Chemical Thermodynamics

Chemical Thermodynamics discusses the fundamental laws of thermodynamics along with their relationships to heat, work, enthalpy, entropy, and temperature. Predicting the direction of a spontaneous change and calculating the change in entropy of a reaction are core concepts. The relationship between entropy, free energy and work is covered. The Gibbs free energy is used quantitatively to predict if reactions or processes are going to be exothermic and spontaneous or endothermic under the stated conditions. Also explored are the enthalpy and entropy changes during a phase change. Finally the Gibbs free energy of a chemical reaction is related to its equilibrium constant and the temperature.

Weak Interactions of the Isomers of Phototrexate and Two Cavitand Derivatives

The interactions of two conformers of newly synthesized photoswitchable azobenzene analogue of methotrexate, called Phototrexate, with two cavitand derivatives, have been investigated in dimethyl sulfoxide medium. Photoluminescence methods have been applied to determine the complex stabilities and the related enthalpy and entropy changes associated to the complex formation around room temperature. Results show opposite temperature dependence of complex stabilities. The structure of the upper rims of the host molecules and the reordered solvent structure were identified as the background of the opposite tendencies of temperature dependence at molecular level. These results can support the therapeutic application of the photoswitchable phototrexate, because the formation of inclusion complexes is a promising method to regulate the pharmacokinetics of drug molecules.

Selective Binding of Cyclodextrins with Leflunomide and Its Pharmacologically Active Metabolite Teriflunomide

The selectivity of encapsulation of leflunomide and teriflunomide by native α-, β- and γ-cyclodextrins was investigated through 1H NMR and molecular modeling. Thermodynamic analysis revealed the main driving forces involved in the binding. For α-cyclodextrin, the partial encapsulation was obtained while deep penetration was characterized for the other two cyclodextrins, where the remaining polar fragment of the molecule is located outside the macrocyclic cavity. The interactions via hydrogen bonding are responsible for high negative enthalpy and entropy changes accompanying the complexation of cyclodextrins with teriflunomide. These results were in agreement with the molecular modeling calculations, which provide a clearer picture of the involved interactions at the atomic level.

Poly(N,N′-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N′-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N′-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van’t Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

On the Role of Entropy in the Stabilization of α-Helixes

Protein folding evolves by exploring the conformational space with a subtle balance between enthalpy and entropy changes which eventually leads to a decrease of the free energy upon reaching the folded structure. <br>A complete understanding of this process requires, therefore, a deep insight into both contributions to the free energy.<br>In this work, we clarify the role of entropy in favoring the stabilization of folded structures in polyalanine peptides with up to 12 residues . We use a novel method referred to as K2V that allows us to obtain the potential energy landscapes in terms of residue conformations extracted from molecular dynamics simulations at conformational equilibrium, and yields folding thermodynamic magnitudes in agreement with the experimental data available. <br>Our results demonstrate that the folded structures of the larger polyalanine chains are stabilized with respect to the folded structures of the shorter chains mostly by an increase of the entropic contribution of the solvent, which compensates the decrease of conformational entropy of the polypeptide, thus unveiling a key piece in the puzzle of protein folding.<br>In addition, the ability of the K2V method to provide the enthalpic and entropic contributions for individual residues along the peptide chain makes it clear that the entropic stabilization is basically governed by the nearest neighbor residues conformations, with the folding propensity being rationalized in terms of triads of residues.<br><br>

On the Role of Entropy in the Stabilization of α-Helixes

Protein folding evolves by exploring the conformational space with a subtle balance between enthalpy and entropy changes which eventually leads to a decrease of the free energy upon reaching the folded structure. <br>A complete understanding of this process requires, therefore, a deep insight into both contributions to the free energy.<br>In this work, we clarify the role of entropy in favoring the stabilization of folded structures in polyalanine peptides with up to 12 residues . We use a novel method referred to as K2V that allows us to obtain the potential energy landscapes in terms of residue conformations extracted from molecular dynamics simulations at conformational equilibrium, and yields folding thermodynamic magnitudes in agreement with the experimental data available. <br>Our results demonstrate that the folded structures of the larger polyalanine chains are stabilized with respect to the folded structures of the shorter chains mostly by an increase of the entropic contribution of the solvent, which compensates the decrease of conformational entropy of the polypeptide, thus unveiling a key piece in the puzzle of protein folding.<br>In addition, the ability of the K2V method to provide the enthalpic and entropic contributions for individual residues along the peptide chain makes it clear that the entropic stabilization is basically governed by the nearest neighbor residues conformations, with the folding propensity being rationalized in terms of triads of residues.<br><br>

Kinetic models of the extraction of vanillic acid from pumpkin seeds

Vanillic acid is used in the food industry and perfumery, and the optimization of its extraction process from the natural source is important for saving time and money. The presence of vanillic acid in pumpkin seeds was proven using HPLC analysis. Computational optimization of the extraction shows that for the concentration of ethanol 40% and solmodul: V/m=20, the optimum condition for the extraction of vanillic acid from pumpkin seeds was 100 min and 450C. The estimation of fitting for each kinetic model to the experimental kinetic data was performed using the root mean square, standard deviation, and the correlation coefficient. Ponomarev model was shown as the most suitable with the highest accuracy among the six considered kinetic models. The enthalpy and entropy changes were positive, while the Gibbs free energy was negative and decreased when temperature increased during the thermodynamic analysis. Therefore, the extraction of vanillic acid from pumpkin seeds was endothermic, spontaneous, and irreversible.

Supramolecular Interaction Between Cucurbit[8]uril and the Quinolone Antibiotic Ofloxacin

The host–guest inclusion complex of cucurbit[8]uril (Q[8]) and ofloxacin (OFLX) has been prepared and characterised by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and UV-vis absorption spectroscopy. The findings demonstrated that a host–guest inclusion complex could be formed through an encapsulation of the methylmorpholine and piperazine rings in OFLX. ITC results indicated that the formation of this inclusion complex (1:1 molar ratio) was primarily dependent on enthalpy and entropy changes. In addition, the release of OFLX from the inclusion complex was increased under acidic conditions.