Radiation- and Photo-Induced Oxidation Pathways of Methionine in Model Peptide Backbone under Anoxic Conditions

Within the reactive oxygen species (ROS) generated by cellular metabolisms, hydroxyl radicals (HO^•) play an important role, being the most aggressive towards biomolecules. The reactions of HO^• with methionine residues (Met) in peptides and proteins have been intensively studied, but some fundamental aspects remain unsolved. In the present study we examined the biomimetic model made of Ac-Met-OMe, as the simplest model peptide backbone, and of HO^• generated by ionizing radiation in aqueous solutions under anoxic conditions. We performed the identification and quantification of transient species by pulse radiolysis and of final products by LC-MS and high-resolution MS/MS after γ-radiolysis. By parallel photochemical experiments, using 3-carboxybenzophenone (CB) triplet with the model peptide, we compared the outcomes in terms of short-lived intermediates and stable product identification. The result is a detailed mechanistic scheme of Met oxidation by HO^•, and by CB triplets allowed for assigning transient species to the pathways of products formation.     

Effect of the unsaturated acyl position in triglycerides on the Hydrogenation rate

Linseed and cod liver oil triglycerides as well as the randomly rearranged triglycerides of soybean and rapeseed oils were subjected to partial hydrogenation in the presence of nickel or platinum catalyst. The hydrogenated triglycerides were subjected to enzymatic hydrolysis with pancreatic lipase in order to determine the changes of their structure. It follows from the data obtained that the position of an unsaturated acyl in a triglyceride affects its hydrogenation rate.     

STABILITY OF COAL - AIR BUBBLE - WATER SYSTEM

Measurements of the detachment force of an air bubble from the surface of various coal ranks were carried out in water. These values were compared with those calculated from the contact angle values measured in coal - air bubble - water system and from the values (from literature) of the dispersion and the nondispersion components of the surface free energy of the studied coals. On the basis of the obtained results we found that there is a good agreement among the measured and calculated values of the detachment force for given coal rank. The coals studied may be divided into three groups according to their ability to form the stable attachment with air bubble. The arithmetic weighted mean of the detachment forces were correlated with the natural flotability of coals and the linear relationship as obtained.     

暴雨荷载下平台布局、上部重量和结构可靠性的关系

通过对暴雨荷载下，西北海湾中简易护套、顶托、布局等坍塌行为的对比，研究了平台布置与结构可靠性的关系。采用两种不同性能分析的方法确定极限强度，对确定的结构，给出了截然不同的RSR值。评估了上部重量变化对结构可靠性的影响，结果发现可靠性与结构的布局有关，这种影响在现行的指南中没有提及。研究也显示在不同的平台布局以及相同环境下，同一组固定荷载和环境荷载因素具有不同的可靠性。而且，在某些情况下，采用工作应力设计方法比荷载一抗力设计系数法可以得到更一致的可靠性结果。     

Effect of concentration of some nickel catalyst poisons in oils on the course of hydrogenation

Hydrogenation has been carried out in a“dead-end” type reactor with various types of nickel catalyst and with automatic recording of hydrogen absorption. Oils with varying concentrations of sulfur and phosporus compounds, fatty acids and their salts, as well as partly oxidized oil have been hydrogenated. The resulting kinetic curves are characteristic for each catalyst type and for each inhibitor. An increase in the poison concentration in the reaction system results in lengthening of the induction period apart from the decrease of catalyst activity.     

INFLUENCE OF CHEMICAL COMPOSITION OF ENAMEL ON A FEW OF ITS PROPERTIES

To enlarge the knowledge and data of the relation between the chemical composition of enamel and its properties, many enamels were melted and their acid resistance, viscosity, and opacity were measured. The results have been assembled in graphs representing measurements of these properties in relation to the systematic change of the composition. On the basis of the results, a revision of information is offered relating to the acid resistance of glass and enamel.     

Adhesion of an Air Bubble to Quartz Surface in Aqueous Solution of Aliphatic Amine Hydrochloride

Measurements of the force of air bubble detachment from a quartz surface in aqueous solutions of decylamine hydrochloride (DAC1), dodecylamine hydrochloride (DDAC1) and tetradecylamine hydrochloride (TDAC1) were made in the concentration range from 0 to 200mg/dm³. At the same time, the instantaneous contact angle for the system quartz-air bubble-aqueous solution of aliphatic amine hydrochloride was photographed at the moment of detachment. Wetting contact angles for the system quartz-air bubble-aqueous solution of DDAC1 were also measured. On the basis of the measured values of the instantaneous angle and surface tension of the solutions studied, the changes of polar components of surface free energy of quartz/monolayer water film under the influence of adsorption of molecules of aliphatic amine hydrochloride (RAC1) from aqueous solution were calculated and compared with the literature data. Using these data, the force of air bubble detachment from the quartz surface in the presence of RAC1 was also calculated and compared with the measured force, obtaining a good agreement. From the studies and calculations it appears that a little addition of RAC1 to water considerably changes the stability of the system quartz-air bubble-solution, and that this change is first of all caused by reduction of a polar component of the surface free energy of quartz/monolayer water film as a result of adsorption of RAC1 molecules in the negative places of such a surface. As a result of RAC1 adsorption this surface becomes similar to that of paraffin.     

Nonenzymatic browning reactions of retro-aldol degradation products of carbohydrates

 Methylglyoxal and glyceraldehyde were used to demonstrate the involvement of retro-aldol degradation products of carbohydrates in browning (A ₄₂₀), the formation of hydroxymethylfurfural (HMF), and the extent to which the mechanism of browning proceeds by homolytic or heterolytic pathways in the caramelization and Maillard reactions of mono- and disaccharides. The amino component of Maillard systems is required for the retro-aldol cleavage of D-glucose. The short-chain α-dicarbonyl compounds so formed appear to dominate not only the formation of brown products but also the mechanism of the browning of D-glucose under these conditions, even if only small amounts are formed. Evidence is presented to link the formation of free radicals in the browning of glucose to reactions of the retro-aldol products. A novel reaction pathway is suggested for the formation of HMF from glyceraldehyde. Received: 9 October 1998 / Revised version: 15 December 1998     

Evaluation of Hydroxyl Radical Reactivity by Thioether Group Proximity in Model Peptide Backbone: Methionine versus S-Methyl-Cysteine

Hydroxyl radicals (HO^•) have long been regarded as a major source of cellular damage. The reaction of HO^• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO^• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH₃C(O)NHCHXC(O)NHCH₃, where X = CH₂CH₂SCH₃ or CH₂SCH₃, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC–MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves transient species of five-membered and/or six-membered ring formations with different heteroatoms present in the backbone as well as quite different rates of deprotonation in forming α-(alkylthio)alkyl radicals.