Oligonucleotide charge states in negative ionization electrospray-mass spectrometry are a function of solution ammonium ion concentration

The charge state distribution for oligonucleotides detected using negative ionization electro-spray-mass spectrometry has been studied as a function of solution counterion concentration. In the absence of added buffer, an average charge state (Z) of ?7.2 is observed for a 10 µM aqueous solution of a 14mer DNA at pH 7.0, with [M ? 8H]^8? the most abundant ion. As the solution concentration of ammonium acetate increases from 0.1 to 33 mM, Z shifts to ?3.8 with [M ? 4H]^4? the most abundant charge state. The shift in most abundant charge state from [M ? 7H]^7? to [M ? 4H]^4? occurs abruptly between 1.0 and 10 mM NH₄OAc. Above 100 mM NH₄OAc, the value of Z plateaus at ?3.1, with [M ? 3H]^3? the most abundant charge state. The addition of 1–50 mM glycine to the analyte solution does not alter Z, suggesting that the changes in charge state observed by using ammonium acetate result from a solution equilibration of cations around the DNA strand, rather than nonspecific gas-phase proton transfers during the electrospray process. The fraction of neutralized phosphate groups reaches a maximum of 0.79 ± 0.03 independent of length and sequence.     

Coordination Compounds of Schiff-Base Ligands Derived from Diaminomaleonitrile (DMN): Mononuclear, Dinuclear, and Macrocyclic Derivatives

Copper(II) and V(IV)O complexes of an open chain (1:2) Schiff-base ligand (H(2)L1), derived by the template condensation of diaminomaleonitrile (DMN) and salicylaldehyde, and dicopper(II) complexes of (2:2) macrocyclic Schiff-base ligands derived by template condensation of diformylphenols and diaminomaleonitrile, have been synthesized and studied. Structures have been established for the first time for mononuclear Cu(II) and V(IV)O derivatives of the open chain ligand H(2)L1 (1:2), a dinuclear macrocyclic Cu(II) complex derived from a 2:2 macrocyclic ligand (H(2)M1), and the half-condensed 1:1 salicylaldehyde ligand (H(2)L2). [Cu(L1)] (1) (L1 = C(18)H(10)N(4)O(2)) crystallized in the monoclinic system, space group P2(1)/n (No. 14), with a = 11.753(6) ?, b = 7.708(5) ?, c = 16.820(1) ?, and Z = 4. [VO(L1)(DMSO] (2) crystallized in the orthorhombic system, space group Pbca (No. 61), with a = 22.534(9) ?, b = 23.31(1) ?, c = 7.694(5) ?, and Z = 8. H(2)L2 (C(18)H(8)N(4)O) (3) crystallized in the monoclinic system, space group P2(1)/c (No. 14), with a = 13.004(6) ?, b = 11.441(7) ?, c = 7.030(4) ?, and Z = 4. [Cu(2)(M3)](CH(3)COCH(3)) (4) (M3 = C(32)H(24)N(8)O(4)) crystallized in the monoclinic system, space group C2/c (No. 15), with a = 38.33(2) ?, b = 8.059(4) ?, c = 22.67(2) ?, and Z = 8. [Cu(L3)(DMSO)] (5) (L3 = C(20)H(14)N(2)O(4)) crystallized in the triclinic system, space group P&onemacr; (No. 2), with a = 10.236(4) ?, b = 13.514(4) ?, c = 9.655(4) ?, and Z = 2. 4 results from the unique addition of two acetone molecules to two imine sites in [Cu(2)(M1)](ClO(4))(2) (M1 = 2:2 macrocyclic ligand derived from template condensation of DMN and 2,6-diformyl-4-methylphenol). 4 has extremely small Cu-OPh-Cu bridge angles (92.0, 92.8 degrees ), well below the expected lower limit for antiferromagnetic behavior, but is still antiferromagnetically coupled (-2J = 25.2 cm(-)(1)). This behavior is associated with a possible antiferromagnetic exchange term that involves the conjugated framework of the macrocyclic ligand itself. The ligand L3 in 5 results from hydrolysis of M1 on recrystallization of [Cu(2)(M1)](ClO(4))(2) from undried dimethyl sulfoxide.     

