Leucine enkephalin—A mass spectrometry standard

The present article reviews the mass spectrometric fragmentation processes and fragmentation energetics of leucine enkephalin, a commonly used peptide, which has been studied in detail and has often been used as a standard or reference compound to test novel instrumentation, new methodologies, or to tune instruments. The main purpose of the article is to facilitate its use as a reference material; therefore, all available mass spectrometry‐related information on leucine enkephalin has been critically reviewed and summarized. The fragmentation mechanism of leucine enkephalin is typical for a small peptide; but is understood far better than that of most other compounds. Because ion ratios in the MS/MS spectra indicate the degree of excitation, leucine enkephalin is often used as a thermometer molecule in electrospray or matrix‐assisted laser desorption ionization (ESI or MALDI). Other parameters described for leucine enkephalin include collisional cross‐section and energy transfer; proton affinity and gas‐phase basicity; radiative cooling rate; and vibrational frequencies. The lowest‐energy fragmentation channel of leucine enkephalin is the MH⁺ → b₄ process. All available data for this process have been re‐evaluated. It was found that, although the published E_a values were significantly different, the corresponding Gibbs free energy change showed good agreement (1.32 ± 0.07 eV) in various studies. Temperature‐ and energy‐dependent rate constants were re‐evaluated with an Arrhenius plot. The plot showed good linear correlation among all data (R² = 0.97), spanned over a 9 orders of magnitude range in the rate constants and yielded 1.14 eV activation energy and 10^11.0 sec^?1 pre‐exponential factor. Accuracy (including random and systematic errors, with a 95% confidence interval) is ±0.05 eV and 10^±0.5 sec^?1, respectively. The activation entropy at 470 K that corresponds to this reaction is ?38.1 ± 9.6 J mol^?1 K^?1. We believe that these re‐evaluated values are by far the most accurate activation parameters available at present for a protonated peptide and can be considered as “consensus” values; results on other processes might be compared to this reference value. © 2010 Wiley Periodicals, Inc., Mass Spec Rev 30:298–320, 2011     

关于△G和△A做为判据的条件的讨论

本文讨论了Gibbs函数变和Helmholtz函数变作为过程方向和限度判据的充分必要条件,以厘清目前各种版本物理化学教材中Gibbs函数变作为判据的混乱。     

Thermodynamic analysis of supercritical water gasification of methanol,ethanol, glycerol,glucose and cellulose

In the present work the Gibbs free energy minimization, using a non-linear programming formulation and an approximation in the gas fugacities, was used to calculate the equilibrium composition for supercritical water gasification of methanol, ethanol, glycerol, glucose and cellulose. The proposed formulation mathematically ensures finding the global optimal solution with no need of an initial estimate and the numerical results are close to the ones calculated using non-ideal gas formulation. Therefore, the proposed approach is reliable and easy to use, without numerical difficulties, such as an undesirable local minimum. The model predictions show a good agreement with the experimental studies in all cases studied in this work.     

自由能变化在有机化学中的应用

本文探讨应用自由能（△Ｇ）判据和吉布斯—亥姆霍兹（Ｇｉｂｂｓ—Ｈｅｌｍｈｏｌｔｙ）方程相结合，分析和解释有机化学中的一些问题。为正确解释有机化学中的一些化学现象，判断一些不常见的化学反应提供了理论依据。     

广义自旋变相过程的不变测度集

关于无穷粒子系统不变测度的讨论已越来越引起人们的兴趣,R.A.Holley和D.W.Stroock在[1]中给出了一维、二维随机Ising模型的不变测度都是GibbS态。唐守正在[2]中给出了有界自旋变相过程在一定的条件下,它的不变测度也是Gibbs志。本文讨论了广义自旋变相过程(参见[3]),当它是有界的或者其速度函数是强局部有限程的情况下,它的不变测度集与Gibbs态集重合,特别地若广义自旋变相过程有唯一的Gibbs态则一定遍历。     

Estimators of sensitivity and specificity in the presence of verification bias: A Bayesian approach

Verification bias can occur if some of the patients with test results are not selected to receive the gold standard procedure. Unverified cases frequently are not suggestive to be positives. Consequently, the set of verified cases overestimates the number of true positives and underestimates the number of true negatives. The sensitivity and specificity estimates based only on the patients with verified disease are often biased. In this article we derive estimators for sensitivity and specificity not subject to verification bias using a Bayesian approach. Marginal posterior densities of all parameters are estimated using the Gibbs sampler algorithm. An application to the study of accuracy of Hybrid Capture II in the diagnosis of cervical intraepithelial neoplasia grades 2 and 3 illustrates the proposed methodology.     

