Electronic structure of Li2Ga and Li9Al4, two solids containing infinite and uniform zigzag chains

The electronic structure of inorganic solids such as Li(2)Ga and Li(9)Al(4) containing infinite zigzag homoatomic chains is discussed. It is shown that Li(2)Ga, a solid for which a Zintl-type electron-counting approach would suggest that a half-filled pi-type band occurs as in trans-polyacetylene, is really a three-dimensional solid with strong covalent interchain connections and small effective charge transfer. The zigzag chains do not play a dominant role as far as the electronic structure near the Fermi level is concerned, and there is no reason for the occurrence of a Peierls distortion despite the possible analogy with trans-polyacetylene. It is suggested that even assuming that a Zintl-type approach is appropriate for electron counting purposes, the infinite zigzag chains in this compound and those in trans-polyacetylene are not isolobal. The bonding in Li(9)Al(4) and Li(2)Ga is very similar, and both phases are predicted to be stable three-dimensional metals.     

Decay of the peroxide intermediate in laccase: reductive cleavage of the O-O bond.

Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.     

Generation and reactivity of the phenyl cation in cryogenic argon matrices: monitoring the reactions with nitrogen and carbon monoxide directly by IR spectroscopy

The phenyl cation 1 has been prepared by co-deposition of iodobenzene 6 or bromobenzene 7 with a microwave-induced argon plasma and characterized by IR spectroscopy in cryogenic argon matrices. The cation can clearly be identified by its strongest absorption at 3110 cm(-1) that is rapidly bleached upon visible light irradiation. This characteristic band is observed neither in the conventional photochemistry of 6 or 7 nor in discharge experiments with alkyl halides or chlorobenzene. The latter finding is in line with energetic considerations. According to density functional theory (DFT) computations, the strongest absorption of 1 is caused by a C-H stretching vibration that involves almost entirely the ortho-hydrogens. This is confirmed by isotopic labeling experiments. Co-deposition of halobenzene/N2 mixtures leads to a decrease of the 3110 cm(-1) absorption, whereas several new signals are detected in the 2200-2400 cm(-1) range of the IR spectrum. Annealing of a matrix that contains 1 and 1% N2 leads to an increase of a broad band at 2260 cm(-1) that is assigned to the benzenediazonium ion 2. A sharp signal at 2327 cm(-1) that had previously been assigned to the N-N stretching vibration of 2 is due to molecular nitrogen. The mechanism that triggers the IR activity of N2 is not yet understood. Annealing of a matrix that contains 1 and 0.5% CO leads to an increase of a broad band at 2217 cm(-1) that is considerably stronger than the 2260 cm(-1) absorption of 2. This signal is assigned to the C-O stretching vibration of the benzoyl cation 12, in excellent agreement with previous investigations of 12 in superacidic media. Some consequences of the measured frequencies with regard to bonding in 2 and 12 are discussed.     

Kinetics and products from reaction of Cl radicals with dioctyl sebacate (DOS) particles in O(2): a model for radical-initiated oxidation of organic aerosols

The reaction of Cl radicals with bis (2-ethylhexyl) sebacate (also known as dioctyl sebacate, DOS) particles in the presence of O(2) is studied as a model of radical-initiated oxidation of organic aerosols. The uptake coefficient as measured from the rate of loss of DOS is gamma(DOS) = 1.7 (+/-0.3) indicating that a radical chain is operative. It is observed that nearly all of the detected products, accounting for 86% (+/-12%) of the reacted DOS, remain in the particles indicating that they are not efficiently volatilized. Correspondingly, the particles do not decrease in volume even after 60% of the DOS has reacted; upon further reaction the volume does decrease by up to 20%. Additionally, the mass of a DOS film increases with reaction indicating that the density increases. The two primary products identified are the ketone (38 +/- 10% yield) and alcohol (14 +/- 4% yield) resulting from reactions of alkylperoxy radicals originating from DOS oxidation. The fact that the ketone/alcohol ratio is >1 implies that the Russell mechanism, the typical fate of alkylperoxy radicals in liquids whereby both a ketone and an alcohol are generated, is not the only source of ketones. In fact, the ketone yield demonstrates a Langmuir-Hinshelwood type dependence on the O(2) concentration indicating that 44% (+/-8%) of the ketone is created from the reaction of alkoxy radicals with O(2) at the surface of the particles (at 20% O(2)). While this is a common reaction in the gas phase, it is generally not considered to occur in organic solvents. Furthermore, the appearance of gas-phase H(2)O(2) suggests that peroxy radicals react to form two ketones and H(2)O(2)via the Bennett and Summers mechanism. The absence of aldehyde products, both in the gas phase and in the particles, indicates that beta-scission of the alkoxy radicals is not significant. The results of this study suggest that organic aerosols in the troposphere are efficiently oxidized by gas-phase radicals but that their chemical transformation does not lead to their removal through volatilization.     