Geminal interaction and effect of “positive charge”

Quantum-chemical calculations of the molecules containing Cl-C-M group (M=C, N, O, F) were carried out using RHF/6-31G(d) and MP2/6-31+G(d) methods. The calculations are aimed at verifying concept on “positive charge” effect of the central atom of the group suggested as understanding of non-inductive effect of heteroatom M on the Cl (Y) atom in Y-Z-M moieties (geminal interaction). It is shown that C atom in such systems bears negative or small positive charge, and hence “positive charge” effect is excluded. The non-inductive effect originates from the polarization of Cl-C (Y-Z) bonds due to the influence of the charge on any M atom directly through space on any Y and Z.     

One metal and forty nitrogens. Ab initio predictions for possible new high-energy pentazolides

High-energy nitrogen-rich pentazolides of groups 6 and 13-16 are studied theoretically. Many of them have experimentally known azide analogues. Our highest nitrogen-to-element ratio of 40:1 is achieved in the systems [M(N5)8](2-) (M=Cr, Mo, W). The thermodynamic and kinetic stability of the studied systems grows with the negative charge on the system and is highest for tetra-pentazolides and hexa-pentazolides of B, Al, and Si. Systems such as B(N5)4- or Si(N5)6(2-) are examples of the most stable candidates for these new species. N(N5)2- is a candidate for a new all-nitrogen system. Neutral and positive systems were less stable. Pentazole derivatives of "dinuclear" C2Hn and N2Hn systems were investigated and were found to be of comparable stability as their "mononuclear" analogues. Pentazole derivatives of benzene, the C6H(6-n)(N5)n (n=2, 3, 6) systems, have a similar stability as the experimentally known phenylpentazole. A borazine analogue, N3B3H3(N5)3 is predicted to be one of the most stable systems of this family.     

New two-dimensional metal-organic networks constructed from 1,2,4,5-benzenetetracarboxylate and chelate ligands

Two novel nickel coordination polymers [Ni(2)(2,2'-bipy)(2)(OH)(2)(H(2)btec)](3)(n)(1) and [Ni(2)(1,10'-phen)(2)(H(2)O)(2)(btec)](n)(2) (btec = 1,2,4,5-benzenetetracarboxylate) have been hydrothermally synthesized and characterized by elemental analyses, IR and XPS spectra, TG analysis, X-ray powder diffraction, and single crystal X-ray diffraction. Crystal data for 1: C(90)H(66)N(12)O(30)Ni(6), monoclinic P2(1)/c, a = 10.905(2) A, b = 18.006(4) A, c = 20.551(4) A, beta = 94.91(3) degrees, Z = 2. Crystal data for 2: C(34)H(22)N(4)O(10)Ni(2), monoclinic P2(1)/n, a = 10.122(2) A, b = 9.3106(19) A, c = 15.690(3) A, beta = 92.03(3) degrees, Z = 2. Compound 1 exhibits a novel one-dimensional chainlike structure, in which the dinuclear Ni centers are linked by the btec ligands. Furthermore, the adjacent chains are linked into a 2-D wavelike layer via the strong OH.O hydrogen bonding interactions. Compound 2 possesses an unusual two-dimensional steplike network with interesting rhombic grids. Both compounds exhibit unprecedented metal-organic ligand construction modes in [M/btec/L] (M = transition metal; L = chelate ligands) systems. The magnetic behaviors of compounds 1 and 2 have been studied.     

