Imidazole acid/base properties

The imidazole ring of histidine acid-base properties... 

A quantitative analysis of acid-base properties of azoles (see Section IV,B) requires the knowledge of thermodynamic quantities necessary to complete the thermodynamic cycle between the gas phase and the solution. Work in progress is aimed to obtain this information for parent azoles. For instance, the cycle has been determined for the imidazole/pyrazole pair (Fig. 3) (86JA3237). 

A thermodynamic analysis (86UP1) of the effect of annelation on the acid-base properties of the couples imidazole/benzimidazole and pyra-zole/indazole in aqueous solution has shown that this effect is essentially determined by the enhanced electronic delocalization in the case of the anions of the benzazoles. The differential steric hindrance to solvation, on the other hand, does not seem to play a significant role. 

The acid-base properties of 4-nitroso-5-aminopyrazoles 50 have been studied. In particular, protonation sites have been determined where salts 51 are formed [66], The effects produced on II, 13C, and 15N chemical shifts by protonation and by hydrogen-bonding solvents on five azoles (imidazole, 4,5-dimethylimidazole, pyra-zole, 3,5-dimethyl-pyrazole, and 4,5-dihydro-3-methyl-2//-benz[g]indazole) have been determined experimentally. Phase effects on the 13C chemical shifts of the C-4 atom of pyrazole were discussed, based both on empirical models and on GIAO calculations of absolute shieldings in different complexes. The special case of the chemical shifts of pyrazoles in the solid state, where they form multiple N-H N hydrogen bonds, was also studied theoretically [88],... 

The basicity of imidazole was thoroughly covered in CHEC(1984) and CHEC-II(1996). The ambivalent acid-base properties of imidazole play an important role in the biochemical activity of the histidine unit. 

The imidazole ring, with a physiological pH value of 7.4, exists in the histidine building blocks of proteins as a free base and as a conjugate acid (pAT = 7.00, see p 166) because of a regulating acid-base equilibrium. Especially in enzymes, the ring can act as a Bronsted base or as a Bronsted acid as the occasion demands, i.e. it acts as a buffer. It is also able to form complexes with metal ions. Such properties are not found in any other proteinogenic amino acids [119]. 

Poiyphosphazene Biends Blends of sulfonated polyphosphazene, for example, sulfonated poly[bis(3-methylphenoxy)phosphazene] or poly[(bisphenoxy)phosphazene] (see Fig. 29.14) with either an inert organic polymer such as poly(vinylidene fluoride) (Pintauro and Wycisk, 2004 Wycisk et al., 2002) or polyacrylonitrile (Carter et al., 2002) or a reactive polymer (e.g., polybenzimidazole) (Wycisk et al., 2005) have been investigated. The resultant membranes had conductivities of 0.01-0.06 S/cm (in water at 25°C) and equilibrium water swelling from 20 to 60% (at 25°C). Blends of poly[(bisphe-noxy)phosphazene] and polybenzimidazole (where acid-base complexation occurred between the sulfonic acid and the imidazole nitrogen) exhibited good mechanical properties and low methanol permeability. MEAs with this membrane material outperformed Nafion 117 in a DMFC at 60°C with concentrated (5-10 M) methanol feeds. With 1.0 M methanol and 0.5 L/min ambient air at 60°C, the maximum power density was 97 mW/cm and the methanol crossover was 2.5 times lower than that with Nafion 117 (Wycisk et al., 2005). 

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