Calculation of reference pH values for standard solutions from the corresponding acid dissociation constants

Making the simplest possible assumption about the activity coefficient of the charged species, pH values of standard buffer solutions have been evaluated from the thermodynamic acidity constants, K, of the weak acids involved. A general equation is given for a triprotic acid, H₃A, as it can be simplified to derive the equations for other systems. A computer program for the solution of the equation was written giving m_H values, species distribution coefficients, α, buffer capacities, β, species activity coefficients, γ, and ionic strength, I. Iteration was continued until agreement between successive values to within 1 ± 10^?6 was reached.The activity coefficients of singly charged ions were taken as equal to γ_Cl, where log γ_Cl=?AI^1/2 (1 + 1.5I^1/2), which is the Bates-Guggenheim convention, and those of doubly and triply charged ions were given by the valence relations of the Debye-Hückel theory as γ⁴_Cl and γ⁹_Cl, respectively.     

Multiwavelength Spectrophotometric Determination of Dissociation Constants of Mercaptopyrimidines

Abstract

The dissociation constants of some 2- and 6-mercaptopyrimidine derivatives with amino or hydroxy substituents are reported. The constants have been determined at 25° C in 0.1 M KCl aqueous solution. The multiwavelength spectrophotometric data obtained have been 3mlyzed by means of the SQUAD program (Stability Quotients from Absorbance Data).     

质谱光电离/解离技术和生物分子结构鉴定

质谱是一种广泛应用于化学、生物医学、药学、环境、农业和能源等各领域的分子结构鉴定技术,这种技术通过准确测定分子离子和碎片离子的质量-电荷比来推导分子结构。如何将试样中待测组分有效气化、离子化,转变为具有不同质-荷比的气态离子是质谱仪器和分析方法研究的关键。基于不同物理化学原理的电离、解离方法各有特点,适合不同分析目的。常见的软电离技术一般产生稳定的偶电子离子,往往需要与其他技术联用才能实现分子离子的进一步解离。除了基于碰撞活化和电子得失的两类常见解离方法,光解离技术利用波长/能量可调控的光辐射来使样品分子电离,并引发特定化学键断裂。本文旨在综述不同电离/解离技术,重点探讨近年来发展的红外和紫外光电离/解离技术基本工作原理、仪器特点及其在生物分子(包括有机小分子、蛋白质、核酸和多糖等)结构鉴定中的应用。     

Applying the concepts of density functional theory to simple systems

Equations are derived for the chemical potential and local hardness of the ground states of helium and the related two electron ions. With these properties it is possible to correct the energies of the simple single‐zeta wave functions to the nearly exact values. The calculations are simple for these simple systems. In principle, it is possible to extend this method to all atoms and molecules. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Acid-Base and Photochemical Reactions of Selenious Acid

Abstract

Spectrophotometric data used in connection with H_A acidity function yield pK₁=?3.3(K₁= [H₂SeO₃][H⁺]/[H₃SeO₃ ⁺]);potentiometric and spectral measurements give for pK₂ and pK₃ data in agreement with the literature. At pH > 10 selenite undergoes a fast photochemical reaction, preceded by a rapidly established acid-base equilibrium. Products of this reaction undergo a slow reaction which is second order in selenite.     

Reversible dissociation of dibromobromates in organic solvents

A study was carried out on the reversible decomposition of dibromobromate anions in organic solvents and we found the equilibrium constants for the processes responsible for the distribution of active halogen and hydrogen halide between their different forms in solution. The capacity of the solvent to form hydrogen bonds with the bromide anion, the feasibility of forming a molecular complex with bromine, and the structure of the cation (in solutions with low ionizing capacity) were all found to affect the reversible decomposition of the dibromobromate anion. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 298–305, September–October, 2008.     

PF(X~3∑~-)的结构、势能函数与光谱常数

运用多种方法、多种基组对PF(X3∑-)的平衡结构进行优化计算.用QCISD/6-311G(df)方法得到的平衡结构为RPF=0.158 9 nm,与实验值RPF=0.158 97 nm进行比较,最为接近,得出QCISD/6-311G(df)基组为最优基组;然后对PF(X3∑-)进行单点能扫描计算,用正规方程组拟合Murrell-Sorbie函数,得到相应电子态的势能函数解析式,由势能函数计算了与PF(X3∑-)态相对应的光谱常数,结果与实验数据较为一致.这些数据为反应动力学提供了理论依据.     

