Monomer–Dimer Control and Crystal Engineering in TASPs

We have reported a template assembled synthetic protein (cavitein Q4) as an unexpected dimer in the solid state and as a monomer–dimer equilibrium in solution. We have since reported an ability to bias a cavitein’s monomer–dimer equilibrium in solution by sequence design involving histidine metal chelation or disulfide incorporation. However, little remains known about the forces contributing to dimeric cavitein crystal nucleation and lattice stabilization. We, therefore, designed glutamine variants to probe factors involved in dimeric cavitein crystallization. It was found that a key glutamate hydrogen-bonding interaction between dimers is integral to crystal formation and stabilization. Additionally, we obtained a crystal structure of a cavitein (Q4-E3H) designed to bias the dimeric structure via histidine metal coordination. The resolved structure indicates a histidine cluster interaction that likely accounts for the biased dimeric form observed in solution.     

蛋白质测定的流动注射荧光法

根据荧光素的单体－二聚体平衡转化的原理，提出了一种检测蛋白质的方法；荧光素在表面活性剂作用下形成二聚体后荧光强度明显降低，加入蛋白质后二聚体解聚使荧光强度增加；结合流动注射法测定牛血清蛋白（BSA），该法简便，灵敏度高，对血清中蛋白质进行测定，结果令人满意。     

罗丹明6G的溶液状态和荧光特性的研究

罗丹明6G的荧光性质和它的分子聚集态研究是一个重要的科研领域,所得之k_d值表明水溶液中形成罗丹明6G的双聚体。本文提出用荧光法测定k_d聚合常数,该法用于测定罗丹明6G的溶液状态.实验结果表明罗丹明6G双聚体解聚的某些规律,当单体与双聚体平衡趋于单体时,荧光灵敏度就会增加。     

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

Herein, we demonstrate that the dimerization behavior of amine‐substituted dicyanomethyl radicals can be switched from σ‐ to π‐dimerization simply by varying the electron‐donating substituents. For dicyanomethyl radicals with a 4,4′‐ditolylamine ( DT^. ) or a phenothiazine ( PT^. ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C−C bond (σ‐dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton ( JD^. ) does not undergo σ‐dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X‐ray single‐crystal analysis revealed that JD^. forms the π‐dimer in the crystalline state, and variable‐temperature spectroscopy showed that JD^. is in equilibrium with the π‐dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π‐dimerization of JD^. because of its polarized structure.     

N=N bond cleavage by a low-coordinate ironII hydride complex

Reaction of the three-coordinate complex LFeCl (L = bulky beta-diketiminate) with KBEt3H gives a dark red iron(II) hydride complex. The complex is a dimer in the solid state, but spectroscopy and kinetics suggest that an orange three-coordinate monomer is in equilibrium with the dimer in solution. The double bond of azobenzene is completely cleaved by heating with the hydride complex, and a hydrazido intermediate can be isolated.     

Reversible Carbon–Carbon Bond Breaking and Spin Equilibria in Bis(pyrimidinenorcorrole)

Reversible homolytic dissociation of the bis(pyrimidinenorcorrole) σ‐dimer was elucidated by means of variable temperature ESR and ¹H NMR spectroscopy, mass spectrometry, and optical spectrocopy. Dehydrogenation of the σ‐dimer yielded another dimer displaying a singlet–triplet equilibrium in solution, strong NIR absorption (1570 nm), and a narrow electrochemical HOMO–LUMO gap (0.74 V).     

Kinetics of equilibrium attainment between molecular glycolaldehyde structures in an aqueous solution

The kinetics of the isomerization and monomerization of the glycolaldehyde dimer in a D₂O solution (pD = 4.3) at 25°C is studied by¹H NMR spectroscopy (360 MHz). The dynamics of the concentrations of seven dimeric and two monomeric glycolaldehyde forms present in the solution is examined when the system attains equilibrium. A kinetic scheme of equilibrium attainment in an aqueous solution of glycolaldehyde is proposed. The apparent rate constants of the transformation of the molecular glycolaldehyde structures into each other are determined     

