抗坏血酸和尿酸在甘氨酸-壳聚糖修饰玻碳电极上的电化学行为及测定

制备了甘氨酸-壳聚糖复合膜修饰玻碳电极(Gly-CTS/GCE),研究了抗坏血酸(AA)和尿酸(UA)在该修饰电极上的电化学行为。结果表明在pH=5.59的磷酸盐缓冲溶液中,AA、UA在Gly-CTS/GCE上均产生灵敏的不可逆氧化峰,其峰电流与浓度在一定范围内呈良好的线性关系。对AA和UA混合溶液平行测定7次,相对标准偏差分别为4.6%、2.9%,表明该电极重现性和稳定性良好。AA、UA在Gly-CTS/GCE电极上的氧化峰峰电位相差340mV,据此可实现对二者的同时检测,并可应用于实际样品测定。     

Density functional theory and RRKM calculations of decompositions of the metastable E‐2,4‐pentadienal molecular ions

The potential energy profiles for the fragmentations that lead to [C₅H₅O]⁺ and [C₄H₆]^+? ions from the molecular ions [C₅H₆O]^+? of E‐2,4‐pentadienal were obtained from calculations at the UB3LYP/6‐311G + + (3df,3pd)//UB3LYP/6‐31G(d,p) level of theory. Kinetic barriers and harmonic frequencies obtained by the density functional method were then employed in Rice–Ramsperger–Kassel–Marcus calculations of individual rate coefficients for a large number of reaction steps. The pre‐equilibrium and rate‐controlling step approximations were applied to different regions of the complex potential energy surface, allowing the overall rate of decomposition to be calculated and discriminated between three rival pathways: C? H bond cleavage, decarbonylation and cyclization. These processes should have to compete for an equilibrated mixture of four conformers of the E‐2,4‐pentadienal ions. The direct dissociation, however, can only become important in the high‐energy regime. In contrast, loss of CO and cyclization are observable processes in the metastable kinetic window. The former involves a slow 1,2‐hydrogen shift from the carbonyl group that is immediately followed by the formation of an ion‐neutral complex which, in turn, decomposes rapidly to the s‐trans‐1,3‐butadiene ion [C₄H₆]^+?. The predominating metastable channel is the second one, that is, a multi‐step ring closure which starts with a rate‐limiting cis—trans isomerization. This process yields a mixture of interconverting pyran ions that dissociates to the pyrylium ions [C₅H₅O]⁺. These results can be used to rationalize the CID mass spectrum of E‐2,4‐pentadienal in a low‐energy regime. Copyright © 2010 John Wiley & Sons, Ltd.     

Solving second-order differential equations with variable coefficients

A method is developed in which an analytical solution is obtained for certain classes of second-order differential equations with variable coefficients. By the use of transformations and by repeated iterated integration, a desired solution is obtained. This alternative method represents a different way to acquire a solution from classic power series techniques and other approaches. It is, at times, more involved than traditional methods.     

Effect of diluents on poly(vinyl acetate) dynamics

The effect of diluents and temperature on segmental motion in poly(vinyl acetate) was investigated by both NMR and ESR spectroscopy. Three classes of diluents were studied: chloroform, a thermodynamically good solvent; water, a poor solvent which slightly swells the polymer and lowers its glass transition temperature; and decane, a nonsolvent so poor it does not appear to swell the polymer nor lower the calorimetric glass transition temperature. At all temperatures investigated each type of diluent increased the segmental motion of the polymer over that of the bulk sample. Under the conditions studied, ¹³C and ²H NMR and nitroxide spin-label ESR data gave similar views of segmental motion of the polymer, indicating that in this amorphous polymer the segmental motion of the polymer may be safely inferred from spin-label data.     

氢原子在镍台阶面上吸附、振动和表面扩散行为的理论研究

本文用对势方法研究了氢原子在Ni(510)台阶面上的吸附和振动, 计算结果与实验符合得很好。并考察了氢原子在Ni(997)台阶面上的吸附和扩散, 结果表明,台阶对下台面上扩散的氢原子开成捕获势阱, 对上台面扩散的氢原子形成反射势,这也很好地支持了实验结果。     

Mechanical and electrical properties of nanostructured ‘plastic metals’

Deformable, supraspherical (SS) aggregates of metal nanoparticles (NPs) connected by long-chain dithiol ligands self-assemble into nanostructured materials of macroscopic dimensions. These materials are both malleable/moldable at room temperature as well as electrically conductive. Upon gentle heating, they harden into polycrystalline metals. The paper discusses qualitative models that rationalize the mechanical and electrical properties of these unusual ‘plastic metals’.     

Development of a fast liquid chromatography/mass spectrometry screening method for angiotensin‐converting enzyme (ACE) inhibitors in complex natural mixtures like snake venom

A new robust high‐performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI‐MS)‐based screening method for angiotensin‐converting enzyme (ACE)‐inhibiting substances in crude samples is described. The ACE assay is carried out in a typical offline setup by incubation of the samples with ACE and angiotensin I (AI), followed by stopping the reaction with acetonitrile containing val⁵‐AI serving as internal standard (I.S.). AI and the product angiotensin II (AII) are extracted from the incubation mixture by turbulent‐flow chromatography (TFC) applied in backflush mode as online solid‐phase extraction and are directly quantified by ESI(+)‐MS. The presence of ACE inhibitors (ACEi) is detected by an increase in AI signal intensity and a corresponding decrease of AII signal, as compared to the blank assay. The overall time of analysis of the TFC/ESI‐MS method was 5 min, thus making the described setup suitable for a rapid screening method. The assay was validated using a known ACE inhibitor and the IC₅₀ values found were in good accordance with a common HPLC/UV method and literature data. The method was successfully applied for the screening of size‐exclusion chromatography fractions of the venom of the pitviper Bothrops moojeni. Three of 18 analyzed fractions inhibited ACE, due to peptides present as components of this snake venom. These compounds were extracted from the two most‐active fractions by means of TFC and isolated by means of HPLC. Three peptides with ACE inhibitory activity were characterized and their structures were elucidated with ESI‐MS/MS‐based de novo sequencing to be ZKWPPGKVPP, ZKWPRPGPEIPP and ZNWPRPGPEIPP, respectively (Z = pyroglutamic acid). Copyright © 2010 John Wiley & Sons, Ltd.