A thermodynamic and kinetic study of hydride transfer of a caffeine derivative

One representative type of heterocyclic compound that can release a hydride ion is 7,8-dihydro-9-methylcaffeine (CAFH). The one-electron oxidation potential of CAFH [-0.294 (V vs Fc(+/0))] and the one-electron reduction potential of CAF(+) [-2.120 (V vs Fc(+/0))] were obtained using two different methods, CV and OSWV. Applying titration calorimetry data in thermodynamic cycles, the enthalpies of CAFH releasing a hydride ion [57.6 kcal/mol] and releasing a hydrogen atom [80.3 kcal/mol] and of its radical cation CAFH(?+) releasing a proton [33.0 kcal/mol] and releasing a hydrogen atom [38.4 kcal/mol] have been determined. Several conclusions can be drawn from the thermodynamic results: (1) CAFH is a very good single-electron donor whose single-electron oxidation potential is much less positive than that of NAD(P)H model compound BNAH [E(ox) = 0.219 V vs Fc(+/0)]. (2) The single-electron reduction potential of CAF(+) is much more negative than that of BNA(+) [E(red) = -1.419 V], which means that CAF(+) is not a good electron acceptor. Furthermore, CAFH is a very good hydride donor compared to BNAH. The results of non-steady-state kinetic studies, for the reaction of CAFH and AcrH(+)ClO(4)(-), show that the ratio of t(0.50)/t(0.05) is larger than 13.5 and the ratio of k(init)/k(pfo) is larger than 1. The pseudo-first-order rate constants obtained at different reaction stages decrease with the time, and the kinetic isotope was observed to be small at a short reaction time and slowly increases to 3.72 with the progress of the reaction. These kinetic results clearly display that the hydride transfer of CAFH to AcrH(+) in acetonitrile is not a one-step mechanism, while the thermodynamic results show that CAFH is a very good electron donor. The combination of the kinetic results with the thermodynamics analysis shows that the hydride transfer of the caffeine derivative CAFH takes place by a two-step reversible mechanism and there is an intermediate in the reaction.     

A New Photoactivatable Ruthenium(II) Complex with an Asymmetric Bis-Thiocarbohydrazone: Chemical and Biological Investigations

The synthesis, photoactivation and biological activity of a new piano-stool Ru(II) complex is herein reported. The peculiarity of this complex is that its monodentate ligand which undergoes the photodissociation is an asymmetric bis-thiocarbohydrazone ligand that possesses a pyridine moiety binding to Ru(II) and the other moiety contains a quinoline that endows the ligand with the capacity of chelating other metal ions. In this way, upon dissociation, the ligand can be released in the form of a metal complex. In this article, the double ability of this new Ru(II) complex to photorelease the ligand and to chelate copper and nickel is explored and confirmed. The biological activity of this compound is studied in cell line A549 revealing that, after irradiation, proliferation inhibition is reached at very low half maximal inhibitory concentration (IC₅₀) values. Further, biological assays reveal that the dinuclear complex containing Ni is internalized in cells.     

波长检测型表面等离子体共振传感器对硫堇电聚合成膜的原位分析

利用波长检测型表面等离子体共振(SPR)传感器对硫堇在金膜表面的电化学聚合成膜过程进行了跟踪分析.结果表明,在固定入射角下SPR共振波长随循环伏安扫描周数的增加而线性红移,表明聚硫堇膜的生长是匀速的;扫描100周后共振波长红移总量为96.6 nm.对该实验结果进行理论拟合,得出聚硫堇膜的厚度约为71 nm.聚硫堇膜在酸性缓冲液中的电化学活性很高,其电化学反应过程受扩散控制,在一个完整的循环伏安扫描过程中SPR共振波长的变化完全可逆,说明聚硫堇膜的氧化反应和还原反应是一对可逆过程.与还原态聚硫堇膜相比,氧化态聚硫堇膜对应的SPR共振波长较大,说明氧化态聚硫堇膜折射率高.     

