正、负和中性TiP10团簇结构与电子性质的密度泛函研究

采用密度泛函理论的B3LYP方法研究了正、负和中性TiP10团簇的几何构型和电子结构. 计算结果表明, 中性TiP10团簇的基态构型为金属夹心结构, 正、负离子团簇同样具有该基态稳定结构. 通过对基态稳定结构的分子轨道分析表明, δ键对形成夹心结构起到重要作用. 理论计算得到的中性TiP10团簇的垂直和绝热电离能分别为7.84和7.68 eV, 垂直和绝热电子亲和势分别为3.18和3.35 eV.     

3-(4′-吡啶基)/苯基-6-芳基-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑的合成及电化学性质表征

由化合物3-(4′-吡啶基)/苯基-4氨基-5-巯基-1,2,4-三唑与芳香酸在三氯氧磷作用下脱水、闭环得到3-(4′-吡啶基)/苯基-6-芳基-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑类新型化合物,产率为42%~72%。该化合物具有较大的共轭平面,其结构经红外光谱、核磁共振氢谱、元素分析测试技术确证。电化学测试的工作窗口为-1.4~1.0V,负向扫描,循环伏安图中显示标题化合物均有还原峰,还原起始电位为-0.35~-0.95V。借鉴有机材料能带表征的方法,结合其紫外-可见光谱计算出了被测化合物的电子亲和势、电离势、带隙等电化学参数。结果表明,与常用的有机电子传输材料PBD相比,标题化合物具有较高的电子亲和势(3.79~4.39eV)和电离势(6.95~7.84eV),较高的电子亲和势有利于电子的传输。     

3-（4′-吡啶基）/苯基-6-芳基-1,2,4-三唑并[3,4-b]-1,3,4-噻二唑的合成及电化学性质表征

由化合物3-（4＇-吡啶基）／苯基4氨基-5-巯基-1，2，4-三唑与芳香酸在三氯氧磷作用下脱水、闭环得到3-（4＇-吡啶基）／苯基-6-芳基-1，2，4-三唑并[3，4-b]-1，3，4-噻二唑类新型化合物，产率为42％～72％。该化合物具有较大的共轭平面，其结构经红外光谱、核磁共振氢谱、元素分析测试技术确证。电化学测试的工作窗口为-1.4～1.0V.负向扫描，循环伏安图中显示标题化合物均有还原峰，还原起始电位为-0.35～-0.95V.借鉴有机材料能带表征的方法，结合其紫外-可见光谱计算出了被测化合物的电子亲和势、电离势、带隙等电化学参数。结果表明，与常用的有机电子传输材料PBD相比，标舾化合物具有较高的电子亲和势（3.79～4.39ev）和电离势（6.95～7.84eV），较高的电子亲和势有利于电子的传输。     

银团簇的形成及性质研究*

本文综述了在固相、液相和气相中形成银团簇的方法, 银团簇的主要性质如吸收光谱、ESR、氧还电位、电子亲和势与电离能, 以及银团簇在光解水和照相显影过程中的催化作用。总结了银团簇的理论计算方法及计算得到的主要性质如稳定几何结构、电子亲和势与电离能。最后展望了今后的发展趋势。     

乙基硫自由基及阴、阳离子低电子激发态从头算研究

采用CASSCF方法和6-311++(3df, 3pd)基组以及Cs对称性优化了乙基硫自由基和阳、阴离子3种分子的12个电子态的几何构型. 利用二级微扰方法(CASPT2)对这12个电子态做了单点能校正. 通过比较自由基与阴阳离子的能量, 得出了绝热电子亲和势和绝热电子电离能, 与实验结果在允许误差范围内基本一致.     

全氟代金刚烷及其自由基的理论研究

采用密度泛函理论(DFT)方法:BHLYP、B3LYP、BP86、BLYP,在全电子的双ζ基组加极化函数和弥散函数(DZP＋)基组下,计算了全氟代金刚烷(C10F16)及其自由基(C10F15)的总能量、优化几何构型、电子亲和势和谐振频率.在B3LYP水平上所得到的可靠绝热电子亲和势(EAad)分别为: C10F16, 1.06 eV; C10F15, 4.11和 3.03 eV.     

