C. A. Matuszak and A. J. Matuszak The University of the Pacific Stockton. California 95204
Imidazole-Versatile Tomorrow
lmidazole is proving to be of very prime importance in biological systems where it is found in the amino acid histidine. Histidine has been implicated in the active site ot'enzymes such as rihonuclease and the serine proteases includinp a-chymotrv~sin,trypsin, and subtilisin, and is bound to metal ions in &era1 mt?talloproteins such as hemoglobin, myoglobin, cytochrome C, carboxypeptidase A, and histidine deaminase(l-4). As the fascinating story of life and death on the molecular level continues to unfold over the next few decades, imidazole chemistry will play an intimate and critical role. Most oreanic chemistrv students are ~ u r s u i n ebioloeically orienred'careera and $11 need to understand'and u& the chemistrv of imidazole in the future but orranic chemistrv courses of today give very little coverage of this important com~ound.Fortunatelv imidazole chemistrv has considera b ~ e - ~ e d a ~ o ~use i c aasl a framework on which to develop .and illustrate manv. kev. fundamental ~ r i n c i ~ l of e s oreanic chemistry and can simultaneously be a direit and relevant link between organic chemistry and biological systems. We have compiled this summary of imidazole chemistry largely from reviews (5) in the literature, many of which are not readily available in some college libraries, t o encourage and facilitate the inclusion of imidazole chemistry into organic chemistry courses and organic chemistry textbooks. lmldazole as an Azole Imidazole can be related to other 5-memhered heterocycles by pointing out that it is one of six azoles derived from furan, pyrrole, and thiophene by replacement of a ring CH by a nitrogen atom. All the azoles are found in important drug molecules.
Today, Prominent
gen in position 3 has an electron pair available and is a "pyridine-like" nitrogen. However imidazole is both somewhat more acidic than pyrrole and somewhat more basic than pyridine. This is an interesting consequence of the two nitrogens being equivalent in each ion. The charge is distributed equally on the two nitrogens in each case.
* Imidazolium Ion, pKa versus Pyridinium Ion, pKn
= =
6.95 (7) 5.2 (7)
Imidazole Anion,pKa = 14.2 (7); 14.5 (5d) versuspKa = 16.5 for Pyrrole (7) Substituent effects on basicity and acidity of imidazoles parallel those for anilines and henzoic acids. Electron withdrawing groups such as nitro, halogen, and phenyl decrease the basicity while electron donating groups such as alkyl increase basicity. As Table 2 indicates, the limited available data show that electron withdrawing substituents increase the weak acidity of imidazoles. 4-Bromoimidazole is reported to have an ac2 for the general hase reaction and ranges from 0.%1.9 for the nucleophilic reaction. The mechanism favored for an individual acyl compound depends on both the ease the X group can leave and the electronegativity of the acyl group. The easier X can leave and the more electronegative the acyl group, the more likely the nucleophilic reaction. Thus the nucleophilic path is followed if X has a pKa below 10 such as for p-nitrophenoxide or acetate. If X is a poor leaving group, as is the case for alkyl esters or acetoxime acetate, the tetrahedral intermediate will lose imidazole instead of X, and the nucleophilic reaction becomes so slow that the general hase mechanism dominates (38). Even though imidazole exhibits unusual properties as a nucleophile there is no evidence, as yet, that imidazole acts as a nucleophilic catalyst in any enzymatic acyl transfer (39). Serine Proteases
The serine proteases are a family of enzymes within the large group of protein-cutting enzymes. The serine proteases are involved in a wide variety of hiological functions (3) including digestion, blood clot formation and dissolution,
fertilization of the ovum, the immune reaction, and cell growth inhibition with possible implications for cancer. I t appears that all the serine proteases prohahly employ the same three amino acid units t o hydrolyze peptide bonds. a-Chymotrypsin is the best studied hut conclusions concerning it may he valid for the others. In a-chymotrypsin the active site includes serine 195, histidine 57, and aspartic acid 102 (3, 40). The imidazole ring of the histidine unit acts as a general base and removes a proton from the hydroxyl group of serine as the oxygen of the latter attacks the carhonyl carbon of the suhstrate. The imidazole is assisted in this role by the aspartate anion as indicated (3,401.
Aspartic Acid 102
Histidine 57
Serine 195
The protons then move in the reverse direction t o protonate the -NH- group of the suhstrate followed by cleavage of the peptide hond and formation of an acyl enzyme intermediate. This latter is hydrolyzed by a very similar process in which a water molecule, instead of serine, is activated by the histidine unit (3,401. Proton Nuclear Magnetic Resonance
As with other heteroaromatics, the aromatic hydrogens of imidazole compounds absorb in the aromatic nmr region with hydrogens alpha to the heteroatom shifted downfield. The C-2 hydrogen is alpha to hoth nitrogens and is furthest downfield (6 7.2-7.9) (41). This downfield position of the C-2 hydrogen was of great value in the study of rihonuclease. Each of the four histidine units has a discernible peak for its C-2 hydrogen and by studying the effect of varying the p H , the pKa of each unit has been determined (15). lmidazole as a Ligand
The electron pair of the pyridine-like nitrogen is available for coordination with metal ions and many important examples are known in nature (42-44). A good example is hemoglohin which has Fez+ octahedrally coordinated with the four nitrogens of heme, with a histidiue unit and with either oxygen or water molecule (45). Myoglohin is similar (45b). Histidine deaminase has a manganese ion hound to the enzyme and to the imidazole ring of the suhstrate histidine (43,46). Carhoxypeptidase A has a zinc ion coordinated with two histidine units and with a glutamic acid unit of the enzyme (47). Insulin hexamer is reported having two zinc ions each hound to imidazoles of three histidine units (48). Cytochrome C has an iron ion coordinated with a porphyrin ring and a histidine unit along with the sulfur of a methionine (49). Conclusion
We feel an understanding of the chemistry of imidazole is essential for elucidation of molecular mechanisms for many biological processes. I t is also valuable in determining how certain drugs exert their effects in the body. Some coverage of this important compound rightly belongs in basic organic chemistry courses. Literature Cited
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