Increase In Average Chain Length Research Articles

Seasonal and canopy height variation in n-alkanes and their carbon isotopes in a temperate forest

The stable carbon isotopic composition (δ13C) of terrestrial leaf wax components, such as n-alkanes (δ13Calk), is used extensively to address questions about past changes in vegetation, climate and the carbon cycle. To interpret sedimentary δ13Calk values, characterization of the environmental and biological controls on carbon isotopic fractionation during plant metabolism is required, especially for fractionation that occurs during photosynthesis (ɛleaf-CO2 or Δleaf) and n-alkane biosynthesis (εalk-leaf). Although much is understood about controls on ɛleaf-CO2, little is known about seasonal or within canopy variation in n-alkane composition and εalk-leaf. To address the gap, we sampled 5 common tree species (Acer rubrum, Acer saccharum, Juniperus virginiana, Sassafras albidum and Ulmus americana) from buds to senescing leaves from within a single temperate forest in southwestern Ohio, USA. We measured n-alkane concentration, δ13Calk, leaf biomass δ13C (δ13Cleaf), δ13C of atmospheric CO2 within the canopy, luminous flux and specific leaf area (SLA). In angiosperms, δ13Calk values were low in the buds, increased substantially (4–6‰) to the highest values in young leaves and then decreased (1–3‰) in mature leaves. Seasonal variation in δ13Calk values generally tracked changes in δ13Cleaf, but the fractionation between leaf and n-alkanes (εalk-leaf) was variable, with lower values in buds (−6.7 ± 1.1‰) than in young leaves (−4.9 ± 2.3‰). Later in the growing season, εalk-leaf values stabilized and ranged between −2.5 and −6.5‰, with a mean of −4.3 ± 1.4‰ in angiosperms. Stabilization of δ13Calk values coincided with settling of SLA and increase in average chain length (ACL). For the conifer, temporal variation in δ13Calk values was smaller (ca. 2‰) compared with the angiosperms, and εalk-leaf values slightly increased when n-alkane production increased. Average εalk-leaf value in the conifer at the end of the growing season was −5.0 ± 0.7‰. We speculate that εalk-leaf variation in both angiosperms and the conifer reflects changes in the source of carbon for plant metabolism, as well as the timing of wax synthesis. We also tested canopy effects on n-alkane concentration and carbon isotopic fractionation at different heights and extent of canopy closure. n-Alkane concentrations were higher at the top of the canopy by 2–8 times than in the lower canopy. The εleaf-CO2 values were more negative at the lower two thirds of the canopy than the upper one third of the canopy by 2–4‰ in two species with closed canopies. The εalk-leaf values did not vary with height and extent of canopy closure. To further explore the link between canopy leaves and leaf litter, we compared δ13Calk values of mature canopy leaves with those in leaf litter collected during senescence, and the offsets were within ca. 1‰. The results confirm indications that leaf litter δ13Calk values reflect those of the upper canopy leaves due to not only high leaf biomass in the upper canopy but also to high n-alkane production.

Abundance and distribution of leaf wax n-alkanes in leaves of Acacia and Eucalyptus trees along a strong humidity gradient in northern Australia

Environmental parameters such as rainfall, temperature and relative humidity can affect the composition of higher plant leaf wax. The abundance and distribution of leaf wax biomarkers, such as long chain n-alkanes, in sedimentary archives have therefore been proposed as proxies reflecting climate change. However, a robust palaeoclimatic interpretation requires a thorough understanding of how environmental changes affect leaf wax n-alkane distributions in living plants. We have analysed the concentration and chain length distribution of leaf wax n-alkanes in Acacia and Eucalyptus species along a 1500km climatic gradient in northern Australia that ranges from subtropical to arid. We show that aridity affected the concentration and distribution of n-alkanes for plants in both genera. For both Acacia and Eucalyptus n-alkane concentration increased by a factor of ten to the dry centre of Australia, reflecting the purpose of the wax in preventing water loss from the leaf. Furthermore, Acacian-alkanes decreased in average chain length (ACL) towards the arid centre of Australia, whereas Eucalyptus ACL increased under arid conditions. Our observations demonstrate that n-alkane concentration and distribution in leaf wax are sensitive to hydroclimatic conditions. These parameters could therefore potentially be employed in palaeorecords to estimate past environmental change. However, our finding of a distinct response of n-alkane ACL values to hydrological changes in different taxa also implies that the often assumed increase in ACL under drier conditions is not a robust feature for all plant species and genera and as such additional information about the prevalent vegetation are required when ACL values are used as a palaeoclimate proxy.

