Ureido anion

The simplest mechanism for transfer of the carboxyl group of carboxyphosphate to biotin would appear to be nucleophilic displacement of the phosphate leaving group by NT of biotin. The enzyme could presumably first catalyze removal of the NT hydrogen to form a ureido anion.59 Another reasonable possibility would be for the terminal phospho group of ATP to be transferred to biotin to form an O-phosphate60/61... 

Either of the foregoing mechanisms requires that the ureido anion of biotin attack the rather unreactive carbon atom of carboxyphosphate. Another alternative, which is analogous to that suggested for PEP carboxylase (Eq. 13-53) is for carboxyphosphate to eliminate inorganic phosphate to give the more electrophilic C02 (Eq. 14-9, step a). The very basic inorganic phosphate trianion P043 that is eliminated could remove the proton from Nl of biotin to create the biotin ureido anion (step b), which could then add to C02 (step c).22... 

Have we checked all of the possibilities for the mechanism of biotin carboxylation Kruger and associates62 63 suggested that biotin, as a ureido anion, might add to bicarbonate to form a highly unstable intermediate which, however, could be phosphorylated by ATP (Eq. 14-10, steps a and b). This intermediate could undergo elimination of inorganic phosphate... 

The macrocyclic receptors 32 [43] and 33 [44] combining two calix[4]arene motifs within the molecule were designed for anion recognition. While 32 creates 1 1 complexes with several anions (halides, HSOj, H2POj), compound 33 is too rigid to efficiently complex halides or benzoate. By contrast, similar cage molecule 34 with the ureido bridges showed complexation ability towards chloride or benzoate. 

Similar structures 37 and 38, bearing ureido functions on the lower rim of calix[4]diquinone or monoquinone moieties, were used for anion recognition with the aid of electrochemistry (cyclic voltammetry) [47,48]. Some of these compounds were found [49] to bind selectively HSOj or H2POj anions. 

Two ureido functions introduced into the upper rim of calix[4]arene [52] or the corresponding calix[4]diquinone derivative 41 exhibit good complexa-tion abilities for various anions. Even in highly competitive solvents such as DMSO-d6, quinone 41 shows [53] very strong complexation for H2POj (K4l= 13,900 M-1) or for benzoate (K41=2,430 M-1). The complexation process can be observed using electrochemical methods (cyclic voltammetry) where the addition of anions generates substantial cathodic shifts. 

Several calix[4]arene [54, 55] or calix[6]arene [56, 57] derivatives 42-44 bearing ureido or thioureido functions on the lower rim have also been evaluated as anion receptors. As indicated by NMR spectroscopy, calixarene 42... 

Another strategy for functionalizing calixarenes is to strap the macrocydes. Several systems bearing a diamido strap have been prepared and have shown a selectivity for acetate anions (e.g., receptor 10 Ka=103M 1 in CDC13) [49]. Similar selectivity preferences were observed for thiacalix[4]arene receptors functionalized with four ureido or thioureido units on the lower rim [50]. 

Very high association constants (>10 M ) for the formation of the capsular 1 1 complex in CD2CI2/CD3OD (2 1) solution were estimated for alkyl chains of C12-C16. The ureido functionality of the bridge at the wider rim of host 11-24 supports the compl-exation in nonprotic solvents by binding with anions such as chloride, picrate, or car-boxylate (see Figure 12.4). Shorter alkyl chains are encapsulated by only a single cavity. 

Anion binding with the assistance of metal ions has become increasingly popular in the last decade. Barboiu et al. reported a dual host heteroditopic (ureido) crown ether... 

An interesting example reported by Barboiu is based on the neutral heteroditopic (ureido)crown ether, 4-phenylurea-benzo-15-crown-5 receptor 22, complexing both anions and cations and being able to give self-organized structures in solution and in solid state. 

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