广义模糊Gibbs随机场与MR图像分割算法研究

为了对图像进行准确、可靠的分割,提出了一种基于广义模糊集的软分割算法,并将广义模糊集和G ibbs场结合起来,提出了广义模糊G ibbs随机场模型,同时建立了广义模糊G ibbs分割(GFGS)算法。该算法是首先把每一个分割类看作是广义模糊类,并以最大后验概率(MAP)为判别准则来决定每一个像素值的归类以及它属于该类的隶属度;然后用广义隶属度函数中负的部分来刻划数据中的异常值,使得该算法能有效地处理异常值;最后用该模糊类的质心来更新类的中心,并以人脑的仿真图像和临床MR图像进行了实验。实验结果表明,该算法能有效地滤除噪声和处理部分容积效应,是一个分割能力强、稳健性好的算法。     

Computational Issues in the Sequential Probit Model: A Monte Carlo Study

We discuss computational issues in the sequential probit model that have limited its use in applied research. We estimate parameters of the model by the method of simulated maximum likelihood (SML) and by Bayesian MCMC algorithms. We provide Monte Carlo evidence on the relative performance of both estimators and find that the SML procedure computes standard errors of the estimated correlation coefficients that are less reliable. Given the numerical difficulties associated with the estimation procedures, we advise the applied researcher to use both the stochastic optimization algorithm in the Simulated Maximum Likelihood approach and the Bayesian MCMC algorithm to check the compatibility of the results. JEL Classifications: C11, C15, C35, C63     

The thermal capacity and enthalpy of gadolinium sesquiselenide over a wide temperature range

Measurements have been made on the thermal capacity of γ-Gd₂Se₃ at 58.88–298.34 K. Values have been obtained for the thermal capacity, entropy, reduced Gibbs energy, and enthalpy under standard conditions: C°_p = 125.87 ± 0.5 J· mole⁻¹ · K⁻¹; S°(298.15 K) = 196.5 · 1.6 J · mole⁻¹ · K⁻¹; Φ°(298.15 K) = 103.6 ± 1.6 J · mole⁻¹ · K⁻¹; H°(298.15 K)-H°(0) = 27681 ± 138 J · mole⁻¹. The enthalpy of Gd₂Se₃ has been measured and the major thermodynamic functions have been calculated for the solid and liquid states over the temperature range 450–2300 K. The temperature dependence of the enthalpy in the ranges 300–1800 K and 2000–2300 K are represented: H°(T)-H°(298.15 K) = = 1.1949 · 10⁻² · T² + 122.38 · T + 347402 · T⁻¹ − 38716 and H°(T)-H°(298.15 K) = 262.81 · T-− 196047, respectively. The calculated temperature, enthalpy, and entropy of melting for Gd₂Se₃ are: T_m = 1925 ± 40 K, Δ_mH° (Gd₂Se₃) = 68.5 kJ · mole^-1, Δ_mS°(Gd₂Se₃) = 35.6 J · mole⁻¹ · K⁻¹. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 56–61, March–April, 2006.     

Using duration models to reduce fragmentation in audio segmentation

We investigate explicit segment duration models in addressing the problem of fragmentation in musical audio segmentation. The resulting probabilistic models are optimised using Markov Chain Monte Carlo methods; in particular, we introduce a modification to Wolff’s algorithm to make it applicable to a segment classification model with an arbitrary duration prior. We apply this to a collection of pop songs, and show experimentally that the generated segmentations suffer much less from fragmentation than those produced by segmentation algorithms based on clustering, and are closer to an expert listener’s annotations, as evaluated by two different performance measures. Editor: Gerhard Widmer