New features in the catalytic cycle of cytochrome P450 during the formation of compound I from compound 0

Density functional theory (DFT) is applied to the dark section of the catalytic cycle of the enzyme cytochrome P450, namely, the formation of the active species, Compound I (Cpd I), from the ferric-hydroperoxide species (Cpd 0) by a protonation-assisted mechanism. The chosen 96-atom model includes the key functionalities deduced from experiment: Asp(251), Thr(252), Glu(366), and the water channels that relay the protons. The DFT model calculations show that (a) Cpd I is not formed spontaneously from Cpd 0 by direct protonation, nor is the process very exothermic. The process is virtually thermoneutral and involves a significant barrier such that formation of Cpd I is not facile on this route. (b) Along the protonation pathway, there exists an intermediate, a protonated Cpd 0, which is a potent oxidant since it is a ferric complex of water oxide. Preliminary quantum mechanical/molecular mechanical calculations confirm that Cpd 0 and Cpd I are of similar energy for the chosen model and that protonated Cpd 0 may exist as an unstable intermediate. The paper also addresses the essential role of Thr(252) as a hydrogen-bond acceptor (in accord with mutation studies of the OH group to OMe).     

Chemically induced contraction and stretching of a linear rotaxane dimer

Copper(I)-induced assembly of two self-complementary identical units, which consist of a ring that incorporates a 1,10-phenanthroline group attached to a small filament containing a second 1,10-phenanthroline (phen) group, leads quantitatively to a doubly threaded complex. Each copper(I) center is four-coordinate and is located inside a ring and bound to a phen from the macrocyle. The two other coordination sites are occupied by a phen from the filament connected to the other ring. An X-ray structure of the dicopper(I) complex unambiguously demonstrates the doubly threaded nature of the system. The molecule has C(2) symmetry in the crystal. This is an extended form with a Cu small middle dot small middle dot small middle dotCu separation of 18.3 A and an overall length close to 40 A. Further synthetic work, which utilizes the two terminal phenolic functions of the previous dicopper(I) complex, gives rise to a more complex system in which both filaments have been prolonged in opposite directions by 2,2':6',2"-terpyridine (terpy) motifs and bulky stoppers. The organic backbone is that of a rotaxane dimer. Although redox cycling of Cu(I) to Cu(II) did not lead to intramolecular rearrangement, simple chemical reactions induced large conformational changes. The rotaxane dimer was set in motion as follows. The dicopper(I) complex, which is in an extended conformation, was demetallated by using KCN. From the free ligand, the dizinc complex was formed quantitatively at room temperature. (1)H NMR data show that a new conformation is obtained: each Zn(II) is five-coordinate (phen + terpy), and the molecule is in a contracted conformation. This process is reminiscent of biological muscles in the sense that the two filaments of this system can be moved along one another in a gliding motion that keeps the whole system together, but which converts a stretched compound (overall length approximately equal to 83 A) into a contracted species (overall length approximately equal to 65 A, according to CPK models). The motion is quantitatively reversed by the addition of an excess of copper(I) to the dizinc complex; this regenerates the extended starting form. Although the motivation of the present contribution was to illustrate that a muscle-like molecule may be stretched or contracted using electrochemistry and coordination chemistry, the main body of the work is organic synthesis. This is testified by the fact that the dicopper(I) rotaxane dimer was obtained in 23 steps from commercially available compounds.     

