Sedimentation chain branching

Shiea J., Brassell S. C., and Ward D. M. (1990) Mid-chain branched mono- and dimethyl alkanes in hot spring cyanobacterial mats a direct biogenic source for branched alkanes in ancient sediments Org. Geochem. 15, 223-231. 

Mineral Oil Hydraulic Fluids. Methods are available for analysis of the hydrocarbon components of mineral oil hydraulic fluids (predominantly straight and branched chain alkanes) in environmental samples. Some of these methods are summarized in Table 6-3. In general, water and sediment samples are extracted with a suitable solvent in a Soxhlet extractor (for solid samples) or in separatory funnel or shake flask (for liquid samples) (Bates et al. 1984 Peterman et al. 1980). The extract is cleaned up on silica gel or Florisil columns using a nonpolar solvent to elute the nonpolar alkanes. Analysis is usually performed by GC/MS (Bates et al. 1984 Kawamura and Kaplan 1983 Peterman et al. 1980). Method performance has not been reported, although 82% recovery of aliphatic hydrocarbons was reported for rainwater (Kawamura and Kaplan 1983). 

Takada and Ishimatari [20] extracted alkylbenzenes with normal C10-C14 and branched Cn-C13 alkyl chains from marine and coastal sediment and suspended matter in benzene methanol. The extract in benzene was then applied to a Florisil column for removal of copper sulphide and polar materials, and then subjected to silica gel column chromatography. Alkyl benzenes were quantified and identified using gas chromatography with flame ionization detection. The recoveries of alkyl benzenes were 81-94%. 

A large variety of hydrocarbons have been identified in marine sediments and petrolemn. These chains of carbon atoms vary greatly in munber and some are branched, as illustrated in Figure 22.8. Some have double or triple bonds. Hydrocarbons with double or triple bonds are said to be unsaturated with respect to hydrogen. 

The structure of concentrated solutions of branched molecules has also received little attention. It is probable that the network formed by entangled branched molecules displays a topological structure which is different from that formed by linear chains. It is known that the sedimentation properties of branched synthetic polymers differ, especially in good solvents, from those of linear polymers. The concentration dependence of the sedimentation coefficient is relatively more pronounced for branched polymers than that for linear ones 20). 

Hoering, T. C., Branched-Chain Fatty Acids in Recent Sediments, ... 

Hudecz, F., Kovacs, P., Kutassi-Kovacs, S., and Kajtar, J. (1984) GPC, CD and sedimentation analysis of poly-Lys and branched chain poly-Lys-poly-DL-Ala polypeptides. Colloid Polym. Sci. 262, 208-212. 

Branching. So far our discussion has been limited to linear polymer chains. The effect of branching on viscosity is still not well understood. The statistics of certain simple types of branched chains has been studied by Zimm and Stockmayer (1949) and Stockmayer and Fixman (1953). Since branching produces a less extended hydrodynamic volume than would be expected for a linear chain of the same molecular weight, conceivably the intrinsic viscosity for a branched polymer would be smaller and the sedimentation coefficient larger than those for a linear polymer. At present quantitative treatments are still scarce. 

Schouten S., Sephton S., Baas M., and Sinninghe Damste J. S. (1998b) Steroid carbon skeletons with unusually branched C-3 alkyl side chains in sulphur-rich sediments. Geochim. Cosmochim. Acta 62, 1127-1132. 

Acids other than n-fatty acids have been shown to be important components in the mono-AME fraction, though they are generally present in much smaller concentration than in the straight-chain acids. The branched-chain acids wo-I4 0 (14 0 refers to the n-fatty acid with 14 C atoms and 0 double bonds iso-l4 0 indicates the iso isomer of 14 0), wo-I5 0, ant-I5 0, iso l7 0, and ant-I7 0 were observed to be present in most samples. As previously mentioned, branched-chain acids are often taken to be representative of a bacterial input to sediments (i, 2i, 24). Since bacteria are known to be active in assisting plant diagenesis, which leads to coalification (40), one would expect to find evidence of their presence. 

Gum arabic (acacia) has been used in pharmacy as an emulsifier. It is a polyelectrolyte whose solutions are highly viscous owing to the branched stmcture of the macromolecular chains its adhesive properties are also believed to be due to, or in some way related to, this branched stmcture. Molecular weights of between 200 000 and 250 000 (MJ have been determined by osmotic pressure, values between 250 000 and 3 x 10 by sedimentation and diffusion, and values of 10 by light scattering, which also points to the shape of the molecules as short stiff spirals with numerous side-chains. Arabic acid prepared from commercial gum arabic by precipitation is a moderately strong acid whose aqueous solutions have a pH of 2.2-2.7. It has a higher viscosity than its salts, but emulsions prepared with arabic acid cream are not as stable as those made with its salts. 

Fatty Acids and Other Organic Acids. Fatty acids were extracted from Lake Haruna sediment humic acid by refluxing with BF3/methanol (Ishiwa-tari, 1975a). The fatty acids consisted of a series of long-chain saturated (Cu-C34 maximum at Cie), unsaturated (C16, Cig, and C24), and branched (C15 and Civ) monocarboxylic acids. The fatty acids amounted to 0.2-0.3% of the humic acid. 

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