萤火虫氧化荧光素及其衍生物的电子结构和光物理性质

通过密度泛函理论(DFT)的MPW3PBE泛函, 对萤火虫生物发光底物氧化荧光素及其衍生物进行了结构全优化. 计算了其电离能、 电子亲和势、 空穴抽取能、 电子抽取能、 空穴和电子重组能, 并评估了其空穴和电子传输能力. 采用含时密度泛函理论(TD-DFT)//MPW3PBE/6-31+G(d)方法计算了吸收光谱, 优化了最低单重态S₁, 研究了其荧光光谱, 进而考察了具有较高发光效率的氧化荧光素作为有机发光二极管(OLED)材料的可能性. 计算结果表明, 氧化荧光素及其衍生物可以同时作为电子传输层和发光层材料.     

(AlN)+n和(AlN)-n(n=1～15)团簇的结构与稳定性

用密度泛函理论(DFT)的B3LYP方法,在6-31G*水平上,对(AlN)+n和(AlN)-n(n=1～15)团簇的几何构型、红外光谱和热力学稳定性进行了研究,并对它们的电离能及电子亲和能进行了讨论.结果表明:(AlN)n团簇的电荷状态对簇合物的结构有较大影响,随着n的增大影响逐渐减小;所有平衡结构中不存在Al-Al和N-N键,Al-N键是惟一键型;(AlN)+n和(AlN)-n结构稳定性幻数与(AlN)n相同,即n=2,4,6,…等偶数结构较为稳定;(A1N)10团簇具有最大的电离能(IP=9.14 eV)和最小的电子亲和能(EA=0.19eV),较其他团簇更稳定.     

萘醌类化合物抗肿瘤活性的理论研究

采用量子化学方法对目前发现的39种天然紫草萘醌类化合物的电离能(IP)、电子亲和势(EA)及羟基氢原子解离能(Bond dissociation energy, BDE)进行了理论研究, 并分析了上述物理量与羟基自由基清除活性之间的关系. 同时研究了5, 7及9位C及14, 16位氧的自旋密度及化合物电子亲和势(EA)对其活性的影响. 研究结果表明, 化合物侧链的增长及不饱和键的存在均可导致其BDE和IP值的减小, EA值的减小, 说明苯氧基自由基自旋密度的增大, 有助于其自由基清除活性的增大, 表明其抗肿瘤作用的增强. 而在支链上引入体积较大的官能团以及羟基和乙酰基, 会导致化合物的BDE和IP值的增大.     

(S)-4-氨基-5-巯基戊酸的简便合成方法

(S)-4-氨基-5-巯基戊酸(Glutamate thiol,Glu SH)是谷氨酸α-羧基被亚甲基巯基取代的衍生物,也可被视为半胱氨酸的衍生物.该氨基酸是海洋抗肿瘤环酯肽Apratoxin E及其结构优化物的重要组成片段.报道(S)-4-氨基-5-巯基戊酸两种简便合成方法.第一种方法是以巯基和氨基均被保护的D-半胱氨酸为原料,先与米氏酸缩合生成β-酮酯,然后还原消除酮羰基、脱羧形成内酰胺,最后去除巯基和氨基上的保护基以及将内酰胺环打开,以4步76.0%的总收率得到所需产物.第二种方法以氨基和γ-羧基被保护的谷氨酸为原料,先将α-羧基还原为伯醇,然后借助Mitsunobu反应引入巯基,最后一锅脱除氨基、巯基和羧基上的保护基得到(S)-4-氨基-5-巯基戊酸.     

Correlation of electronic structures of three cyclic dipeptides with their photoemission spectra