Chitin and chitin-cellulose composite hydrogels prepared by ionic liquid-based process as the novel electrolytes for electrochemical capacitors

This paper reports on the preparation and electrochemical performance of chitin- and chitin-cellulose-based hydrogel electrolytes. The materials were prepared by a casting solution technique using ionic liquid-based solvents. The method of chitin dissolution in ionic liquid with the assistance of dimethyl sulfoxide co-solvent was investigated. The obtained membranes were soaked with 1-M lithium sulfate aqueous solution. The prepared materials were preliminarily characterized in terms of structural and physicochemical properties. Further, the most promising biopolymer membranes were assembled with activated carbon cloth electrodes in symmetric electrochemical capacitor cells. The electrochemical performances of these devices were studied in a 2-electrode system by commonly known electrochemical techniques, such as cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The devices operated at a maximum voltage of 0.8 V. All the investigated materials have shown high efficiency in terms of specific capacitance, power density, and cyclability. The studied capacitors exhibited specific capacitance values in the range of 92–98 F g⁻¹, with excellent capacitance retention (ca. 97–98%) after 20,000 galvanostatic charge and discharge cycles. Taking into account the above information and the eco-friendly nature of the biopolymer, it appears that the prepared chitin- and chitin-cellulose-based hydrogel electrolytes can be promising components for green electrochemical capacitors.

     

Contact angle determination of nanoparticles: film balance and scanning angle reflectometry studies

St?ber silica nanoparticles of diameter about 45, 60 and 100 nm and different hydrophobicity are used to produce monolayers at a water-air interface. Both the surface pressure-area isotherms and the reflectivity angle of incidence curves of the layers have been measured in a Wilhelmy film balance. The contact angle of the as-prepared particles have been determined from the isotherms by two different evaluation methods, and compared to those obtained from in situ scanning angle reflectometry (SAR) measurements. SAR is proved to be an effective tool for the estimation of contact angles on nanoparticles of different wettability, using a modified version of the previously published gradient layer model (E. Hild, T. Seszták, D. V?lgyes and Z. Hórv?lgyi, Prog. Colloid Polym. Sci., 2004, 125, 61, ref. 1) for evaluation. The results are in fairly good agreement with those determined from the non-dissipative part of the isotherms of the as prepared particles, assuming a weakly cohesive film model (S. Bordács, A. Agod and Z. Hórv?lgyi, Langmuir, 2006, 22, 6944, ref. 2). It seems that the traditional way to calculate the contact angle from the film balance experiments (J.H. Clint and N. Quirke, Colloids Surf., A, 1993, 78, 277, ref. 3) results in unreasonably high contact angles for the investigated systems and the homogeneous layer optical model gives unrealistic film thickness values in the case of hydrophobic particles.     

Eigenmobilities in background electrolytes for capillary zone electrophoresis: III. Linear theory of electromigration

A mathematical model of capillary zone electrophoresis (CZE) based on the conception of eigenmobilities, which are the eigenvalues of a matrix M tied to the linearized governing equations is presented. The model considers CZE systems, where constituents, either analytes or components of the background electrolyte (BGE), are weak electrolytes--acids, bases, or ampholytes. There is no restriction on the number of components nor on the valence of the constituents nor on pH of the BGE. An electrophoretic system with N constituents has N eigenmobilities. In most BGEs one or two eigenmobilities are very close to zero so their corresponding eigenzones move very slowly. However, there are BGEs where no eigenmobility is close to zero. The mathematical model further provides: the transfer ratio, the molar conductivity detection response, and the relative velocity slope. This allows the assessment of the indirect detection, conductivity detection and peak broadening (distortion) due to electromigration dispersion. Also, we present a spectral decomposition of the matrix M to matrices allowing the assessment of the amplitudes of system eigenpeaks (system peaks). Our model predicted the existence of BGEs having no stationary injection zone (or water zone, gap, peak, dip). A common practice of using the injection zone as a marker of the electroosmotic flow must fail in such electrolytes.     

Singlet diradical complexes of chromium, molybdenum, and tungsten with azo anion radical ligands from M(CO)6 precursors

The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.     