GC-MS identification of styrene and oligomers in polystyrene plastic for milk packaging. Headspace and solution injection technique

Twenty compounds have been determined by GC-MS in polystyrene used for milk packaging. The major components identified in milk were styrene and its dimer at concentrations of 13 and 43 mg/kg, respectively. The detection limit for styrene by the equilibrium headspace method was 0.29 mg/kg. Another method used to determine the concentration of styrene and its dimer in milk was the use of acetone to precipitate the proteins and extract both the fat and the residues from the plastic packaging. The solution obtained was then directly injected into the GC. The detection limits were found to be 0.16 mg/kg for styrene and 0.28 mg/kg for the dimer. The solution injection method thus proved to be much more sensitive.     

溶液中芳香单酮的聚集态

本文对九种芳香单酮(苯乙酮、对氯苯乙酮、对甲氧基苯乙酮、对氨基苯乙酮、对硝基苯乙酮、间硝基苯乙酮、2.4-二甲基苯乙酮、1-萘乙酮和1-萘基苯基甲酮)在丙酮溶液中的吸收光谱进行了比较。发现芳香单酮在溶液中的分散是不均匀的, 随浓度增加, 有芳香单酮的聚集体生成。且芳香单酮分子的共轭体系越发达或苯环上电子云密度越犬, 分子就越易聚集。同时提出芳香单酮聚粲时的分子排列是苯环对苯环“面对面”取向的推断。     

Mechanistic studies of the lithium enolate of 4-fluoroacetophenone: rapid-injection NMR study of enolate formation, dynamics, and aldol reactivity

Lithium enolates are widely used nucleophiles with a complicated and only partially understood solution chemistry. Deprotonation of 4-fluoroacetophenone in THF with lithium diisopropylamide occurs through direct reaction of the amide dimer to yield a mixed enolate-amide dimer (3), then an enolate homodimer (1-Li)(2), and finally an enolate tetramer (1-Li)(4), the equilibrium structure. Aldol reactions of both the metastable dimer and the stable tetramer of the enolate were investigated. Each reacted directly with the aldehyde to give a mixed enolate-aldolate aggregate, with the dimer only about 20 times as reactive as the tetramer at -120 °C.     

碘化四(4-三甲胺苯基)卟啉铁(II)配合物的聚合及其轴向配位的研究

本文通过元素分析、磁化率、红外光谱、紫外可见光谱进一步确定了碘化四(4-三甲胺苯基)卟啉铁(III)配合物的组成和结构. PH滴定曲线证明它在不同PH水溶液中存在单体和二聚体平衡. 通过PH光度法求出在恒定离子强度下(μ=0.1FNaNO3)聚合物的聚合度、聚合平衡常数及其热力学参数. 用分光光度法测定了该二聚体与生物轴向配体(咪唑、1-甲咪唑、L-组氨酸及吡啶)反应平衡常数及热力学参数, 其配位能力大小顺序为:咪唑>1-甲基咪唑>L-组氨酸>吡啶. 还求出上述反应在水-二氧六圜混合溶剂中的平衡常数, 得出lgK与1/8呈线性关系(8为介电常数), 并对热力学参数进行了讨论.     

Dimerization of merocyanine dyes. Structural and energetic characterization of dipolar dye aggregates and implications for nonlinear optical materials

Aggregation of polar merocyanine dyes has been identified as an important problem in the fabrication of organic materials for photonic applications. In this work, a series of merocyanine dyes is synthesized, and their aggregation is investigated by a combination of several experimental techniques to reveal structure-property relationships. These studies provide clear evidence for the formation of centrosymmetric dimers for all investigated merocyanines in concentrated solution and in the solid state. The thermodynamics of dimerization in liquid solution is studied by concentration-dependent permittivity measurements, UV-vis spectroscopy, and electrooptical absorption experiments. A centrosymmetric dimer structure with antiparallel ordering of the dipole moments is observed in solution by 2D NMR spectroscopy as well as in the solid state by X-ray crystallography and interpreted in terms of dipolar and pi-pi interactions. The optical properties of the dimer aggregates are satisfactorily explained by an excitonic coupling model. The effect of an external electric field on the dimerization equilibrium is considered and quantitatively determined by electrooptical absorption measurements. Implications of the observed findings on the design of nonlinear optical and photorefractive materials are discussed.     

