Barend van Lagen

Composition of plant tissues and soil organic matter in the first stages of a vegetation succession

Differences in soil organic matter (SOM) composition in consecutive soil profiles (under algae, grass, moss, heather and pine) from a primary vegetation succession (central Netherlands) were investigated by Cross Polarization-Magic Angle Spinning (CPMAS) 13C NMR, chemical degradation (acid hydrolysis of polysaccharides) and pyrolysis-GC/MS (Py-GC/MS) in relation to the vegetation. All mineral soil horizons had lower contents of polysaccharides than the ectorganic layer and/or the fresh plant material as suggested by the diminishing O-alkyl peaks by NMR. The monosaccharide distribution pointed to an increase of microbial sugars in the soil as suggested by the raising C6/C5 ratios. Soil polysaccharides may be accompanied by a significant contribution of carbohydrates from mosses in the first stages, because the undecomposed moss tissues have high C6/C5 ratios, which falsify the usual interpretation that such high ratios may be due to microbial sugars. Fresh litter and ectorganic layers under heather and pine had similar contents of aromatics, but the contribution of lignin (guaiacols and syringols) was lower in the ectorganic horizons. The degree of oxidation, as determined by the acid/aldehyde ratio, increased with progressive humification. All Ah horizons contained less aromatics, more oxidized lignin remnants and higher amounts of aliphatic compounds than the litter. One or two macromolecules, which produced n-alkenes/n-alkanes and branched alkenes/alkanes upon pyrolysis and were derived from mosses, were mainly responsible for the accumulation of aliphatics in soil, although other aliphatic polymers, such as suberin, might also play an important role. Preservation of components from earlier vegetation might significantly influence SOM composition.

GM3, GM2 and GM1 mimics designed for biosensing : chemoenzymatic synthesis, target affinities and 900 MHz NMR analysis

Undec-10-enyl, undec-10-ynyl and 11-azidoundecyl glycoside analogues corresponding to the oligosaccharides of human gangliosides GM3, GM2 and GM1 were synthesized in high yields using glycosyltransferases from Campylobacter jejuni. Due to poor water solubility of the substrates, the reactions were carried out in methanol-water media, which for the first time were shown to be compatible with the C. jejuni alpha-(2-->3)-sialyltransferase (CST-06) and beta-(1-->4)-N-acetylgalactosaminyltransferase (CJL-30). Bioequivalence of our synthetic analogues and natural gangliosides was examined by binding to Vibrio cholerae toxin and to the B subunit of Escherichia coli heat-labile enterotoxin. This bioequivalence was confirmed by binding mouse and human monoclonal antibodies to GM1 and acute phase sera containing IgM and IgG antibodies to GM1 from patients with the immune-mediated polyneuropathy Guillain-Barré syndrome. The synthesized compounds were analyzed by 1D and 2D 900 MHz NMR spectroscopy. TOCSY and DQF-COSY experiments in combination with 13C-1H correlation measurements (HSQC, HMBC) were carried out for primary structural characterization, and a complete assignment of all 1H and 13C chemical shifts is presented.

Hydrogen-bond stabilized columnar discotic benzenetrisamides with pendant triphenylene groups

A series of 1,3,5-benzenetrisamide derivatives with three hexaalkoxytriphenylene pendant groups were prepared, in which the triphenylene groups are connected to the central 1,3,5-benzenetrisamide core through a flexible spacer. The length of this spacer, as well as the size of the ortho-substituent at the triphenylene core, influences the columnar packing of these molecules. All compounds show liquid crystalline behavior with high isotropization temperatures. Different columnar hexagonal phases (Colh, Colhp, Colobl and Colr) have been identified using optical polarization microscopy, differential scanning calorimetry and X-ray diffraction. A chiral 1,3,5-benzenetrisamide derivative forms columnar stacks with a single helical sense, both in an apolar solvent and in a film, as observed by circular dichroism studies. Pulse radiolysis time-resolved microwave conductivity (PR-TRMC) studies show that the high ordering in a Colhp phase results in high charge carrier mobilities. On the other hand, the highest mobility was observed for the chiral compound, which has a Colr ordering.