Analysis and modeling of flow in rotating spiral microchannels: towards math-aided design of microfluidic systems using centrifugal pumping

This paper describes the experimental measurement and mathematical modeling of centrifugally-pumped flow in spiral microchannels. Here, the liquid is delivered by the rotation of a circular microchip as depicted before (X. Y. Peng, P. C. H. Li, H. Z. Yu, M. Parameswaran and W. L. Chou, Sens. Actuators, B, 2007, 128, 64-69). The spiral microchannel in it was specially designed to produce a constant centrifugal force component. From experimental measurements, it was found that the flow velocity inside the spiral microchannels was associated with the rotation speed only, but not with the length of the liquid column. The mathematical modeling of liquid flow was constructed based on solving the Navier-Stokes equations of incompressible flow formulated in a new orthogonal curvilinear coordinate system aligned with the channel geometry. The governing equations were simplified under various assumptions, rendering a mathematically-tractable physical model. In addition, a commercial computational fluid dynamics (CFD) program was used to simulate the flow in the spiral microchannel. The predicted liquid flow velocities from the mathematical model and the CFD program showed reasonable agreement with the experimental data. Under proper assumptions, the mathematical model gave a flexible and rather accurate analytical solution using much less computing power. The proposed study demonstrated the effectiveness of the spiral microchannel design in microfluidic applications using centrifugal force. With modifications, this study could be adapted to the simulation and modeling of other centrifugal-pumping microflow systems.     

Synthesis and characterization of diverse coordination polymers. Linear and zigzag chains involving their structural transformation via intermolecular hydrogen-bonded, interpenetrating ladders polycatenane, and noninterpenetrating square grid from long, rigid N,N'-bidentate ligands: 1,4-bis[(x-pyridyl)ethynyl]benzene (x = 3 and 4)

The long, rigid ligands 1,4-bis[(3-pyridyl)ethynyl]benzene (L1) and 1,4-bis[(4-pyridyl)ethynyl]benzene (L2) were used in the synthesis of 10 new organic-inorganic coordination frameworks, each of them adopting different structural motifs. Synthesis, single-crystal X-ray structure determination, and spectroscopic and thermogravimetric analyses are presented. The reactions between M(NO3)2 x xH2O; M = Cd(II), Cu(II), and Co(II); x = 3-6 and Cu(hfac)2 x H2O [hfac = bis(hexafluoroacetylacetonato)] with L1 afforded the following one-dimensional zigzag chain structures: [Cd(C20H12N2)0.5(NO3)(CH3OH)]n (1, monoclinic, C2/c; a = 7.586(1) A, b = 23.222(1) A, c = 13.572(1) A, beta = 92.824(1), Z = 4); [{Cu(C20H12N2)(NO3)2(CH3OH)} x CH3OH]n (2, orthorhombic, P2(1)2(1)2(1); a = 8.589(1) A, b = 15.766(1) A, c = 17.501(1) A, Z = 4); [Co(C20H12N2)2(NO3)2(H2O)2] (5, triclinic, P1; a = 7.493(1) A, b = 8.948(1) A, c = 14.854(1) A, alpha = 100.427(1), beta = 97.324(1), gamma = 110.901(1), Z = 1); [Cu(C20H12N2)(hfac)2]n (4, monoclinic, C2/c, a = 18.828(1) A, b = 14.671(1) A, c = 13.427(1) A, beta = 90.447(1) degrees, Z = 4). Moreover, the minority phase compound formed from Cu(NO3)2 x 3H2O and L1 yielded a metallocyclic chain structure, [Cu(C20H12N2)(NO3)]n (3, triclinic, P; a = 8.728(1) A, b = 10.018(1) A, c = 11.893(1) A, alpha = 109.991(1), beta = 97.109(1), gamma = 115.542(1), Z = 1). In addition to the dinuclear coordination complex 5, all other polymeric structures (1-4) from L1 are composed of interpenetrating 2D and 3D cross-linked zigzag chains via hydrogen-bonding interactions. The reactions between M(NO3)2 x xH2O; M = Cd(II), Cu(II), and Co(II); x = 3-6 and Cu(hfac)2 x H2O [hfac = bis(hexafluoroacetylacetonato)] and L2 were dependent on the nature of the metal center and resulted in the formation of four different interpenetrating and noninterpenetrating compounds (6-10): [Co(C20H12N2)1.5(NO3)2]n (6, triclinic, P; a = 14.172(1) A, b = 15.795(1) A, c = 18.072(1) A, alpha = 115.380(1), beta = 101.319(1), gamma = 93.427(2), Z = 4), which consists of T-shaped building blocks assembled into three-dimensional interpenetrating polycatenated ladders; [Cd(C20H12N2)2(NO3)2]n (7, monoclinic, I2/a; a = 11.371(1) A, b = 20.311(2) A, c = 15.240(2) A, beta = 100.201(2) degrees, Z = 4), which adopts a two-dimensional noninterpenetrating square-grid motif; [Cu(C20H12N2)(hfac)2]n (8, monoclinic, I2/a; a = 11.371(1) A, b = 20.311(2) A, c = 15.240(2) A, beta = 100.201(2) degrees, Z = 4), composed of three sets of distinct one-dimensional linear chains; [Cu(C20H12N2)(EtOH)(NO3)2] [Cu(C20H12N2)1.5(NO3)2] x 2EtOH (9, triclinic, P; a = 12.248(2) A, b = 13.711(3) A, c = 18.257(4) A, alpha = 108.078(4) degrees, beta = 97.890(4) degrees, gamma = 103.139(5) degrees, Z = 2) and [Cu(C20H12N2)(MeOH)(NO3)2] [Cu(C20H12N2)1.5(NO3)2] x 2MeOH (10, triclinic, P; a = 12.136(1) A, b = 13.738(2) A, c = 17.563(3) A, alpha = 107.663(3) degrees, beta = 94.805(4) degrees, gamma = 104.021(4) degrees, Z = 2). Both 9 and 10 stack into infinite interpenetrating ladders through bundles of infinite chains and are described in our preliminary communication.     

