chemicals on the physicochemical properties of the lipid bilayers and on the formation and functionality of the reconstituted ion channels induced by antimicrobial agents was performed. It was found that digitonin, tribulosin, and dioscin substantially reduced the boundary potential of the phosphatidylcholine membranes. We concluded that saponins might affect the membrane boundary potential by restructuring the membrane hydration layer. Moreover, an increase in the conductance and lifetime of gramicidin A channels in the presence of tribulosin was due to an alteration in the membrane dipole potential. Differential scanning microcalorimetry data indicated the key role of the sapogenin core structure (steroid or triterpenic) in affecting lipid melting and disordering. We showed that an alteration in pore forming activity of syringomycin E by dioscin might be due to amendments in the lipid packing. We also found that the ability of saponins to disengage the fluorescent marker calcein from lipid vesicles might be also determined by their ability to induce a positive curvature stress.

Heat denaturation of ribonuclease under high pressure: Differential scanning microcalorimetry study

Scanning microcalorimetry was used to study heat denaturation of ribonuclease A at high pressure. The pressure dependences of temperature and enthalpy of denaturation were measured at two levels of pH. It was shown that pressure destabilizes the RNase structure and significantly reduces the enthalpy of denaturation. Pressure does not affect the cooperativity of transition. The results allowed estimating of the partial increment of volume ΔVm as well as increments of isothermal compressibility and thermal expansibility associated with denaturation. Increasing pressure increases the magnitude of the volume increment ΔVm. The second-order pressure derivatives of ΔVm must be taken into account to retrieve dependencies between temperature / enthalpy of denaturation vs. pressure. Proportional change of ΔVm and its partial derivatives with respect to pressure and temperature provides a basis for approximation of experimental data. The nature of the volume increment decrease accompanying the denaturation of proteins and the driving mechanism of the change of denaturation enthalpy with increasing pressure are discussed.

Determination of SADT and TMRad of 3-bromo-1-(3,5-dichloropyridin-2-yl)-4,5-dihydro-1H-pyrazole-5-carboxylic acid: Applying thermal decomposition kinetics

The aim of this study was the evaluation of the safety parameters: Time to Maximum Rate under adiabatic conditions (TMRad) and self-accelerating decomposition temperature (SADT) for 3-bromo-1-(3,5-dichloropyridin-2-yl)-4,5-dihydro-1H-pyrazole −5-carboxylic acid (BDPCA) using evaluated kinetic parameters. The required decomposition kinetics was determined from the results of differential scanning calorimetry (DSC), microcalorimetry (C600), and adiabatic accelerating calorimeter (ARC). AKTS Thermokinetics (TK) and Thermal Safety (TS) Software were used for evaluation of the kinetic parameters of a decomposition reaction. The kinetic analysis was based on the isoconversional Friedman method. The values of TMRad amount to 110.2 °C, 107.3 °C, and 104.9 °C when calculated from the data collected by DSC, C600, and ARC, respectively. The simulated values of SADT for a 50 kg package of BDPCA amount to 101 °C and 96 °C when elaborating the DSC and C600 signals, respectively. Obtained results indicate that the application of kinetic parameters evaluated from the experiments carried out by three different calorimetric devices resulted in the evaluation of consistent safety parameters.