Methenylation

Folic acid, 4-amino-4-deoxy-10-methyl-, 1, 164 3, 325 as anticancer drug, 1, 263 biological activity, 3, 325 Folic acid, 4-amino-10-methyl-toxicity, 1, 141 Folic acid, 7,8-dihydro-biosynthesis, 3, 320 synthesis, 1, 161, 3, 307 Folic acid, 4-dimethylamino-hydrolysis, 3, 294 Folic acid, 5-formiminotetrahydro-biological activity, 3, 325 Folic acid, 5-formyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 occurrence, 3, 325 Folic acid, 10-forfnyltetrahydro-biological activity, 3, 325 Folic acid, 5,10-methenyl-5,6,7,8-tetrahydro-biological activity, 3, 325 chirality, 3, 281 Folic acid, 5-methyl-chirality, 3, 281 Folic acid, 9-methyl-toxicity, 1, 141... 

L-Folic acid, 5,10-methenyl-5,6,7,8-tetrahydro-conformation, 3, 281 L-Folic acid, 5,6,7,8-tetrahydro-conformation, 3, 281 Folpet... 

Homofolic acid, 5,11-methenyl-tetrahydro-biological activity, 3, 327 Homofolic acid, tetrahydro-biological activity, 3, 327 Homoisoflavanones occurrence, 3, 722 thermoisomerization, 3, 722 thermolysis, 3, 728 Homolytic reactions heterocyclic compounds reviews, 1, 74 Homophthalic acid isocoumarins synthesis from, 3, 830 synthesis, 3, 830 Homophthalic anhydride isochroman-l-one synthesis from, 3, 860 20a-Homoporphyrin nomenclature, 1, 30 Homopterocarpin isolation, 4, 998 ( )- D- Homotestosterone synthesis, 1, 453 Homer-Emmons reaction chromene synthesis by, 3, 749 Hortiacine isolation, 3, 149 Hortiamine isolation, 3, 149... 

The porphyrins found in nature are compounds in which various side chains are substituted for the eight hydrogen atoms numbered in the porphin nucleus shown in Figure 32-1. As a simple means of showing these substitutions, Fischer proposed a shorthand formula in which the methenyl bridges are omitted and each pyrrole ring is shown as indicated with the eight... 

Figure 32-1. The porphin molecule. Rings are labeled I, II, III, and IV. Substituent positions on the rings are labeled 1,2,3,4,5,6,7, and 8. The methenyl bridges (—HC=) are labeled a, (3,y, and 5.

In birds and amphibia, the green biliverdin IX is excreted in mammals, a soluble enzyme called biliverdin reductase reduces the methenyl bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment (Figure 32-12). 

Tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino, or methyl groups), N-10 (formyl group), or bridging N-5 to N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharma-... 

Methanogenesis from CO2 via methenyl H MPT, methylene H MPT, and methyl H MPT using coenzyme F420H2 (5,6,7,8-tetrahydrofolate). 

To prepare 10-formyl TF1F (formyl donor for the formylation of fMet-tRNAfMet), 10 mg of 5-formyl-tetrahydrofolate (5-formyl THF) are dissolved in 1 ml of 0.1 MHC1 that has been flushed extensively with N2 and kept overnight at 4° in a closed vial. The resulting yellow suspension of 5,10-methenyl THF is agitated and an aliquot withdrawn for spectropho-tometric determination of the concentration (6 mM solution = 85 A355). The expected 5,10-methenyl THF concentration is approximately 20 mM. The cyclic methenyl THF can be stored for several weeks at —20°. The 10-formyl THF is stable for only a few weeks and is freshly prepared hydrolyzing the cyclic methenyl THF. For this purpose, the stock solution is diluted in 100 mM Tris-HCl (pH 8.0) extensively flushed with N2 and supplemented with 100 mM 2-mercaptoethanol. After 15 min incubation at 20°, the solution is divided in aliquots, which are stored at —20°. 

By comparison, the conversion of methenyl-THF (—N—CH=N—R, state 1) to methylene-THF (—N—CH2—N—, state 2) requires a reducing agent, NADPH. 

Second, leucovorin is known to dehydrate under acidic conditions to form anhydroleucovorin, 5,10-methenyl H, F, which absorbs at 352-353 nm. In the absence of interfering species, leucovorin may be analyzed by acidification with 0.1N HC1 followed by uv measurement after 1.5-2.0 hours. Purity may be determined relative to a sample of known purity or relative to literature values e352 = 2.39-2.41 x 10 M 1 cm"1.21... 

Proposed mechanism for the reversible reaction of N, N -methenyltetrahydromethanopterin (methenyl-H4MPT ) with H2 to N, N -methylenetetrahydromethanopterin (methylene-H4MPT) and a proton catalysed by the metal-free hydrogenase from methanogenic archaea... 

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