Mimicking the Silicon Surface: Reactivity of Silyl Radical Cations toward Nucleophiles

Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si-H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si-Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of organic monolayers onto hydrogen-terminated Si surfaces via hydrosilylation leads to the conclusion that the delocalized Si radical cation (a surface-localized hole) can initiate the hydrosilylation chain reaction at the Si surface. Comparison to monolayer experiments shows that this reaction only plays a significant role in the initiation, and not in the propagation steps of Si-C bond making monolayer formation.

Synthesis and Optical Properties of all-trans-Oligodiacetylenes

A new series of pure and highly soluble oligodiacetylenes (ODAs) was synthesized in high yield and on a multi-milligram scale by a sequence of Sonogashira reactions with a strongly reduced level of homocoupling. The lambda max and epsilon max of these ODAs show an increase with both chain elongation and solvent polarity. A plot of lambda max absorption versus 1/CL (CL=conjugation length) was shown to be linear. The lambda max converges to 435 nm for the longest members of the series at micromolar concentration. This reveals that the longest wavelength absorption observed for PDA chains (lambda max up to 700 nm) is due to aggregation effects. The fluorescence quantum yield increased from monomer to trimer and decreased for longer ODAs. A similar trend is found for the lifetime of fluorescence with a maximum of 600 ps for the trimer. The observed linearity of the rotational correlation time with the oligomer length implies that the ODA chains in solution lack significant geometrical changes. This implies that the ODAs in solution are fully stretched molecular rods of up to 4 nm in length.

Postnatal development of depth-dependent collagen density in ovine articular cartilage

BackgroundArticular cartilage (AC) is the layer of tissue that covers the articulating ends of the bones in diarthrodial joints. Adult AC is characterised by a depth-dependent composition and structure of the extracellular matrix that results in depth-dependent mechanical properties, important for the functions of adult AC. Collagen is the most abundant solid component and it affects the mechanical behaviour of AC. The current objective is to quantify the postnatal development of depth-dependent collagen density in sheep (Ovis aries) AC between birth and maturity. We use Fourier transform infra-red micro-spectroscopy to investigate collagen density in 48 sheep divided over ten sample points between birth (stillborn) and maturity (72 weeks). In each animal, we investigate six anatomical sites (caudal, distal and rostral locations at the medial and lateral side of the joint) in the distal metacarpus of a fore leg and a hind leg.ResultsCollagen density increases from birth to maturity up to our last sample point (72 weeks). Collagen density increases at the articular surface from 0.23 g/ml ± 0.06 g/ml (mean ± s.d., n = 48) at 0 weeks to 0.51 g/ml ± 0.10 g/ml (n = 46) at 72 weeks. Maximum collagen density in the deeper cartilage increases from 0.39 g/ml ± 0.08 g/ml (n = 48) at 0 weeks to 0.91 g/ml ± 0.13 g/ml (n = 46) at 72 weeks. Most collagen density profiles at 0 weeks (85%) show a valley, indicating a minimum, in collagen density near the articular surface. At 72 weeks, only 17% of the collagen density profiles show a valley in collagen density near the articular surface. The fraction of profiles with this valley stabilises at 36 weeks.ConclusionsCollagen density in articular cartilage increases in postnatal life with depth-dependent variation, and does not stabilize up to 72 weeks, the last sample point in our study. We find strong evidence for a valley in collagen densities near the articular surface that is present in the youngest animals, but that has disappeared in the oldest animals. We discuss that the retardance valley (as seen with polarised light microscopy) in perinatal animals reflects a decrease in collagen density, as well as a decrease in collagen fibril anisotropy.