Reactions of ions of iron and cobalt acetylacetonates with macrocyclic thioiurea derivatives in the gas phase

Ion-molecule reactions of (acac)₂M⁺ ions (M = Fe, Co) with macrocyclic thiourea derivatives (L) were studied. The formation of acacML⁺ and acacM(L – H)⁺ were observed. AcacM(L – H)⁺ ions arise from loss of the hydrogen atom of the NH group. The ion peak intensity ratios, Z = [acacML⁺]/[acacM(L – H)⁺], for M = Co are higher than those for M = Fe, in accordance with the easier reduction of Co(III) to Co(II) than that of Fe(III) to Fe(II). It is proposed that the metal-containing ions preferentially attack the thiourea fragment of the macro-cycles studied.     

电解质和阳离子表面活性剂对MMH/粘土体系胶体性能的影响

通过流变参数、电泳淌度和pH值的测定,研究了NaCl、MgCl₂、Na₂SO₄等电解质和阳离子表面活性剂(十六烷基三甲基溴化铵)对铝镁混合金属氢氧化物(MMH)/钠土(Mt)悬浮体胶体性能的影响.发现所研究的电解质和表面活性剂都能使MMH/Mt悬浮体的Bingham屈服值降低,但不同处理剂影响pH值和滤失量的变化趋势却不同.认为电解质中的阳离子以影响粘土颗粒间的缔合为主,而阴离子以影响MMH的荷电性能为主,前者使滤失量上升,pH值下降;后者使滤失量下降,pH值上升.电解质对MMH/Mt悬浮体滤失量和pH值的影响取决于二者相对能力的大小.阳离子表面活性剂由于在粘土上吸附后不仅影响颗粒之问的缔合,也能使其润湿性反转,导致MMH/Mt悬浮体的滤失量和Bingham屈服值的变化幅度明显高于无机电解质.     

