Symbiotic modifications

C-Methylation also features in the biosynthesis of usnic acid (Figure 3.41), an antibacterial metabolite found in many lichens, e.g. Usnea and Clado-nia species, which are symbiotic combinations of alga and fungus. However, the principal structural modification encountered involves phenolic oxidative coupling (see page 28). Two molecules of methylphloracetophenone are incorporated, and these are known to derive from a pre-aromatization methylation reaction and not by... 

The Symbiotic Function of the Unique Structural Features and Modifications of Rhizobial Lipopolysaccharides... 

Fraysse, N., Jabbouri, S., Treilhou, M., Couderc, F., Poinsot, V. Symbiotic conditions induce structural modifications of Sinorhizobium sp. NGR234 surface polysaccharides. Glycobiology 12 (2002) 741-748. 

A modification of the HSAB approach was first explained by C. K. Jprgensen. We will consider an actual example to make this idea clear. The Co3+ ion is a hard Lewis acid. However, when Co3+ is bonded to five cyanide ions, a more stable complex results when the sixth group is iodide than when it is fluoride. On other words, [Co(CN)5I]3 is stable, whereas [Co(CN)5F]3 is not. At first this seems like a contradiction that the soft I- bonds more strongly to the hard acid, Co3+. However, the five CN- ions have made the Co3+ in the complex much softer than an isolated Co3+ ion. Thus, when five CFT ions are attached, the cobalt ion behaves as a soft acid. This effect is known as the symbiotic effect, and it indicates that whether a species appears to be hard or soft depends on the other groups attached and their character. 

Detoxification of dietary alleochemicals, which can be achieved by symbiotic bacteria or protozoa living in the rumen or intestines, or by liver enzymes which are specialized for the chemical modification of xenobiot-ics. This evolutionary trait is very helpful for Homo sapiens, since it endowed us with a means to cope with our man-made chemicals which pollute the environment. Carnivorous animals, such as cats, are known to be much more sensitive toward plant poisons (505). It was suggested that these animals, which do not face the problem of toxic food normally, are thus not adapted to the handling of allelochemicals. 

A variety of steroidal natural products have been isolated from insects, even though, as mentioned previously, insects are not able to carry out de novo steroid biosynthesis. The steroidal nucleus, as it occurs in insect primary and secondary metabolites thus must ultimately come from dietary or symbiotic microbial sources. For many phytophagus insects, C28 and C29 phytosterols are converted into cholesterol (C27) through a series of dealkylation pathways, with cholesterol subsequently serving as the starting point for further metabolic transformations, and resulting in a wide variety of steroid-based natural products. In other cases, dietary phytosterols are sequestered and deployed unmodified, and as with other compound classes, the relative importance of dietary sequestration versus modification is not always clear. 

Sterol patterns in marine invertebrates reflect the complexity of mixtures of sterols arising through food chains. Even in the same species, the sterol fractions show different patterns depending upon the location where the organisms have been collected. The capability of further biochemical modification of the dietary sterols makes the sterol mixtures even more complex. The symbiotic relationship between organisms also complicates the sterol compositions. These conditions are quite different from those affecting sterols of terrestrial organisms. Many sterols of unprecedented structures have now been isolated from marine sources. 

Generally, one can expect the selection of hexagonal patterns due to the symbiotic" mechanism described above in the case where the nonlinear interaction coefficient Mmn is smaller than for wavevectors km and k with a 60° angle between them. However, the ubiquity of hexagonal patterns has another explanation. In order to describe it, let us consider some modifications of the models described in Section 2. 

Acanthifolidn is the episulfide in 9-10 of okadaic acid. Isolated from the sponge Pandaros acanthifolium, it is not yet known whether this compormd results from a modification of okadaic acid by the sponge, or whether it is a new derivative of an epiphyte or symbiotic dinoflagellate. Its toxidty and biological properties are similar to those of okadaic acid and dinophysistoxins (Schmitz et al, 1981). 

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