Histidine complex formation

Imidazole is of particular relevance to biological mimic ligands due to the presence of histidine as a coordinating group for zinc in biological systems and has been a particular target for zinc complex formation. 

Bioinorganic chemists have been attracted by the complex formations of NHC because the imidazolin-2-ylidene motif is encountered frequently in living organisms. The imidazole moiety is part of the purin bases in both DNA and RNA as well as the amino acid histidine which appears in proteins and enzymes and is in many cases considered to play a decisive role within the catalytically active center. The possible formation of NHC complexes under physiological conditions or in vivo has been addressed by investigation of A-confused caffeine 73 or purine 74 complexes. 

The metalloporphyrin complexes and their derivatives have been studied from the standpoint of model compounds of hemoglobin. The polymer-metalloporphyrin complexes are also formed by the reaction in Scheme 8, and a few qualitative investigations have been made with poly(L-lysine)9,10, poly(L-histidine)11, and poly(vinylimidazole)12 as the polymer ligand. Blauer9 has studied the complex formation of heme with poly(L-lysine) and has discussed the effects of the molecular weight and secondary structure of poly(L-lysine) on complex formation. 

Figure 6.2 The mechanism of cytochrome-c-peroxidase complex formation, (a) Native enzyme, (b) Activated complex with the acid-base catalytic function of distal histidine (His) and stabilization of negative charge by arginine (Arg) residue of the active site, (c) Hypothetic intermediate oxene complex, (d) Complex I after intramolecular electron regrouping of oxene complex with Fe4+ and free radical X fragment formation.

Fig. 12 Substances used for complex formation and nanoparticles, (retinoic acid) All-trans retinoic acid, (PLA) poly(-L-arginine), (PLH) poly(-L-histidine), (PLL) poly(-L-ly-sine). As dispersing agent a tri-block copolymer was used. It consisted of ethylene oxide and propylene oxide (Poloxamer 188). Reprinted with permission from [142]. Copyright 2000 American Chemical Society...

Jones, A.D. and Williams, D.R. (1970) Thermodynamic considerations in co-ordination. Part VIII. A calorimetric and potentiometric study of complex formation between some lanthanide(III) ions and histidine. Journal of the Chemical Society A, 3138-3144. 

Fig. 7. (A) The oxidation states of Mn in the various S-states. The model incorporates a histidine radical formation in the Sj-state with no oxidation of Mn in the Sj- Sa transition the model also accommodates the 1 0 1 2 proton-release pattern in the Kok cycle (B) a proposed topological model for the photosynthetic water-oxidizing Mn-complex based on XAS and EPR studies. Figure source (A) [adapted] and (B) Sauer, Yachandra, Britt and Klein (1992) The photosynthetic water oxidation complex studied by EPR and X-ray absorption spectroscopy. In VL Pecararo (ed) Manganese Redox Enzymes, pp 141-175. VCH Publ.

It can be similarly attributed to the formation of the complex that dialkyldi-thiocarbamates can be antagonised with histidine, which also has complex-forming properties. Monoalkyldithiocarbamates are not antagonised, since the complex-formation does not play any role in their effect. 

Glennon JD and Sarkar B (1982) Nickel(II) transport in human blood serum. Studies of nick-el(II) binding to human albumin and to native-sequence peptide, and ternary-complex formation with L-histidine. Biochem J 203 15-23. 

Co(II) or Cu(II) histidine or imidazole complexes were immobilized in porous matrices (montmorillonite and MCM-41) via two methods (introduction of preformed complex or complex formation within the ion-exchanged host substances). It was found that immobilization in general and the latter method in particular increased catalytic activity and catalyst life time in the decomposition reactions of hydrogen peroxide relative to the matrix-free complexes. The immobilized materials were characterized by experimental and computational methods and the structures of the guest molecules inside the hosts were also investigated. 

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