N-苄基甘氨酸-N-乙基磷酸离解常数及其有关配合物稳定常数的确定

用 p H滴定法测定了标题化合物在 2 5± 0 .1℃、3 5± 0 .1℃和 4 5± 0 .1℃ KNO3 存在下的离解常数 ;计算了 2 5± 0 .1℃离解反应的焓变Δr Hm和熵变Δr Sm。确定了 2 5± 0 .1℃ KNO3 存在下 ,它与 Co( )、Ni( )及Cu( )形成的配合物的稳定常数     

Protonation acidity constants for benzotoluidides in sulfuric acid solutions

The protonation of o-, m-and p-benzotoluidide in sulfuric acid solutions is studied by UV spectroscopy in the 190–350 nm region. Principal component analysis is applied to estimate the contributions of the effect of protonation and the medium effect. For the substances studied in this work, the first principal component (PC) captures about 98 % of the variance and the second PC ∼100 % of the cumulative percentage variance in the 210–350 nm region. The same spectral region is used for calculation of the ionization ratio from the coefficients of the first PC and mole fractions of the base and its conjugate acid. Using these data and Hammett's equation (pK _{BH +} = H _X + log I), the pK _{BH +} values for the protonation reaction are obtained. The dissociation constants as well as the solvent parameters m* (∼0.43) and ϕ (∼0.60) are calculated using the Excess Acidity Method (-pK _{BH +}= 2.28–2.30) and the Bunnett-Olsen Method (-pK _{BH +}= 2.24–2.28). The probable sites of protonation are discussed.     

聚天冬氨酸酸解离常数的测定

采用pH电位滴定法，利用先进的BEST程序，针对聚羧酸在水溶液中解离及平衡常数测定的复杂性，构建了聚天冬氨酸（PASP）在水溶液中解离的H4L和H2L两种模型，分别测得了相应的各级酸解离常数，并得到了PASP在不同pH条件下的物种分布曲线。在35℃，I=0．1时，对于H4L模型，各级酸解离常数的pK,值为：2．27、3．66、4．22、5．40；对于H2L模型，相应的pK,值为2．92、4．84。研究结果表明，两种模型计算结果均能与实验结果很好吻合，但H4L模型误差更小，因此是一种更为合理的模型。     

Ligand–Receptor Binding Affinities from Saturation Transfer Difference (STD) NMR Spectroscopy: The Binding Isotherm of STD Initial Growth Rates

The direct evaluation of dissociation constants (K_D) from the variation of saturation transfer difference (STD) NMR spectroscopy values with the receptor–ligand ratio is not feasible due to the complex dependence of STD intensities on the spectral properties of the observed signals. Indirect evaluation, by competition experiments, allows the determination of K_D, as long as a ligand of known affinity is available for the protein under study. Herein, we present a novel protocol based on STD NMR spectroscopy for the direct measurements of receptor–ligand dissociation constants (K_D) from single‐ligand titration experiments. The influence of several experimental factors on STD values has been studied in detail, confirming the marked impact on standard determinations of protein–ligand affinities by STD NMR spectroscopy. These factors, namely, STD saturation time, ligand residence time in the complex, and the intensity of the signal, affect the accumulation of saturation in the free ligand by processes closely related to fast protein–ligand rebinding and longitudinal relaxation of the ligand signals. The proposed method avoids the dependence of the magnitudes of ligand STD signals at a given saturation time on spurious factors by constructing the binding isotherms using the initial growth rates of the STD amplification factors, in a similar way to the use of NOE growing rates to estimate cross relaxation rates for distance evaluations. Herein, it is demonstrated that the effects of these factors are cancelled out by analyzing the protein–ligand association curve using STD values at the limit of zero saturation time, when virtually no ligand rebinding or relaxation takes place. The approach is validated for two well‐studied protein–ligand systems: the binding of the saccharides GlcNAc and GlcNAcβ1,4GlcNAc (chitobiose) to the wheat germ agglutinin (WGA) lectin, and the interaction of the amino acid L ‐tryptophan to bovine serum albumin (BSA). In all cases, the experimental K_D measured under different experimental conditions converged to the thermodynamic values. The proposed protocol allows accurate determinations of protein–ligand dissociation constants, extending the applicability of the STD NMR spectroscopy for affinity measurements, which is of particular relevance for those proteins for which a ligand of known affinity is not available.