NMR spectroscopic characterization of copper(II) and zinc(II) complexes of indomethacin

Molecular diffusion constants were studied by NMR spectroscopy to provide information about the solution structures of a variety of Cu(II) and Zn(II) monomeric and dimeric complexes of indomethacin (IndoH). These studies showed that monomeric Zn(II)-Indo complexes substantially dimerize in DMF-d7 and DMSO-d6 solutions at room temperature, whereas the Cu(II) and Zn(II) dinuclear complexes remain largely intact in these solutions. There is evidence of an equilibrium between monomers and dimers for the Zn(II) complexes in solution, as shown by a reduced diffusion constant and lower average radius compared to the Cu(II) dimer. Such an equilibrium between monomers and dimers for the Zn(II) complexes is also consistent with previous results obtained from XAFS analysis of DMF solutions of such complexes. The greater lability and lower thermodynamic stability of the Zn(II) dimer complex compared to the Cu(II) analogue, as determined from the NMR experiments, is likely to result in the more ready release of free Indo in the GI tract. This is consistent with the previously observed higher GI toxicities of the Zn-Indo pharmaceutical preparations compared to the Cu(II)-Indo counterparts.     

Monomer-Dimer Control and Crystal Engineering in TASPs

We have reported a template assembled synthetic protein (cavitein?Q4) as an unexpected dimer in the solid state and as a monomer-dimer equilibrium in solution. We have since reported an ability to bias a cavitein's monomer-dimer equilibrium in solution by sequence design involving histidine metal chelation or disulfide incorporation. However, little remains known about the forces contributing to dimeric cavitein crystal nucleation and lattice stabilization. We, therefore, designed glutamine variants to probe factors involved in dimeric cavitein crystallization. It was found that a key glutamate hydrogen-bonding interaction between dimers is integral to crystal formation and stabilization. Additionally, we obtained a crystal structure of a cavitein (Q4-E3H) designed to bias the dimeric structure via histidine metal coordination. The resolved structure indicates a histidine cluster interaction that likely accounts for the biased dimeric form observed in solution.     

Precise investigation of the axial ligand substitution mechanism on a hydrogenphosphato-bridged lantern-type platinum(III) binuclear complex in acidic aqueous solution

Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the "lantern-type" platinum(III) binuclear complex, [Pt(2)(mu-HPO(4))(4)(H(2)O)(2)](2)(-), with halide and pseudo-halide ions (X(-) = Cl(-), Br(-), and SCN(-)) were carried out in acidic aqueous solution at 25 degrees C with I = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with -log K(h1) = 2.69 +/- 0.04. The consecutive formation constants of the aquahalo complex () and the dihalo complex () were determined spectrophotometrically to be log = 2.36 +/- 0.01 and log = 1.47 +/- 0.01 for the reaction with Cl(-) and log = 2.90 +/- 0.04 and log = 2.28 +/- 0.01 for the reaction with Br(-), respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (C(X)()- > C(Pt)), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k(obs)) depended on C(X)()- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.     

Factors influencing Al(3+)-dimer speciation and stability from density functional theory calculations

