Natural methylation

Methylmercury in the marine environment may originate from industrial discharges or be synthesised by natural methylation processes. Fish do not themselves methylate inorganic mercury [62,64], but can accumulate methyl mercury from sea water [63]. Methylmercury has been detected in sea water only from Minamata Bay, Japan, an area with a history of gross mercury pollution from industrial discharge. It has been found in some sediments but at very low concentrations, mainly from areas of known mercury pollution. It represents usually less than 1% of the total mercury in the sediment, and frequently less than 0.1% [65-67]. Microorganisms within the sediments are considered to be responsible for the methylation [65,68], and it has been suggested that methylmercury may be released by the sediments to the sea water, either in... 

In view of the lack of success in discovering a plausible natural methylating source for the methylgermanium species14, some efforts have been made to investigate model methylating systems in laboratories. 

Rhew, R.C. Miller, B.R. Weiss, R.F. (2000) Natural methyl bromide and m yl chlcnide anissions from coastal salt marshes. Nature, 403, 292-8. 

Hydroxybenzoic acid is obtained as white crystals, fine needles, or fluffy white crystalline powder. It is stable in air and may discolor gradually in sunlight. The synthetic form is white and odorless. When prepared from natural methyl salicylate, it may have a light yellow or pink tint and a faint, wintergreen-like odor. w-Hydroxybenzoic acid crystallizes from water in the form of white needles and from alcohol as platelets or rhombic prisms. -Hydroxybenzoic acid crystallizes in the form of monoclinic prisms. Various physical properties of hydroxybenzoic acids are listed in Tables 1—4. 

Petitgrain oils are obtained by steam distillation of the leaves of citrus trees. The oils derived from the bitter orange tree are the most important. Other petitgrain oils (mandarinier, citronnier, and bergamottier) are less important. Petitgrain oil mandarinier is a source of natural methyl A -methylanthranilate, which is present at a concentration of nearly 50%. Petitgrain oils are essential constituents of eau de cologne. 

Rhew RC, Miller BR, Weiss RF (2000) Natural Methyl Bromide and Methyl Chloride Emissions from Coastal Salt Marshes. Nature 403 292... 

Isoeugenol Indolic Indol Naturals Methyl isoeugenol... 

Walden 2 has directed attention to another fact of a similar nature. Methyl sulphide boils at a temperature 60 degrees higher than methyl ether, and ethyl sulphide 56 degrees higher than ethyl ether. Thus... 

If sulfated oligosaccharides, such as the carrabiitol derivatives 21 and 22, are formed as the result of partial reductive hydrolysis directed to determination of the absolute configuration of 3,6-anhydrogalactose, they may be desulfated and acetylated simultaneously.244 Naturally methylated agars give rise to methylated disaccharide alditols. 

Colourless acicular crystals or a white crystalline powder with a sweetish acrid taste. The synthetic form is white and odourless but if prepared from natural methyl... 

Results for other aromatic substances are compiled in Table 6.5, but are not discussed in detail, as the available data do not justify a discussion. Methyl salicylate, the main component of natural wintergreen oil is often used in cosmetic products. The quite negative 8 C-values of the natural product (5 C-values from -32.6 to -33.7%o) are probably partially caused by the extreme depletion of the methyl groups (see 6.2.2.4.4) and permit perhaps a discrimination from the nature-identical product (8 C-values from -27.7 to -30.8%o). The 5 H-values of natural methyl salicylate -130 to - 63%o [3, 270[ are, with the exception of one sample (-56%o), in the range of other natural aromatic substances (e.g. benzaldehyde, cinnamic aldehyde, anethole). Nature-identical analogues show more positive 5 H-values. 

The values measured for natural methyl cinnamate (8 C-values between -25.7 and -30.6%o and 8 H-values between -85 and -191%o, [265, 271[) are in line with those for other natural aromatic substances. The nature-identical substance seems to have similar 8 C- and partially very high 8 H-values. 

The metals are elements and, as such, are not produced by man. However, the occurrence of metals and their distribution in nature are often related to anthropogenic activities, resulting in high levels in the surrounding environment and/ or bioavailable forms. Of great concern is the natural methylation of Hg, Pb... 

Coenzyme F430 is a nickel-containing tetrapyrrole that is required by methyl-coenzyme M reductase (Figure 29). It is one of a class of nickel chlorins found in nature.Methyl-coenzyme M reductase catalyzes the last step in methanogenesis, the reduction of methyl-coenzyme M to methane and coenzyme M (step 8, Figure 1). The biosynthesis of F430 follows the common tetrapyrrole biosynthetic pathway from glutamate to... 

Hydroxylation occurs naturally in position 3, 5, 7, 3 , 4 and 5 . Glycosidation with L-rhamnose, D-glucose, glucorhamnose, galactose or arabinose in position 3 or 7 is frequent in nature. Methyl ethers and acetyl esthers of the alcohol groups are known to occur naturally. 

Some polysaccharides contain naturally methylated sugars. Analyses of the lipopolysaccharides from Myxococcus fulvus and Rhodopseudo-... 

Poulter et al. have shown that the absolute configuration of natural santolinyl alcohol (62) is S, the same as that of natural methyl chrysanthemate (R, because of the change in priorities), by ring-opening dihydrochrysanthemyl alcohol (69)... 

In Nature, methyl groups are selectively introduced into reactive aromatic rings by methyltransferases (Mtases), in particular with SAM as the cofactor. Furthermore, enzyme-catalyzed reactions are important for access to isoprenoids. SAM can act as an electrophile that transfers a methyl group to a specific nucleophilic atom. 

The goal of introducing a nucleophile to an enone 1,4 enantioselectively has been reported by the use of both chiral enones and chiral nucleophiles. The former strategy is exemplified by a synthesis of natural (-)-methyl jasmonate by the addition of an acetate enolate equivalent to a chiral... 

To impose specific properties, natural methylation has to be selective, that is, in most cases chemo- and regioselective, more rarely diastereo- and enantioselective. 

Why is enzymatic methylation not yet used more commonly in production processes The bulk availability of the natural methyl donating cofactor SAM (see below) is the major bottleneck and cost factor. Also, compared to, for example, lipases, the toolbox of stable, easy-to-use MTs is still incomplete, and many methylations can be performed very cheaply by classical chemical methods. 

The methyl group in SAM 1 has been described in the literature as a natural methyl iodide [29]. It acts as an electrophile and facilitates Sj 2 reactions. In the methyl transfer reaction, the methyl group of SAM 1 is transferred to the substrate catalyzed by a MT resulting in a methylated product and the thioether S-adenosyl-L-homocysteine (SAH) 2, which is much more stable than the sulfonium compound (Scheme 18.4). The sulfonium center is chiral as a result of three different substituents and the electron lone pair at sulfur. 

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