We have investigated the electronic structure of three cyclic dipeptides: cyclo(Glycyl-Glycyl) (cGG), cyclo(Leucyl-Prolyl) (cLP), and cyclo(Phenylalanyl-Prolyl) (cPP). These compounds are biologically active and cLP and cPP are derived from cGG (also known as diketopiperazine), by the addition of the respective functional groups of the amino acids, namely, phenyl, alkyl or a fused pyrrolidine ring (proline). Experimental valence and core level spectra have been interpreted in the light of theoretical calculations to identify the basic chemical properties associated with the central ring, and with the additional functional groups in cLP and cPP. The theoretically simulated spectra of all three cyclic dipeptides in both valence and core spaces agreed reasonably well with the experimental spectra. The three molecules displayed similarities in their core spectra, suggesting that the diketopiperazine structure plays an important role in determining the inner shell spectrum. The experimental C 1s spectra of cLP and cPP are analogous but differ from cGG due to the side chains attached to the diketopiperazine structure. Single spectral peaks in the N 1s (and O 1s) spectra of the dipeptides indicate that the chemical environment of the nitrogen atoms (and oxygen atoms) are very similar, although they show a small splitting in the simulated spectra of cPL and cPP, due to the reduction of their point group symmetry. Valence band spectra of the three dipeptides in the frontier orbital region of 9-11 eV exhibit similarities; however theoretical analysis shows that significant changes occur due to the involvement of the side chain in the frontier orbitals of cPP, while lesser changes are found for cLP.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Amphipathic helices from aromatic amino acid oligomers

Synthetic helical foldamers are of significant interest for mimicking the conformations of naturally occurring molecules while at the same time introducing new structures and properties. In particular, oligoamides of aromatic amino acids are attractive targets, as their folding is highly predictable and stable. Here the design and synthesis of new amphipathic helical oligoamides based on quinoline-derived amino acids having either hydrophobic or cationic side chains are described. Their structures were characterized in the solid state by single-crystal X-ray diffraction and in solution by NMR. Results of these studies suggest that an oligomer as short as a pentamer folds into a stable helical conformation in protic solvents, including MeOH and H(2)O. The introduction of polar proteinogenic side chains to these foldamers, as described here for the first time, promises to provide possibilities for the biological applications of these molecules. In particular, amphipathic helices are versatile targets to explore due to their importance in a variety of biological processes, and the unique structure and properties of the quinoline-derived oligoamides may allow new structure-activity relationships to be developed.     

Self-assembly of brush-like amphiphilic statistical polymers: the effect of salt stimulus on the molecular associative mode

The solution properties of amphiphilic statistical polymers, originating from the nanostructures of macromolecular assemblies, strongly depend on the associative behaviours of the polymers. In this paper, we report the effect of salt stimulus on the associative mode of the 2-(acrylamido)-2-methylpropanesulphonic acid–2-(acrylamido)-dodecane sulphonic acid brush-like amphiphilic statistical polymers, employing steady-state fluorescence, dynamic light scattering and transmission electron micrograph methods. It was found that the stimulus effects work more intensely for the polymers with less hydrophobic side chains. As the salt concentration increases, the salt valence state increases or the salt structure adds an organic group, the polymer molecules associate at a lower critical concentration. Meanwhile the molecules undergo an intramolecular associative mode in preference to an intermolecular associative mode, forming hundreds of times smaller unimers rather than giant multipolymer aggregates.     

Dual substrate recognition of aminotransferases

Pyridoxal 5'-phosphate-dependent aminotransferases reversibly catalyzes the transamination reaction in which the alpha-amino group of amino acid 1 is transferred to the 2-oxo acid of amino acid 2 (usually 2-oxoglutarate) to produce the 2-oxo acid of amino acid 1 and amino acid 2 (glutamate). An aminotransferase must thus be able to recognize and bind two kinds of amino acids (amino acids 1 and 2), the side chains of which are different in shape and properties, from among many other small molecules. The dual substrate recognition mechanism has been discovered based on three-dimensional structures of aromatic amino acids, histidinol phosphate, glutamine:phenylpyruvate, acetylornithine, and branched-chain amino acid aminotransferases. There are two representative strategies for dual substrate recognition. An aromatic amino acid aminotransferase prepares charged and neutral pockets for acidic and aromatic side chains, respectively, at the same place by a large-scale rearrangement of the hydrogen-bond network caused by the induced fit. In a branched-chain aminotransferase, the same hydrophobic cavity implanted with hydrophilic sites accommodates both hydrophobic and acidic side chains without side-chain rearrangements of the active-site residues, which is reminiscent of the lock and key mechanism. Dual substrate recognition in other aminotransferases is attained by combining the two representative methods.     