Solvation dynamics under a microscope: single giant lipid vesicle

Picosecond spectroscopy under a confocal microscope is employed to study solvation dynamics of coumarin 153 (C153) inside a single giant lipid vesicle (1,2-dilauroyl-sn-glycero-3-phosphocholine, DLPC) of diameter 20 μm. Fluorescence correlation spectroscopy (FCS) indicates that the diffusion coefficient (D(t)) of the probe (coumarin153, C153) in the immobilized vesicle displays a wide distribution from ~3 to 21 μm(2) s(-1). The distribution of D(t) suggests that the microenvironment of the probe (C153) is highly heterogeneous and the local friction is different for probe molecules in different regions. The values of D(t) is significantly smaller than that for the same dye in bulk water (550 μm(2) s(-1)). This suggests that the probe is located in the interface or membrane region rather than in the water pool of the vesicle. The solvation time of C153 in different regions of the lipid vesicle varies between 750 to 1200 ps. This result clearly shows that a confocal microscope is able to resolve the spatial heterogeneity in local friction (i.e., D(t)) and solvation dynamics within a lipid vesicle.     

Six-coordinate titanium complexes of a tripodal aminetris(phenoxide) ligand: synthesis, structure, and dynamics

The five-coordinate titanium(IV) alkoxide LTi(O(t)Bu) (LH(3) = tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine) is protonolyzed readily by the conjugate acids of monoanionic bidentate ligands, both symmetrical (tropolone, acetylacetone, di-p-toluoylmethane) and unsymmetrical (8-hydroxyquinoline, salicylaldehyde, 2,6-diformyl-p-cresol, anthrarufin). The geometry of these complexes, which is pseudo-octahedral with the tripodal ligand adopting a chiral, propeller-like conformation, has been confirmed in four cases by X-ray crystallography. Variable-temperature NMR spectroscopy indicates that the six-coordinate complexes undergo two dynamic processes. First, the ligands undergo a twisting motion that results in racemization, a process which is over 10(4) times faster than in five-coordinate complexes. The rate acceleration upon binding of an equatorial ligand is ascribed to steric repulsions with one of the cis phenoxides; the dynamics of a binuclear dibenzyl phosphate-bridged compound, which has a unique conformation of the tripodal ligand, indicates that flexing the cis phenoxide is the rate-limiting step in racemization. Second, the complexes undergo a process that interchanges the inequivalent arms of the tripodal ligand. This process involves a trigonal twist that shifts the bidentate ligand between clefts in the tripod. The intermediate geometry in the reaction appears to be a transition state and not a long-lived intermediate, as judged from the relative rates of interconversion of tripod arms and chelate ends in the ditoluoylmethane complex. Tripod arm interchange takes place without partial dissociation of the bidentate chelate, a reaction that has been observed on a slower time scale in one case.     

Intense photoluminescence from mixed solutions of C70 and palladium octaethylporphyrin: A supramolecular heavy atom effect

Unusually intense near-infrared (near-IR) photoluminescence has been observed from mixed solutions of C70 and palladium octaethylporphyrin (PdOEP). The novel emission has a spectrum similar to C70 phosphorescence and an intensity that is approximately 20 times greater than that of C70 fluorescence. The emitting species is identified as a noncovalently bound, short-lived triplet exciplex of C70 with PdOEP. The emission is essentially C70 phosphorescence intensified by spin-orbit coupling from the Pd atom in the nearby metalloporphyrin. This supramolecular heavy atom effect increases the C70 emissive quantum yield to approximately 1 x 10(-2) in degassed hexane solution at room temperature. The radiative rate constant is enhanced by a factor of 10(5), to approximately 7 x 10(4) s(-1), which is a value that exceeds the phosphorescence rate constant of PdOEP. Comparative studies in a rigid poly(methyl methacrylate) (PMMA) matrix show that the excited state of the static C70-PdOEP complex decays in approximately 150 ns. A Job's plot analysis shows that the complex has a 1:1 stoichiometry. It forms dynamically in solution and is relatively weakly bound, with an estimated equilibrium constant near 100 M(-1). Qualitatively similar supramolecular heavy atom effects were also observed for complexes of PdOEP with C60 and fullerene derivatives.     

