Tyrosine phenoxide

Amino acid analysis (196) and a complete amino acid sequence of the 33-kDa protein from spinach (201) indicated that no histidine was present. Thus, Mn ligation to this polypeptide must be to tyrosine phenoxide and/or to the carboxylate functionalities of aspartate and glutamate. 

Fe3+ also shows a strong affinity for the oxygen donor atoms of carboxylates as well as the phenoxide of tyrosine. Like cysteine, the sulfur ligand of methionine is often found bound to iron, for example in electron-transfer haemoproteins such as cytochrome c. 

Recently interest has been shown in studies of the biochemical processes, which include the action of enzymes with vanadium-containing centers (like those of serine or tyrosine) that catalyze or inhibiting the redox and phosphorylation reactions, and the alkoxides were investigated as the structural models for the latter [1319, 424], A bimetallic phenoxide of vanadium (II, HI) was found to be an efficient reducive agent for molecular nitrogen in the methanol solutions [1034],... 

Another approach to ring closure reaction is the o-arylation of substituted phenoxide ions by o-bromobenzonitrile followed by Si02-catalysed lacto-nization. The phenoxide ions of the amino acid (S)-tyrosine, protected as A, O-diacetyl methyl ester, does not racemize under the standard SRN1 conditions and can be used to obtain the optically active benzo[c]chromen-6-one (the O-acetyl is hydrolyzed in the reaction media to furnish the phenoxide ion) (Sch. 43). Racemic dibenzopyranones are obtained by the reaction of the anion from the TV-acetyl methyl ester of (R)-hydroxyphe-nylglycine with o-bromobenzonitrile 2-cyano-4,5-dimethoxybromobenzene (65 and 79% respectively) [110]. 

Mn(III). Structurally characterized higher valent manganese complexes with phenoxide-type ligation are limited to Mn(III) and Mn(IV) Schiff base complexes (128-135), Mn(III) and Mn(IV) catecholates (136-139), Mn(III) and Mn(IV) salicylates (140-142), and Mn(III) bi-phenoxides (143). However, of these ligands, only biphenoxide is similar electronically to tyrosine Mn(III) complexes of these ligands, in general, lack the intense phenoxide - metal charge transfer band centered at 435 nm. 

The fact that the four metal ligands provided by the transferrin apoprotein involve three different amino acid residues, i.e., histidine, tyrosine, and aspartate, is most interesting. Inorganic mononuclear complexes with such a variety of ligand types are not often encountered however, a Mn(III) complex with imidazole, carboxylate-, and phenoxide-type ligation has been structurally characterized. The complex [Mn(sal)2(ImH)2] (Fig. 7) (sal = salicylate) possesses two trans imidazoles and two salicylates disposed trans to one another (142). 

Another unconventional deprotonation is seen with tyrosine. Gas-phase experiments by Kass indicate that deprotonation of tyrosine leads to a 70 30 mixture of the phenoxide 16 to carboxylate 17 anions." Kass optimized the structures of tyrosine and its two possible conjugate bases 16 and 17 at B3LYP/aug-cc-pVDZ, and computed the two DPEs at G3B3.17 is predicted to be 0.2 kcal mol lower in energy than 16 at B3LYP and slightly more stable at G3B3 (0.5 kcal mol ). However, both computational methods fail to predict the DPEs of acetic acid and phenol... 

Model hemes coordinated to phenoxides and aryl thiolates have been investigated as models of catalase and hemoglobins M (which have tyrosine substituted for either the proximal or distal histidine) and cytochromes P450 (which have the thiolate of cysteine coordinated to... 

Sage and Singer (1958, 1962) showed that ribonuclease could not only be recovered from neutral ethylene glycol into aqueous solution with essentially full retention of enzymatic activity, but that this was so even after all six of its tyrosine residues had been converted to the phenoxide ion form in ethylene glycol. This is in contrast to the situation in water solutions of this protein, in which the titration of more than three of the six tyrosines results in an essentially instantaneous irreversible loss of enzymatic activity (Sela and Anfinsen, 1957). This suggests the interesting possibility that the irreversible transition that occurs in aqueous solutions... 

A related procedure giving a 90% yield has been described in which phosphorus oxychloride in chloroform was added to a stirred paste of chloroform, water and sodium phenoxide containing a catalytic quantity of polyethyleneglycol 400 (ref.36). L-Tyrosine has been phosphorylated quantitatively by addition to a stirred mixture of phosphorus pentoxide and 85% orthophosphoric acid followed by heating at 80°C for 24 hours, after which water was introduced, the mixture was cooled, diluted with n-butanol and kept at 0°C for 3 hours to precipitate the product (ref.37). 

The one-electron oxidation of tyrosine by a copper(II) center (step B of Figure 7) is modeled by two systems (Figure 9). Treatment of (1) with sodium 2,6-di(tertbutyl)phenolate yields copper(I) products, which can be trapped with an isocyanide to give (50) 3,3, 5,5 -tetr<2 w(tertbutyl)-4,4 -dibenzoquinone is formed in this reaction in its maximum theoretical yield of 25%. Similar reactions using less sterically hindered phenolates instead yield the stable copper(II) phenoxide compounds (4)-(7). Similarly, (51) and (52) decompose rapidly at room temperature to give... 

Phenoxides undergo substitution for chloride [58-60]. The first substitution in a dichlorobenzene complex occurs at -20 °C [61]. This allowed synthesis of tyrosine derivative 33 without the loss of optical purity [62a]. 

In addition to inductive and field effects, resonance effects can also play an important role in determining the strength of organic acids and bases. For example, the pX of a simple alkyl alcohol is 15, but that of the hydroxyl of tyrosine is 9.11. By analogy, the pX of the hydroxyl of phenol is 9.8 (see Table 2.1). Such may be understood by the realization that once the ionization has occured, the phenoxide anion may be stabilized by electron resonance ... 

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