Synthesis and Optoelectronic Properties of Nanometer-Sized and Highly Soluble Homocoupled Oligodiacetylenes

A new series of pure, nanometer-sized and highly-soluble homocoupled oligodiacetylenes (HODA) consisting of two symmetrical oligodiacetylene units was synthesized with high yield and on a multi-milligram scale under mild, catalytic Sonogashira conditions. The lambda(max) and the epsilon(max) of absorption for these HODAs show an increase with the chain elongation. The lambda(max) converges to 450 nm for the longest members of the series at micromolar concentration and to 462 nm for thin drop-casted films. An additional red-shifted absorption is observed in the solid state and in solution at low temperatures, which is caused by aggregation. The lambda(max) of the fluorescence emission increases with the chain length and reaches 492 nm for the longest oligomer. The fluorescence quantum yield has its maximum for the shortest oligomer and decreases rapidly for the longer ones. A similar trend is found for the fluorescence lifetime with a maximum of 100 ps for the homocoupled monomer. The rotational correlation time shows a linear increase with the oligomer length. This reveals a significant persistence length and indicates that the HODA molecules are fully stretched molecular rods (up to 8.2 nm).

Organic monolayers from 1-alkynes covalently attached to chromium nitride: alkyl and fluoroalkyl termination.

Strategies to modify chromium nitride (CrN) surfaces are important because of the increasing applications of these materials in various areas such as hybrid electronics, medical implants, diffusion barrier layers, corrosion inhibition, and wettability control. The present work presents the first surface immobilization of alkyl and perfluoro-alkyl (from C6 to C18) chains onto CrN substrates using appropriately functionalized 1-alkynes, yielding covalently bound, high-density organic monolayers with excellent hydrophobic properties and a high degree of short-range order. The obtained monolayers were characterized in detail by water contact angle, X-ray photoelectron spectroscopy (XPS), ellipsometry, and infrared reflection absorption spectroscopy (IRRAS).

Chemoenzymatic synthesis of biotin-appended analogues of gangliosides GM2, GM1, GD1a and GalNAc-GD1a for solid-phase applications and improved ELISA tests

Biotinylated analogues of gangliosides GM2, GM1, GD1a and GalNAc-GD1a were synthesized in high yields using glycosyltransferases from Campylobacter jejuni. The presence of a biotin moiety in the aglycone part of these mimics allows for attachment of these materials onto various streptavidin-coated surfaces. Analysis of the interaction of biotin-appended GM1 with the B subunit of Escherichia coli heat-labile enterotoxin performed in a modified ELISA procedure shows the potential of this compound to replace the natural GM1 in toxin detection.

Hybrid Conjugated Organic Oligomers Consisting of Oligodiacetylene and Thiophene Units: Synthesis and Optical Properties

Novel and highly soluble hybrid conjugated organic oligomers consisting of oligodiacetylene and thiophene units have been synthesized in high purity through iterative and divergent approaches based on a sequence of Sonogashira reactions. The series of thiophene-containing oligodiacetylenes (ThODAs) and homocoupled ThODAs (HThODAs) show--both in solution and in the solid state--a strong optical absorption, which is progressively red shifted with increasing chain length. The linear correlation of the absorption maximum (lambda(A)(max)) with the inverse of conjugation length (CL = number of double and triple bonds) shows that the effective conjugation length of this system is extended up to at least CL = 20. Furthermore, absorption measurements of dropcast thin films display not only a bathochromic shift of the absorption maxima but also a higher wavelength absorption, which is attributed to increased pi-pi interactions. The wavelength of the maximum fluorescence emission (lambda(E)(max)) also increases with CL, and emission is maximal for oligomers with CL=7-12 (fluorescence quantum yield Phi(F) = approximately 0.2). Both longer and shorter oligomers display marginal emission. The calculated Stokes shifts of these planar materials are relatively large (0.4 eV) for all oligomers, and likely due to excitation to the S(2) state, thus suggesting that the presence of enyne moieties dominates the ordering of the lowest excited states. The fluorescence lifetimes (tau(F)) are short (tau(F,max) = <<1 ns) and closely follow the tendency obtained for the fluorescence quantum yield. The anisotropy lifetimes show a near-linear increase with CL, in line with highly rigid oligomers.