Lipid polymethylene chains

At present there is considerable interest in the way in which the con-stituents of membranes are associated to form the dynamic complex entity of the cell membrane. Speculations range from the Danielli-type structure, first advanced in 1935, to structures which now place greater emphasis on the so-called hydrophobic bonding of the lipid polymethylene chains and amino acids of the membrane protein (10). Various other speculations about the associations of the membrane components are built around these two main themes. In a field of research where there are such a considerable speculation and divergence of opinion, this usually indicates a shortage of experimental information rather than variations in perspicacity. This seems to be true of our present knowledge of membrane structure. 

We have examined the effect of temperature and enzymes and other materials on the spectrum of the membrane fragment (4). As the temperature increases up to 120°C. the lipid chain [CH2]n signal becomes more prominent, suggesting that, at lower temperatures, the polymethylene chain is somehow inhibited in its molecular freedom. Treatment with phospholipase C removes the peak at 6.7t which we associate with the choline protons. Urea, which is known to have an unfolding effect on proteins, produces new peaks in the spectrum of the membrane which can be assigned to some of the constituent amino acids of the protein. There is, however, no marked increase in the polymethylene chain signal. The effect of trifluoroacetic acid on the membrane is to increase the intensity of the peak associated with the constituent amino acids of the protein, and now the polymethylene chain of the lipid is also clearly visible. 

The appearance in the spectrum of the membrane of peaks assigned to amino acids of the protein, only after the membrane has been placed in urea or trifluoroacetic acid, shows that the segmental motion of these amino acid groups is inhibited by high local viscosity when the membrane is present in normal D20. The spectroscopic evidence suggests that the polymethylene chains of the lipid and some of the amino acids of the protein may be mutually interlocked. However, both lipid-lipid and amino acid-amino acid interactions could also be occurring. 

When the membrane is washed with ether to remove all of the cholesterol, the resultant PMR spectrum shows little change in the intensity of the polymethylene chain signal compared with that of the original membrane spectrum. This appears to rule out lipid-cholesterol interaction in this membrane as having a dominant effect upon the polymethylene chain freedom. In the membrane fragments either lipid chain-chain interactions have increased as a result of the protein interaction... 

The distribution pattern of a,w-dicarboxylic acids for lipids resembled those for humic acid and humin (Fig. 3). This fact clearly indicates the common origin for the polymethylene chains in lipids, humic acid, and humin, which means that phytoplankton-derived lipids actively took part in the formation of humic acid and humin. The relative abundance of polymeth-ylene chains in lipids and humic substances was estimated on the assumption that the yield of production of aliphatic acids from polymethylene chains by alkaline permanganate oxidation was the same for these organic fractions. The following estimations resulted 42% (% of the total amount of polymethylene chains in the sediment) for humin, 38% for lipids, 19% for humic acid, and 1% for fulvic acid. 

Ishiwatari and Machihara (1983) estimated roughly the degree of contribution of lipids to humic acid and humin by assuming that all polymethylene chains (C4-C14) in these fractions were derived from sedimentary lipids. Surprisingly, by this calculation 43% of the humic acid carbon and 74% of the humin carbon were derived from lipids. These extremely high values are in conflict with 8 C calculations. Using S C data, the degree of contribution of lipid to humic acid and humin was estimated on the assumption that (1) humic acid and humin were formed from lipids and nonlipid materials and (2) S C of humic acid and humin were the simple sum of 8 C of lipid (-30.56... 

In multicomponent systems difficulties arise from the overlapping of C-H features due to chemically different lipids, or in hpid-protein arrays, from protein C-H stretching bands. In these systems it becomes impossible to monitor a single lipid component. Mendelsohn et al. [64] showed how this problem csai be overcome by the use of deuterated components. They inserted a completely deuterated fatty acid into a model membrane system and followed the C-D stretching vibrations in the spectral window 2000-2220 cm which is uncluttered by modes from other components. As the membrane passed through a gel-liquid crystal transition the line-width of the C-D stretching vibrations of the bound fatty add was found to be a sensitive probe of membrane polymethylene chain order. 

In summary, the dimeric lipids (29a-29h) with low m-value (3-4) and high m-value (20-22) showed exceptional thermal, lipid-packing and cholesterol-association properties. Obviously the introduction of a polymethylene spacer chain at the level of headgroup brought about dramatic effect on the aggregation behavior, membrane organization and lipid packing of 29. 

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