Preparation of protected syn-alpha,beta-dialkyl beta-amino acids that contain polar side chain functionality

We report the synthesis of syn-alpha,beta-dialkyl beta-amino acid derivatives suitably protected for solid-phase synthesis that give rise to residues containing positively charged lysine-like side chains. These amino acids, as well as syn-alpha,beta-dialkyl beta-amino acids that contain diverse hydrophobic side chains, are prepared in good de and ee. The key step in this route involves Davies's protocol for the conjugate addition of a chiral lithium amide to alpha,beta-unsaturated tert-butyl esters (Davies, S. G.; Ichihara, O.; Walters, I. A. S. J. Chem. Soc., Perkin Trans. 1 1994, 9, 1141). syn-alpha,beta-Dialkyl beta-amino acids are interesting building blocks because of their sheet-forming propensity and because of their presence in bioactive compounds.     

Formation of negative ions <Emphasis Type="Italic">via</Emphasis> resonant low-energy electron capture by cysteine and cystine methyl esters

The processes of resonance low-energy free electron attachment to methyl esters of some sulfur-containing amino acids were studied. The long-lived molecular negative ions of cystine dimethyl ester formed in the valence state via the Feshbach nuclear excited resonance mechanism were detected by mass spectrometry. The reactions of disulfide bond dissociation were identified in an electron energy range of 0—1 eV. They can be considered as model reactions regarding processes of peptide decomposition due to the resonance interaction with low-energy electrons. Predissociation of short-lived molecular ions of cysteine methyl ester formed by capture of electrons with energies of ~1.6 eV is accompanied by the intra-ionic transfer of negative charge from the carbonyl group to the sulfur atom leading to the elimination from the latter of hydrogen atom.     

Chiral discrimination of β‐3‐homo‐amino acids using the kinetic method

Chiral discrimination of seven enantiomeric pairs of β‐3‐homo‐amino acids was studied by using the kinetic method and trimeric metal‐bound complexes, with natural and unnatural α‐amino acids as chiral reference compounds and divalent metal ions (Cu²⁺ and Ni²⁺) as the center ions. The β‐3‐homo‐amino acids were selected for this study because, first of all, chiral discrimination of β‐amino acids has not been extensively studied by mass spectrometry. Moreover, these β‐3‐homo‐amino acids studied have different aromatic side chains. Thus, the emphasis was to study the effect of the side chain (electron density of the phenyl ring, as well as the difference between phenyl and benzyl side chains) for the chiral discrimination. The results showed that by the proper choice of a metal ion and a chiral reference compound, all seven enantiomeric pairs of β‐3‐homo‐amino acids could be differentiated. Moreover, it was noted that the β‐3‐homo‐amino acids with benzyl side chains provided higher enantioselectivity than the corresponding phenyl ones. However, increasing or decreasing the electron density of the aromatic ring by different substituents in both the phenyl and benzyl side chains had practically no role for chiral discrimination of β‐3‐homo‐amino acids studied. When copper was used as the central metal, the phenyl side chain containing reference molecules (S)‐2‐amino‐2‐phenylacetic acid (L ‐Phg) and (S)‐2‐amino‐2‐(4‐hydroxyphenyl)‐acetic acid (L ‐4′‐OHPhg) gave rise to an additional copper‐reduced dimeric fragment ion, [Cu^I(ref)(A)]⁺. The inclusion of this ion improved noticeably the enantioselectivity values obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump-probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short-lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab-based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time-resolved photoelectron spectroscopy with ultraviolet (UV) probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuum-ultraviolet (VUV) region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time-resolved VUV photoelectron spectroscopy. Finally, the importance of soft X-ray free-electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond time-resolved soft X-ray emission spectroscopy or resonant inelastic X-ray scattering for mapping the electronic structure during chemical reactions is discussed.     

Transamination reactions with multiple turnovers catalyzed by hydrophobic pyridoxamine cofactors in the presence of polyethylenimine polymers

Pyridoxamines carrying hydrophobic side chains reversibly bind into the hydrophobic core of polyethylenimines and transaminate ketoacids to amino acids with as much as a 725000-fold rate acceleration. Turnover catalysis was achieved by sacrificial oxidative decarboxylation of C-substituted amino acids, which reconverted the pyridoxals to pyridoxamines.