Effects of Initiator Homolysis Rate Constant on Kinetics and Chain Length Distribution in Living Free-Radical Polymerization

The kinetics, chain length and distribution of living free-radical polymerization with initiators of different homolytic rate constant, kd, were simulated by a Monte Carlo algorithm. The results show that, for the system with a lower kd (10−5–10−4s−1), the polymerization rate is much faster and chain length increases linearly with monomer conversion. However, for the system with higher kd, nonlinear increase in average chain length, and lower reaction rate are found in the simulation results. Low polydispersity indices were obtained for higher and lower kd.

Structural changes during development in the amylose and amylopectin fractions (separated by precipitation with concanavalin A) of starches from maize genotypes

The starches from four maize genotypes (waxy, normal, amylose-extender, and ae ae/wx wx) have been sampled at three stages of development, and fractionated by precipitation of the amylopectin with concanavalin A. The resulting amylose and amylopectin fractions were examined by gel chromatography before and after treatment with debranching enzyme. Both mature and immature samples were efficiently fractionated. The proportion of amylose in the starches of the n and ae genotypes, measured by concanavalin A precipitation, increased on development. A small proportion of the ae/wx mutant starch was not precipitated by concanavalin A, and its properties were those of an “intermediate fraction” of low molecular weight with a degree of branching and average chain-length intermediate between amylose and amylopectin. The amylose of n starch increased in molecular size during development, whereas that of ae decreased. No differences in molecular size and chain-length distribution were detected within a genotype for the amylopectins of wx, n, and ae/wx starches during development, but the component from ae starch decreased in molecular size and increased in average chain-length. Comparison of the amylopectins from wx, n, and ae starches showed a decrease in molecular size and an increase in average chain-length, in that order.

Human intestinal mucin in cystic fibrosis.

Human intestinal mucins from six subjects with Cystic Fibrosis (CF) and eight subjects without CF were prepared from tissue obtained at surgery (one case) and postmortem. Subjects were not age-matched, but the nonCF mucin was obtained from subjects with ages which bracketed those of the CF subjects. Cesium chloride analytical gradient ultracentrifugation showed that CF mucins were generally denser than nonCF mucins. Sedimentation coefficients were also higher in the CF samples. CF mucins were enriched in fucose, galactose, N-acetylglucosamine and total carbohydrate per mg protein and per oligosaccharide chain (mole/mole GalNAc). Fucose/sialic acid molar ratios were significantly higher in CF mucins, and the average oligosaccharide chain length was approximately three residues greater in CF as compared with nonCF mucins. There was no difference in amino acid profiles or the number of side chains per molecule. The mean sulfate content was higher in the CF mucins but not to a level of significance; however, in the eight mucins, sulfate content correlated positively with total carbohydrate, N-acetylglucosamine and galactose, and therefore increased with oligosaccharide chain length. CF intestinal mucin was therefore denser and more highly glycosylated than nonCF musin and probably contained more sulfate. The increase in glycosylation resulted from a rise in fucose, galactose, and N-acetylglucosamine without a concomitant rise in sialic acid.

Morphology of uniaxially oriented poly(ethylene terephthalate)

AbstractThe morphological character of uniaxially oriented poly(ethylene terephthalate) (PET) films was investigated as a function of draw ratio. Dynamic mechanical, infrared, and crystallite‐size measurements were made on the samples. In addition, selective degradation experiments and molecular weight determinations were employed. The dynamic mechanical measurements indicated a sharp decrease in irregular folds for draw ratios of 3.0 and higher, which also coincided with the essentially complete disappearence of regular folds (from the 988 cm−1band in the infrared spectra) in unannealed samples. Infrared studies of drawn samples annealed under different conditions gave evidence in support of a structure in which the chains are stretched out. Apparent crystallite‐size measurements showed a sudden increase in length of the crystals in the direction of the draw beyond a draw ratio of 3.0. Molecular weight measurements showed a large increase in average chain length in the residue after selective degradation of amorphous material and folds; undrawn and slightly drawn samples gave a much lowerMn. Based on these observations, it is postulated that for higher draw ratios and present drawing conditions, the crystals are of the straight chain type, somewhat similar to the fringed‐micelle crystal concept.