Kinetic analysis of a photosensitive chelator and its complex with Zn(II)

The reversible sequestration and release of metal ions is an important objective in biological and environmental research. Unfortunately, although there have been dramatic examples of metal ion activity control, there are very few quantitative investigations of stoichiometry, equilibria and kinetics. A significant contributor to this lack of quantitative work is the complexity of many photochromic systems. Therefore, we have attempted to create a simple, reversible photochromic metal-ion chelator that can be analyzed quantitatively. The chelator should have certain other attributes as well, namely, that it binds to divalent metal ions (because of their extreme biological importance) and that it binds metal ions in the dark so that light is used to release metal ions rather than sequester them. The photochromic chelator (1) binds to divalent metal ions [Zn(II), Cu(II), Pb(II), Hg(II), Fe(II), Co(II) and Cd(II); other metal ions have not yet been tested] in the dark with a significant binding strength. In both methanol (by spectrophotometry) and methanol-water (by voltammetry), the stoichiometry of the 1-Zn(II) complex is 2:1. The binding constant (K1K2) is on the order of 10(12)-10(14) M(-2) in methanol and 5.0 x 10(8) M(-2) in 50% aqueous methanol. The chelator 1 is photolabile, yielding 2 with a quantum efficiency of 0.91. In a solution containing excess Zn(II), so that over 99% of the ligand exists as the monodentate complex, photolysis produces 2 with a quantum efficiency of 0.15. A kinetic analysis leads to the conclusion that the complex itself is photolabile.     

Alkyne haloallylation [with Pd(II)] as a core strategy for macrocycle synthesis: a total synthesis of (-)-haterumalide NA/(-)-oocydin A

The rarely used haloallylation reaction, first described by Kaneda and Teranishi in 1974, employs a Pd(II) catalyst to join an alkyne with an allylic halide to produce a 1-halo-1,4-diene subunit. It is shown here that functionalized and tertiary allylic chlorides perform well as substrates in this reaction under the action of PdCl2(PhCN)2 in THF solution. When the alkyne is added slowly to the reaction mixture, the two reactants can be used in a nearly equimolar ratio. This fact means (i) that reasonably complex pairs of alkyne and allylic halide substrates are tolerated and, therefore, (ii) that an intramolecular version of the reaction is suitable as a core strategy for complex molecule construction. The latter is demonstrated in the macrocyclization of 2b to 17b, which is the central step in the total synthesis of (-)-haterumalide NA/(-)-oocydin A (1) that is reported. The final key to the completion of the synthesis was the choice of the acid-labile PMB ester of 1 as the penultimate intermediate.     

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.     

液态Al80Fe20在快速冷却中的MD模拟

通过Tight binding (TB)势的分子动力学模拟分析了Al80Fe20合金熔体的中程有序结构以及快速凝固过程中体系微观结构的演变规律，发现在倒空间，其结构因子的小角部分都出现了一个预峰.在平衡态（1450 K），模拟结果得到了X射线衍射实验的进一步印证，这被认为是体系中存在中程有序的标志.随着温度的降低，预峰的高度逐渐增大，说明体系中原子团簇尺寸越来越大.通过运用键对分析技术和键取向序参数，发现体系中存在着大量的二十面体短程有序单元.在对平衡态化学短程序参数α的计算过程中，得到了负值的α，证实了熔体中存在着较强的化学序.在FZ偏结构因子中， SAl Fe(Q)在400 K的第二峰较之SAl Al(Q)和SFe Fe(Q)发生了更为明显的劈裂，表明在非晶形成能力方面， Al、Fe元素之间的轨道杂化作用比Al元素或Fe元素单独作用要强.从BT偏结构因子中我们也发现，表征化学序的SCC(Q)在17.5 nm－1左右处出现了第一峰，而这个峰位恰是总结构因子中出现预峰的峰位.因此可推断，正是体系中的化学序导致了中程有序结构的产生.     

1,2-bis(4-Hydroxyphenyl)-3,4-dihydro-6-hydroxynaphthalene, a photofluorogenic ligand for the estrogen receptor

