Bases analogues

Mutation. For industrial appHcations, mutations are induced by x-rays, uv irradiation or chemicals (iiitrosoguanidine, EMS, MMS, etc). Mutant selections based on amino acid or nucleotide base analogue resistance or treatment with Nystatin or 2-deoxyglucose to select auxotrophs or temperature-sensitive mutations are easily carried out. Examples of useful mutants are strains of Candida membranefaciens, which produce L-threonine Hansenu/a anomala, which produces tryptophan or strains of Candida lipolytica that produce citric acid. An auxotrophic mutant of S. cerevisiae that requires leucine for growth has been produced for use in wine fermentations (see also Wine). This yeast produces only minimal quantities of isoamyl alcohol, a fusel oil fraction derived from leucine by the Ehrlich reaction (10,11). A mutant strain of bakers yeast with cold-sensitive metaboHsm shows increased stabiUty and has been marketed in Japan for use in doughs stored in the refrigerator (12). 

Among the polyurethane, polyester, and polyamide thermoplastic elastomers, those with polyether-based elastomer segments have better hydrolytic stabihty and low temperature flexibiUty, whereas polyester-based analogues are tougher and have the best oil resistance (43). Polycaprolactones and aUphatic polycarbonates, two special types of polyesters, are used to produce premium-grade polyurethanes (12). 

More recently, these authors have reported the synthesis of a new thiophene-based analogue of (I ,i )-Me-DuPHOS called UlluPHOS. The facial recognition and enantioselection associated with ruthenium complexes of UlluPHOS and Me-DuPHOS were shown to be similarly high in various hydrogenations of p-keto esters (Scheme 8.32). The most important difference between these two ligands was found by comparing the reaction rates. Indeed, the authors have observed that the use of UlluPHOS considerably increased the activity of the complexes. 

Six-membered heterocycles have also been extensively used to mimic the central pyrazole scaffold of limonabant (382). Merck and Co. has utilised pyiidines (510) [314], and pyrimidines (511) [315] in this capacity. Sanofi-Aventis has also claimed a series of pyridine-based analogues [316], as exemplified by (512) and additionally the non-heteroatom containing terphenyl (513) [317]. 

The most important physical characteristics of polyurethane elastomers are their elasticity, low permanent set, high tear strength, and good abrasion resistance. Polyester-based elastomers have higher tear strength than polyether-based analogues. Polyether-based elastomers have better recovery. Important chemical characteristics include stability when exposed to the elements and resistance to oil and grease. 

Ren RXF, Chaudhuri NC, Paris PL, Rumney S IV, Kool ET (1996) Naphthalene, phenan-threne, and pyrene as DNA base analogues synthesis, structure, and fluorescence in DNA. J Am Chem Soc 118 7671-7678... 

Many of the mutations caused by artificially produced base analogues are transitions. Mutations are produced by base analogues in one of two different ways. On entering the cell, a base analogue is converted to a nucleoside triphosphate that base pairs, perhaps incorrectly, with a DNA template and is inserted into the nucleotide chain. This is one way in which the mutation can be produced. The other requires an additional round of replication so that an improper base pair forms as a result of the previously incorporated analogue. The result in both cases is a permanently modified DNA. 

Several complexes containing these types of ligands including triazol-based analogues have been reported over the last 2 decades. 132,133,149,364,370 However,... 

Heat treatment of carbon black impregnated with a salt of [Fe(phen)3] gives an oj gen reduction catalyst. i The compounds [Fc2(88)Cl4], [Fe(89)Cl2] and [Fe(90)Cl2] are potential ethene polymerization catalysts they all contain five-coordinated iron(II). The chiral terpy ligands (91) (R R = all three combinations of H, Pr ) and the Schiff base analogue (92), also... 

Figure 6.44 Replication of a nucleic acid in the presence of the base analogue bro mo uracil. After the first replication, Bu enters opposite G, giving a ""I G C->A T transition. If G enters opposite Bu already C r, incorporated, an A T—>G C transition results. Abbre-4 gu viations Bu, bromouracil G, guanine. Source 4 Bu Adapted from Ref. 12.

Bu may pair with G, however, as if it were C (Fig. 6.44), with the consequence that G C to A T and A T to G C transitions result. The first type of transition occurs because of erroneous incorporation and the second results from erroneous replication against the incorporated Bu. Other base analogues, some of which are used as anticancer drugs, may produce lethal mutations in mammalian cells, particularly in cancerous cells, which are involved in rapid DNA synthesis. 

---