Impact of Covalent Modifications on the Hydrogen Bond Strengths in Diaminotriazine Supramolecules

Melamine (M) is a popular triamine triazine compound in the field of supramolecular materials. In this work, we have computationally investigated how substituents can be exploited to improve the binding strength of M supramolecules. Two types of covalent modifications were studied: the substitution of an H atom within an amine group −NHR, and the replacement of the whole −NH₂ group (R=H, F, CH₃ and COCH₃). Through our dispersion-corrected density functional theory computations, we explain which covalent modification will show the best self-assembling capabilities, and why the binding energy is enhanced. Our charge density and molecular orbital analyses indicate that the best substituents are those that generate a charge accumulation on the endocyclic N atom, providing an improvement of the electrostatic attraction. At the same time the substituent assists the main N−H⋅⋅⋅N hydrogen bonds by interacting with the amino group of the other monomer. We also show how the selected group notably boosts the strength of hexameric rosettes. This research, therefore, provides molecular tools for the rational design of emerging materials based on uneven hydrogen-bonded arrangements.     

Investigating CN- cleavage by three-coordinate M[N(R)Ar]3 complexes

Three-coordinate Mo[N((t)Bu)Ar]3 binds cyanide to form the intermediate [Ar((t)Bu)N]3Mo-CN-Mo[N((t)Bu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2]3Mo/CN- system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol(-1), accounting for why C-N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN- system where the intermediate and products are closer in energy but not enough for CN- cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN- /W[NH2]3 and [H2N]3Re+/CN-/Ta[NH2]3 systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN /W[NH2]3 system are more favourable than those for the [H2N]3W/CN- system, the final C-N cleavage step is still endothermic by 32 kJ mol(-1) when symmetry constraints are relaxed. The resistance of these systems to C-N cleavage was examined by a bond decomposition analysis of [H2N]M-L1[triple bond]L2-M[NH2]3 intermediates for L1[triple bond]L2 = N2, CO and CN which showed that backbonding from the metal into the L1[triple bond]L2 pi* orbitals is significantly less for CN than for N2 or CO due to the negative charge on CN- which results in a large energy gap between the metal d(pi), and the pi* orbitals of CN-. This, combined with the very strong M-CN- interaction which stabilises the CN intermediate, makes C-N bond cleavage in these systems unfavourable even though the C[triple bond]N triple bond is not as strong as the bond in N2 or CO.     

The role of electronic coupling in linear porphyrin arrays probed by single-molecule fluorescence spectroscopy

Single-molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single-molecule fluorescence detection techniques. Butadiyne-linked arrays (Z(N)B) with extensive π-conjugation perform as photostable one-quantum systems. This demonstration has been suggested by the long-lasting initial emissive state and subsequent discrete one-step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one-quantum system, Z(N)B shows anti-bunching data in the coincidence measurements. On the other hand, in directly-linked arrays (Z(N)) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of Z(N) do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron-hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single-molecule level.     

Salt-induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl-borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.     

Ion Trap Collision-Induced Dissociation of Human Hemoglobin α-Chain Cations

Multiply protonated human hemoglobin alpha-chain species, ranging from [M + 4H]4+ to [M + 20H]20+, have been subjected to ion trap collisional activation. Cleavages at 88 of the 140 peptide bonds were indicated, summed over all charge states, although most product ion signals were concentrated in a significantly smaller number of channels. Consistent with previous whole protein ion dissociation studies conducted under similar conditions, the structural information inherent to a given precursor ion was highly sensitive to charge state. A strongly dominant cleavage at D75/M76, also noted previously in beam-type collisional activation studies, was observed for the [M + 8H]8+ to [M + 11H]11+ precursor ions. At lower charge states, C-terminal aspartic acid cleavages were also prominent but the most abundant products did not arise from the D75/M76 channel. The [M + 12H]12+-[M + 16H]16+ precursor ions generally yielded the greatest variety of amide bond cleavages. With the exception of the [M + 4H]4+ ion, all charge states showed cleavage at the L113/P114 bond. This cleavage proved to be the most prominent dissociation for charge states [M + 14H]14+ and higher. The diversity of dissociation channels observed within the charge state range studied potentially provides the opportunity to localize residues associated with variants via a top-down tandem mass spectrometry approach.