Abstract— 1,2-bis(4-Hydroxyphenyl)-3,4-dihydro-6-hydroxynaphthalene (6) is a non-steroidal ligand for the estrogen receptor that displays photofluorogenic properties. It binds to the receptor with an affinity greater than that of estradiol (150%), and it is esentially non-fluorescent. However, upon brief UV irradiation in protic media it is converted efficiently to an aryl tetraenedione system (8) that is highly fluorescent, but has low affinity for the receptor (0.35%). The aryltetraenedione has a complex electronic spectrum, with absorbance (and fluorescence excitation) bands extending beyond 450 nm. Both the maxima (ε_max) and the intensity (φ_f of the fluorescence emission of the aryl tetraenedione are solvent dependent. The tetraenedione (8) is structurally related to a tetraenedione (13) obtained by photocyclization of diethylstilbestrol(l0); since tetraenedione 13 is essentially non-fluorescent, it appears that the aryl dienone chromophore in 8 is responsible for its fluorescence. Photolysis of 6 in non-protic media results in a direct photocyclization-oxidation to benzodihydrochrysene system 9 that is unstable and has less desirable fluorescent properties. The formation of 8 from 6 in protic media (as contrasted to the formation of 9 in non-protic media) is thought to arise from the competition that operates on the primary photocyclization product, dihydrophenanthrene (7): In protic media, ketonization proceeds more rapidly than oxidation (aromatization), while in aprotic media, oxidation is more rapid. This photofluorogenic ligand for the estrogen receptor should enable interesting fluorescence binding studies with the estrogen receptor to be performed.     

New aspects of the photo-oxidation and photo-stabilization of polymers

The mechanism of the photo-oxidation of an ethylene-propylene random copolymer (EPM) has been compared with that of a styrene-butadiene-styrene block copolymer (SBS). The former is considered as a model for polyolefins that undergo chain scission on irradiation and the latter as a model for unsaturated rubbers that become cross-linked. A hindered amine light stabilizer (HALS) that is a better inhibitor of the absorption of oxygen in SBS than in EPM is, however, less efficient to prevent formation of carbonyl groups and change of the mechanical properties. These results are discussed and an explanation is proposed.     

Electrokinetic transport of rigid macroions in the thin double layer limit: a boundary element approach

A boundary element (BE) procedure is developed to numerically calculate the electrophoretic mobility of highly charged, rigid model macroions in the thin double layer regime based on the continuum primitive model. The procedure is based on that of O'Brien (R.W. O'Brien, J. Colloid Interface Sci. 92 (1983) 204). The advantage of the present procedure over existing BE methodologies that are applicable to rigid model macroions in general (S. Allison, Macromolecules 29 (1996) 7391) is that computationally time consuming integrations over a large number of volume elements that surround the model particle are completely avoided. The procedure is tested by comparing the mobilities derived from it with independent theory of the mobility of spheres of radius a in a salt solution with Debye-Huckel screening parameter, kappa. The procedure is shown to yield accurate mobilities provided (kappa)a exceeds approximately 50. The methodology is most relevant to model macroions of mean linear dimension, L, with 1000>(kappa)L>100 and reduced absolute zeta potential (q|zeta|/k(B)T) greater than 1.0. The procedure is then applied to the compact form of high molecular weight, duplex DNA that is formed in the presence of the trivalent counterion, spermidine, under low salt conditions. For T4 DNA (166,000 base pairs), the compact form is modeled as a sphere (diameter=600 nm) and as a toroid (largest linear dimension=600 nm). In order to reconcile experimental and model mobilities, approximately 95% of the DNA phosphates must be neutralized by bound counterions. This interpretation, based on electrokinetics, is consistent with independent studies.     

An intraline of conical intersections for methylamine

In this article are considered the conical intersections (ci's) related to the N-H bond in the methylamine, CH(3)NH(2), molecule. The novel feature that was revealed is that the two lowest states 1A(') and 1A(") are coupled by a line of cis located in HC-NHH plane-a line that is formed by moving a single hydrogen on that plane while fixing the (six) other atoms. The validity of this line was proven first by studying the singularities of the (angular) nonadiabatic coupling terms and then by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). A theoretical analysis indicated that the line has to be a finite closed line. We also calculated the Berry phase for a contour that surrounds this line and found it to be 3.127 rad, namely, a value reasonably close to pi. The existence of such lines of cis-instead of isolated cis (as exhibited by other n-atomic (n>3) molecules such as HNCO or C(2)H(2))-may enhance significantly the transition rate from an upper adiabatic state to a lower one. There are also numerical advantages in such situations, that is, if such a line is properly placed in that plane (like in the present case) the wave-packet treatment of the nuclei can be carried out employing a single diabatic PES instead of having to consider two coupled PESs.