HUMIC MACROMOLECULES IN NATURAL WATERS

Abstract

Humic substances are the major organic constituents of soils and sediments. They also occur in small concentrations in natural surface waters and groundwaters. They form through the breakdown of plant and animal tissues by chemical and biological processes that tend to produce complex chemical structures that are more stable than the original material from which they were derived. One of the more important characteristics of humic substances is their ability to form water-soluble and water-insoluble complexes with metal ions and hydrous oxides and to interact with clay minerals and various organic compounds such as alkanes, fatty acids, and toxic organic substances such as pesticides.     

Interaction between brush layers of bottle-brush polyelectrolytes: molecular dynamics simulations

Interactions between tethered layers composed of aggrecan (charged bottle-brush) macromolecules are responsible for the molecular origin of cartilage biomechanical behavior. To elucidate the role of the electrostatic forces in interaction between bottle-brush layers, we have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates. In the case of charged bottle-brush layers, the disjoining pressure P between two brush layers in salt-free solutions increases with decreasing distance D between substrates as P ∝ D(-1.8). A stronger dependence of the disjoining pressure P on the surface separation D was observed for neutral bottle-brushes, P ∝ D(-4.6), in the same interval of disjoining pressures. These scaling laws for dependence of disjoining pressure P on distance D are due to bending energy of the bottle-brush macromolecules within compressed brush layers. The weaker distance dependence observed in polyelectrolyte bottle-brushes is due to interaction between counterion clouds surrounding the bottle-brush macromolecules preventing strong brush overlap.     

Competitive FRET-Aptamer-Based Detection of Methylphosphonic Acid,a Common Nerve Agent Metabolite

Competitive fluorescence resonance energy transfer (FRET)-aptamer-based assay formats are described for one-step detection of methylphosphonic acid (MPA; a metabolite of several organophosphorus (OP) nerve agents). AminoMPA was attached to tosyl-magnetic beads and used for DNA aptamer selection from which one dominant aptamer sequence emerged. Two different FRET approaches were attempted. In one approach, the complementary DNA sequence was used as a template for labeling the aptamer with Alexa Fluor 546 (AF 546)-14-dUTP by asymmetric PCR. Following 3-dimensional (3-D), molecular modeling of the aptamer-MPA complex, a series of three fluoresceinated aptamers labeled at positions 50, 51, and 52 in the putative optimal binding pocket were synthesized. In both FRET formats, aminoMPA was linked to Black Hole Quencher (BHQ-1 or BHQ-2)-succinimides and allowed to bind the fluorescein or AF 546-labeled MPA aptamer. Following gel filtration to purify the labeled MPA aptamer-BHQ-aminoMPA FRET complexes, the complexes were competed against various concentrations of unlabeled MPA, MPA derivatives, and unrelated compounds in titration and cross-reactivity studies. Both approaches yielded low microgram per milliliter detection limits for MPA with generally low levels of cross-reactivity for unrelated compounds. However, the data suggest a pattern of traits that may effect the direction (lights on or off) and intensity of the FRET.     

The single-mode rate equations for semiconductor lasers with thermal effects

A mathematical model describing the coupling of electrical,optical and thermal effects in semiconductor lasers is introduced.Numerical and asymptotic solutions are derived, including expressionsfor key physical quantities such as the initial time delay,the frequency of spike oscillation and the temperature rise,together with its influence on the photon density, the electronconcentration and the threshold current. The consequences ofthermal effects in reducing efficiency are thus quantified.     

Global weak solutions for the initial–boundary-value problems Vlasov–Poisson–Fokker–Planck System

This work is devoted to prove the existence of weak solutions of the kinetic Vlasov–Poisson–Fokker–Planck system in bounded domains for attractive or repulsive forces. Absorbing and reflection-type boundary conditions are considered for the kinetic equation and zero values for the potential on the boundary. The existence of weak solutions is proved for bounded and integrable initial and boundary data with finite energy. The main difficulty of this problem is to obtain an existence theory for the linear equation. This fact is analysed using a variational technique and the theory of elliptic–parabolic equations of second order. The proof of existence for the initial–boundary value problems is carried out following a procedure of regularization and linearization of the problem. © 1998 B. G. Teubner Stuttgart—John Wiley & Sons, Ltd.     

Darstellung,Kristallstrukturen und Schwingungsspektren von Verbindungen mit den linearen Dipnictidoborat(3–)‐Anionen [P–B–P]3–, [As–B–As]3– und [P–B–As]3–

Synthesis, Crystal Structure, and Vibrational Spectra of Compounds with the Linear Dipnictidoborate (3–) Anions [P–B–P]^3–, [As–B–As]^3–, and [P–B–As]^3– The alkali metal boron compounds M₃[BX₂] with X = P, As are synthesized from the alkali metals M and the binary components MX or M₄X₆ and BX in sealed steel ampoules (phosphides) or niobium ampoules (arsenides) at 1000 K. The compounds are obtained as bright yellow prisms (M₃[BP₂]) or plates (K₂Na[BP₂]) and yellow‐red prismatic crystals (M₃[BAs₂], Cs₃[BPAs]) which are very sensitive against oxidation and hydrolysis. Three different structure types are formed, namely K₂Na[BP₂] (C2/m (No. 12); Z = 4; a new mC24 structure type); Na₃[BP₂] (P2₁/c (No. 14); Z = 4, β‐Li₃[BN₂] type), M₃[BX₂] with M = K, Rb, Cs and X = P, As and Cs₃[P–B–As] (C2/c, (No. 15); Z = 4, K₃[BP₂] type). The bond lengths of the linear [BX₂]^3– anions are hardly changed and correspond to a Pauling bond order PBO = 1.9 (d(B–P) = 176.7–177.1 pm; d(B–As) = 186.5–188.0 pm). The vibrational spectra confirm the existence of unmixed and mixed units [P–B–P]^3–, [As–B–As]^3– and [P–B–As]^3– with D_∞h and C_∞v symmetry, respectively. The valence force constants f(B–X) and the corresponding Siebert bond orders, calculated from the frequencies, are discussed and compared with those of the isoelectronic anions and molecules.     

Diorganotin(IV) compounds derived from n‐methyl‐2,2′‐diphenolamine and 2,2′‐diphenolamine

The reaction of N‐methyl‐2,2′‐diphenolamine 1 and 2,2′‐diphenolamine 2 with some diorganotin(IV) oxides [R1/2SnO: R¹ = Me, n‐Bu, t‐Bu and Ph] led to the syntheses of diorgano[N‐methyl‐2,2′‐diphenolato‐O,O′,N]tin (IV) 3–6 and diorgano[2,2′‐diphenolato‐O,O′,N]tin (IV) 7–9 . All compounds (except 7 ) studied in this work were characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, infrared, and mass spectroscopy. Their ¹¹⁹Sn NMR data show that the tin atom is tetracoordinated in CDCl₃ but penta and hexacoordinated in DMSO‐d₆. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 133–139, 1999