Low Molecular Weight Homologues Research Articles

Effects of source, thermal maturity, and biodegradation on the distribution and isomerization of homohopanes in petroleum

The distributions of C 31–C 35 17α,21β(H)-homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment. For example, high C 35-homohopane indices [C 35/(C 31–C 35)] are typical of petroleums generated from source rocks deposited under anoxic marine conditions, where the C 35 backbone of the precursor bacteriohopanetetrol is preferentially preserved. Homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock. Homohopane distributions are altered by secondary processes. For example, the C 35-homohopane index decreases with increasing API gravity and thermal maturity of related oils from the Monterey Formation, offshore California. Similar decreases in the homohopane index are observed for expelled oils from Monterey source rock during hydrous pyrolysis. Biodegradation can result in selective loss of high or low molecular weight homologs, apparently depending on the bacterial population in the reservoir. Bacterial demethylation of homohopanes appears necessary to explain 25-norhopanes in at least one biodegraded oil. Hydrous pyrolysis studies of the Phosphatic and Siliceous members in the Monterey Formation show that identical heating conditions result in different homohopane C-22 epimer ratios. Furthermore, a “reversal” in the trend of the epimer ratio at high experimental temperatures is observed for the Phosphatic, but not for the Siliceous lithology. This could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures, possibly mediated by rock mineralogy.

The nucleation of n-eicosane crystals from solution in n-dodecane in the presence of homologous impurities

Studies have been made of the nucleation of crystals of n-eicosane from solutions in n-dodecane both in the pure state and in the presence of the neighbouring homologues; n-octadecane, n-nonadecane, n-heneicosane and n-docosane. Impurity incorporation studies yielded distribution coefficients, λ ⪢ 1 in the early stages of precipitation decreasing to λ ⩽ 1 in the growth stage. This high degree of incorporation in the early stages implies that the homologues should play a significant part in the nucleation process. This conclusion is confirmed by nucleation studies which show that the addition of the homologous impurities yields an increase in the width of the metastable zone and in the evaluated surface free energy of the resulting solid. The higher molecular weight homologues yield a greater effect than the lower molecular weight homologues. The homologues are thus defined as efficient nucleation inhibitors. The characteristics of the product crystals are in accord with this interpretation.

Structural characterization of polyprenols by 13C-n.m.r. spectroscopy: Signal assignments of polyprenol homologues

The 13C-n.m.r. spectra of geraniol, nerol, four isomers of farnesol and eight isomers of geranylgeraniol were investigated as low molecular weight homologues of polyprenol. The aliphatic carbon signals were assigned by considering the correlation of chemical shifts between these compounds. The C-1 methylene carbona tom in trans units exhibited four signals around 39 ppm reflecting the cis- trans, cis- trans (α), trans- trans and trans- trans (α) linkages. The C-1 methylene carbon atom is cis units showed two signals around 32 ppm reflecting cis- cis and cis- cis linkages. Here, the θ terminal unit showed the same shielding effect as the trans unit on the subsequent unit. The geometric isomerism of the unit linked to the θ terminal unit was distinguished by the chemical shift of the C-1 methyl and C-4 methylene carbon signals. The geometric isomerism of the α terminal unit was determined from the chemical shift of the C-4 CH 2OH carbon signal. These correlations were independent of the molecular weight of polyprenol homologues.

Analysis of Low Molecular Weight Homologues of Fiber‐Forming Polycondensates

AbstractOligomers belong to the gray area between low molecular weight chemistry and macromolecular chemistry. Although they represent an undesirable “natural impurity” in fiber‐forming polycondensates, they serve as useful model compounds for the corresponding polymers in fundamental research. Whereas for many years new classes of oligomers were being made preparatively accessible and the isolation of higher oligomers in pure form was being pursued, at the present time the emphasis is on analysis. By a combination of classical chemical and instrumental methods of analysis from polymer and organic chemistry, the identification of oligomers of unknown structure, the analytical control of their synthesis and the determination of their content in technical polymers has meanwhile become a routine task.

Skin irritants of the sun spurge (Euphorbia helioscopia L.).

Euphorbia helioscopia is a common herbaceous week found in the British Isles and parts of Europe. It has been responsible for poisoning of livestock resulting in severe inflammation particularly of mucous membranes and the eyes. Four esters of 12-deoxyphorbol were isolated from the fresh aerial parts of the plant and their irritant doses 50% (I.D.50) ascertained using female L.A.C.A. mice, 12-Deoxyphorbol-13-phenylacetate-20-acetate was found to be the major component of the toxic fraction and occurred in yields of 0.0012% w/w. This compound was also the most irritant substance isolated and exhibited an I.D.50 of 0.038microgram, and is accordingly considered to be responsible for the major part of the toxicity of E. helioscopia. A high molecular weight aliphatic ester, 12-deoxyphorbol-13-dodecdienoate-20-acetate, was also obtained in low yields. This compound exhibited an I.D.50 of 0.12microgram. The final two esters obtained from the toxic fraction of the plant were the low molecular weight homologues, 12-deoxyphorbol-13-[2-methyl-cis-2-butenoate]-20-acetate and 12-deoxyphorbol-13-[2-methyl-cis-2-butenoate]. These compounds exhibited I.D.50 of 3.09 and 0.72 microgram, respectively and were the least irritant of the series of compounds isolated.