We have investigated aqueous Al-dimer complexes using density functional theory methods (e.g. the B3LYP exchange-correlation functional and the 6-311++G(d,p) basis set). In these calculations interactions between the Al(3+) cations and the H(2)O or OH(-) coordinating ligands are considered explicitly while the second hydration shell and remaining solvent are treated as a continuum under the IEF-PCM formalism. The Al-dimer chemical reactivity is discussed by analysis of changes in geometry, electronic structure and Gibbs free energy of formation, relative to two independent Al(H(2)O) monomers, as a function of water and hydroxide coordination. Our results indicate that the mechanism of cooperativity, i.e. decreased Al-water bond stability with increasing OH(-) coordination and increased water ligand hydrolysis as complex CN decreases, is operating on the dimer species and that, therefore, a wide variety of dimer species are available. While the stability of these species is observed to be dependent on the number of water and hydroxide ligands, the hydroxide bridging structure (singly, doubly and triply bridged species are considered) does not appear to correlate with dimer stability. Interestingly, intra-molecular H-bonds (in the form of the well known H(3)O bridge as well as two bridging structures, H(4)O(2) and H(2)O, that have not, to our knowledge, been previously considered) are observed to influence dimer stability. The evaluation of the equilibrium mole fraction of the dimer species in equilibrium with the aqueous Al(3+) monomer species of our previous study displays the qualitatively correct trend of solution composition as pH increases, namely monomeric aqueous Al(3+) and Al(OH) complexes dominate at low and high pH, respectively, and all remaining monomer and dimer species exist at intermediate pH. Further refinement of our data set by eliminating dimer complexes with OH/Al ratios greater than 2.6 brings our predicted equilibrium mole fraction distributions into excellent agreement with experimental observations. The triply bridged dimer is observed in low amounts while the singly and doubly bridged dimers dominate our model system at pH = ～4-7.     

Dimesitylborane monomer-dimer equilibrium in solution, and the solid-state structure of the dimer by single crystal neutron and X-ray diffraction

Dimesitylborane dimer has been shown to exist in equilibrium with dimesitylborane monomer in solution. This equilibrium has been investigated by variable concentration and variable temperature multinuclear NMR spectroscopy and values for the dissociation constant, enthalpy and entropy of dissociation were found to be K_diss=(3.2±0.4)×10⁻³ M, ΔH=70 kJ mol⁻¹, and ΔS=212 J K⁻¹mol⁻¹, respectively. Ab initio methods have been used to investigate the gas-phase structures and energies of both monomer and dimer, and calculated ¹¹B-NMR shifts are also presented. The solid-state structure of dimesitylborane dimer has been investigated by single crystal X-ray diffraction at 100 K and the position of the bridging hydrogen atoms (B---H=1.340(2), 1.342(2) Å, H---B---H=92.46(14)°) has been determined accurately, for the first time, by single crystal neutron diffraction at 20 K.     

Acetic acid vapor: 1. Statistical/quantum mechanical models of the ideal vapor

Two statistical mechanical/quantum computational models are developed for the ideal acetic acid vapor. One describes the equilibrium of the cis-monomer and the ring dimer and the other the equilibrium of a mixture of oligomers. Ten quantum computational models of the acetic acid ring dimer have been compared in developing these models. The end product of this work is a critical tool for assessing experimental vapor density studies of acetic acid reported in the literature.     

Catechol Thioesters: Ligands for Hierarchically Formed Lithium-Bridged Titanium(IV) Helicates and Helicate-Based Switches

The thioester moiety is introduced as a lithium binding unit for the hierarchical formation of titanium(IV) catecholate-based lithium-bridged helicates. In solution, the coordination compounds show a monomer–dimer equilibrium which —in comparison to the oxo esters— is significantly shifted towards the monomers. In addition, the influence of the thioester side chain on the dimerization behavior is investigated and an expansible/compressible molecular switch is synthesized. In the latter case expansion and compression are performed reversibly in methanol, whereas in DMSO spontaneous expansion occurs.     

Ir study of the structure of palladium(ii) acetate in chloroform, acetic acid, and their mixtures in solution and in liquid-solid subsurface layers

It is found that cyclic trimer I and linear dimer II are the molecular states of palladium(II) acetate in its 1. 10^-3-0.7 M chloroform solutions. The mole fraction of dimer II is higher in dilute solutions. Even with small acetic acid (HOAc) additions the I↔II equilibrium is shifted toward I because of the variation of the composition of the outer-spheric solvation shells. The content of II in the 0.1 M solution of PdAc₂ in 50 vol. % HOAc+CHCl₃ is 10%. However, in the thin subsurface layers of this solution at the interface CaF₂ -solution, the I↔ equilibrium is slowly shifted toward II whose concentration after 60 min is 22%. This may be caused by structuring of the liquid phase in the subsurface layers during the measured time. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 540-546, May–June, 2000.