Proton affinity of proline and modified prolines using the kinetic method: role of the conformation investigated by ab initio calculations

The proton affinities of proline, cis-3-methylproline and cis-3-ethylproline have been measured by the kinetic method using an ion trap instrument; the values obtained are 936, 940.5, and 943 kJ mol(-1), respectively. The experimental values are consistent with those obtained by high-level ab initio calculations (B3LYP/6-31+G*//B3LYP/6-31G* and B3P86/6-31+G*//B3LYP/6-31G*). Several conformations of neutral and protonated proline were considered, in particular the endo and exo ring structure and the position of the carboxyl group. These results show the importance of the position of the hydrogen atom of the carboxyl group in determining the most stable protonated proline structure.     

Polymer modifiers for bituminous waterproofing materials

Issues of improving the quality of waterproofing bituminous materials by modifying them using chlorosulfonated polyethylene and epoxy oligomer are considered. The presented results confirm that it is possible to retain the stability of the properties of a modified bituminous binder in the process of its operation and increase the working-temperature range.     

Optimization of STEM-HAADF Electron Tomography Reconstructions by Parameter Selection in Compressed Sensing Total Variation Minimization-Based Algorithms

A novel procedure to optimize the 3D morphological characterization of nanomaterials by means of high angle annular dark field scanning-transmission electron tomography is reported and is successfully applied to the analysis of a metal- and halogen-free ordered mesoporous carbon material. The new method is based on a selection of the two parameters (μ and β) which are key in the reconstruction of tomographic series by means of total variation minimization (TVM). The parameter-selected TVM reconstructions obtained using this approach clearly reveal the porous structure of the carbon-based material as consisting of a network of parallel, straight channels of ≈6 nm diameter ordered in a honeycomb-type arrangement. Such an unusual structure cannot be retrieved from a TVM 3D reconstruction using default reconstruction values. Moreover, segmentation and further quantification of the optimized 3D tomographic reconstruction provide values for different textural parameters, such as pore size distribution and specific pore volume that match very closely with those determined by macroscopic physisorption techniques. The approach developed can be extended to other reconstruction models in which the final result is influenced by parameter choice.     

3D-QSAR Study on Diindolylmethane and Its Analogues with Comparative Molecular Field Analysis (CoMFA)

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Proton affinity ladder for uridine and analogs: influence of the hydroxyl group on the sugar ring conformation

A ladder of relative proton affinities (PA) for a series of modified uridines (e.g. araU, ddU, 5BrU, 5BrdU and 5IU) was established from competitive dissociations of proton-bound heterodimers using Cooks and co-workers' kinetic method. The studied heterodimers are constituted of a modified nucleoside and either an amino acid or a nucleoside with known PA value. These non-covalent heterodimers were prepared under electrospray conditions to be selected and dissociated into the ion-trap analyzer. These results allowed our PA ladder of uridine and deoxyuridine analogs substituted at the C-5 position in the uracil ring to be extended. From this scale, it was showed that the substitution of hydrogen atom at the C-2' position in the sugar ring by a hydroxyl group involves a decrease of about 7 kJ mol(-1). The experimental values for U, 5MeU, dU, 5MedU, ddU and araU are consistent with those obtained by DFT calculations (B3P86/6-31+G//B3LYP/6-31G(.)). Several neutral and protonated conformations of these compounds were considered, in particular the ring conformation of furanose and the orientation of the base with respect to the sugar ring. These calculated results showed the influence of sugar substituent on the conformation of the neutral form of theses nucleosides. However, the most stable protonated structure is the same for all the studied nucleosides except for araU, where the position of the anti 2'-OH group imposes a specific conformation.     

Host-guest inclusion complex between β-cyclodextrin and paeonol: A theoretical approach

Host-guest interactions of β-cyclodextrin (β-CD) with paeonol (PAE) were simulated using semi-empirical PM3 and both ONIOM2 [(B3LYP/6-31G*:PM3), (HF/6-31G*:PM3)] methods. The results obtained with PM3 method clearly indicate that the complexes formed are energetically favored with or without solvent, the model 1 (PAE entering into the cavity of β-CD from its wide side by OCH3 group) is found more favored than the model 2 (PAE entering into the cavity of β-CD from its wide side by COCH3 group). Finally, natural bonding orbital (NBO) analysis was performed based on ONIOM2 optimized complexes to quantify the donor–acceptor interactions between PAE and β-CD.     