Some peculiarities of polymer modification by filling

A qualitative description is given to account for general picture of changes in properties of two types of polymer compositions–filled with fillers of high free surface energy and with polymeric fillers, or polymer-polymer mixtures. It was shown that the properties of the surface layers are different in comparison to polymer in bulk as a result of interactions at the interface and changes in conditions of mass transfer during the process of structure formation. Depending on the conditions of surface layers formation, their structure and polymer-filler ratio, various types of compositions arise. For polymer-polymer mixtures there are various states of their structure depending on the thermodynamic compatibility of components in melt and cooling conditions and phase separation. An important role of small amount of the addition polymer in the properties of binary composition is shown as well as the role of low molecular weight homologues and admixtures in the processes of the structure formation.

Investigations on polysaccharide components of cells of Herpetomonas samuelpessoai grown on various media

Herpetomonas samuelpessoai, when cultured in various media, forms a linear beta-D-(1 leads to 2)-mannopyranan and a branched-chain glucuronoxylan containing D-glucopyranosyluronic acid nonreducing end units connected with alpha- and beta-linked D-xylopyranose units, as depicted in fragmental structures I, II, and III. Traces of amylose and alpha-linked mannopyranose moieties are also present. Mannose-containing materials predominate over glucuronoxylan in cells grown with proline as the carbon source (medium C). In a medium with sucrose as the carbon source and complex supplements (medium A) the proportion of glucuronoxylan is greater, and when the supplements were all chemically defined (medium B), a galactose-containing component is also formed. Glucuronoxylan is liberated from cells with hot aqueous alkali and it could be freed from lower molecular weight mannan by fractional precipitation. Mannan was obtained as the only polymeric component, with lower molecular weight homologues, by extraction of cells with hot water. In terms of ratios of component xylose, mannose, and galactose, the flagella resemble whole cells. However, flagella contain polysaccharide with alpha-D-linked mannopyranose side chains rather than beta-linked linear forms. These data are considered from standpoints of immunochemistry and electron microscopy.

Solvent Effects on Charge-Transfer Complexes in Solid Solutions

Abstract Electrical and optical properties of a great variety of charge transfer complexes (CT) of organic compounds have been investigated in the recent years. The interest in this area is evidenced by large number of reviews and original reports. The studies of dark conductivity and photoconductivity have been recently extended to polymers acting as donors or acceptors, although more papers are related to the systems in which either the donor or acceptor consists of a low molecular weight species. Some studies have been carried out .in which CT-complexes of polymeric species have been compared with the low molecular weight homologs. Little difference was found between these two types of complexes. The equilibrium constants of CT-complexes formation as determined from spectrophotometric studies, for polymeric species and for monomeric molecules were higher for the monomer than for the polymer (e.g. Refs 1-5). The charge transfer bands exhibited by the polymeric complexes did not always coincide with thos...

Viscosity and molecular behaviour of narrow polyoxyethylene fractions in melt

A study was made of the temperature dependence of viscosity and the glass transition points were measured of narrow polyoxyethylene (POE) fractions and low molecular weight homologues with terminal hydroxyl groups or chlorine atoms. From thermodynamic considerations it was shown that the “actual” glass transition point of high molecular weight POE is ∼ 216°K. Parameters of the Vogel and Adam-Gibbs equations were determined. It was found that the characteristic temperature T 0, which corresponds to the disappearance of free volume of the system and T 2 at which the conformation and excess entropy of POE becomes zero ( T 0 = T 2 = 180°K) are identical. The “critical” molecular weight corresponding to the formation of a “structural network of contacts” in a POE melt ( M cr = 3500), was determined. It was shown that the abnormal dependence of the viscosity of POE oligomers on molecular weight in the range of M< M cr is due to the existence of associates, in which macromolecules are linked with intermolecular hydrogen bonds.

Infrared spectra and molecular conformations of poly‐γ‐benzyl‐L‐glutamate

AbstractThe infrared spectra and chain conformations of films of poly‐γ‐benzyl‐L‐glutamate and the N‐deuterated derivative were studied. The IR dichroism of oriented films of the high moleculare weight polymer was measured, and also the IR spectra of three samples of low molecular weight homologs. The amide‐V‐band was found to be useful for studying the chain conformations of polypeptides, particularly for distinguishing between the α‐helical form and the disordered form.

Detecting low molecular weight homologs in poly(ethylene glycols) by gel permeation chromatography

Detecting low molecular weight homologs in poly(ethylene glycols) by gel permeation chromatography