Effect of UV radiation and hydrogen peroxide on the antiradical and antioxidant activities of DOPA-melanin and melanosomes from retinal pigment epithelial cells

The effect of continuous UV radiation and hydrogen peroxide on destruction and antioxidant properties of synthetic DOPA-melanin (prepared by oxidation of 3,4-dihydroxyphenylalanine (DOPA)) and melanosomes isolated from cells of the retinal pigment epithelium (RPE) was investigated. The kinetics of melanin destruction was recorded based on the accumulation of fluorescent low-molecular-weight reaction products, the antiradical activity of melanin was determined by chemiluminescence method, the concentration of free radical products was measured by electron paramagnetic resonance, and the antioxidant activity of melanins was estimated by their inhibitory effect on lipid peroxidation. It was shown that UVC—UVA irradiation (up to 5 hours) of DOPA-melanin and melanosomes of retinal pigment epithelium decreased neither the latency period of luminol chemiluminescence nor the inhibitory action of pigments on Fe²⁺- and UV-induced peroxidation of cardiolipin liposomes. However, very long UV irradiation gave rise to fluorescent destruction products, decreased the concentration of paramagnetic centers in the pigment (especially light-dependent ones), and decreased the antiradical and antioxidant activities. For example, UV irradiation of DOPA-melanin during 52 h resulted in approximately a 2-fold decrease in the concentration of paramagnetic centers and decline of antiradical and antioxidant activities. However, even with such a hard irradiation the pigment retained significant inhibitory activity against lipid peroxidation. The oxidative destruction of DOPA-melanin in the presence of hydrogen peroxide in the dark resulted in complete destruction of the polymer and loss of its protective properties. It is assumed that destruction of RPE cell melanin is caused mainly by oxidative processes.     

Improved proton affinity measurements for proline and modified prolines using triple quadrupole and ion trap mass spectrometers

Proton affinity (PA) of compounds such as proline, cis-3-methylproline, cis-3-ethylproline, cis-3-isopropylproline and cis-3-isopentanylproline was determined by kinetic method with amines as the reference bases. The effective temperatures determined using ion trap and triple quadrupole mass spectrometers were found to be significantly different. In the case of the triple quadrupole instruments, the effective temperature depends significantly on the collision energy. The influence of the apparent basicity (GBapp) on the effective temperature may be used to estimate the difference in protonation entropy (DeltaDeltaS degrees) between the sample and reference compounds. In case of the ion trap mass spectrometer, the variation of the effective temperature as a function of the excitation amplitude is small, so it is difficult to account for the contribution of the entropy effects to the proton affinity value. A better estimation of the PA and DeltaDeltaS degrees values for the investigated molecules is obtained by combining the GBapp and Teff data pairs that are obtained from both the mass spectrometers.     

Proton affinity of diastereoisomers of modified prolines using the kinetic method and density functional theory calculations: role of the cis/trans substituent on the endo/exo ring conformation

The proton affinity (PA) of cis/trans-3-prolinoleucines and cis/trans-3-prolinoglutamic acids have been studied by the kinetic method and density functional theory (DFT) calculations. Several conformations of the neutral and the protonated modified prolines, in particular the endo and exo ring conformations, were analyzed with respect to their contribution to the PA values. When the substituent is an alkyl, both the diastereoisomers have the same PA value. However, the PA values for the diastereoisomers are different when the substituted chain contains functional groups (e.g. a carboxyl group). This variation in PA values could be attributed to the existence of intramolecular hydrogen bonds.     

Protease-catalyzed semisynthesis of human neuropeptide Y

Human neuropeptide Y was semisynthesized by enzymatic condensation of des-Tyr36-NH2 human neuropeptide Y and H-Tyr-NH2 using Pseudomonas aeruginosa elastase, a metalloenzyme possessing a hydrolytic specificity for the imino side of hydrophobic amino acids. The optimum pH for this enzymatic synthesis was judged to be around 7 in a high concentration of an organic solvent.