Secondary Products Synthesized from L-Histidine
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Imidazole
Side chain
Alanine
Replacement of the axial histidine ligand with exogenous imidazole has been accomplished in a number of heme protein mutants, where it often serves to complement the functional properties of the protein. In this paper, we describe the effects of pH and buffer ion on the crystal structure of the H175G mutant of cytochrome c peroxidase, in which the histidine tether between the heme and the protein backbone is replaced by bound imidazole. The structures show that imidazole can occupy the proximal H175G cavity under a number of experimental conditions, but that the details of the interaction with the protein and the coordination to the heme are markedly dependent on conditions. Replacement of the tethered histidine ligand with imidazole permits the heme to shift slightly in its pocket, allowing it to adopt either a planar or distally domed conformation. H175G crystallized from both high phosphate and imidazole concentrations exists as a novel, 5-coordinate phosphate bound state, in which the proximal imidazole is dissociated and the distal phosphate is coordinated to the iron. To accommodate this bound phosphate, the side chains of His-52 and Asn-82 alter their positions and a significant conformational change in the surrounding protein backbone occurs. In the absence of phosphate, imidazole binds to the proximal H175G cavity in a pH-dependent fashion. At pH 7, imidazole is directly coordinated to the heme (dFe-Im = 2.0 Å) with a nearby distal water (dFe-HOH = 2.4 Å). This is similar to the structure of WT CCP except that the iron lies closer in the heme plane, and the hydrogen bond between imidazole and Asp-235 (dIm-Asp = 3.1 Å) is longer than for WT CCP (dHis-Asp = 2.9 Å). As the pH is dropped to 5, imidazole dissociates from the heme (dFe-Im = 2.9 Å), but remains in the proximal cavity where it is strongly hydrogen bonded to Asp-235 (dIm-Asp = 2.8 Å). In addition, the heme is significantly domed toward the distal pocket where it may coordinate a water molecule. Finally, the structure of H175G/Im, pH 6, at low temperature (100 K) is very similar to that at room temperature, except that the water above the distal heme face is not present. This study concludes that steric restrictions imposed by the covalently tethered histidine restrain the heme and its ligand coordination from distortions that would arise in the absence of the restricted tether. Coupled with the functional and spectroscopic properties described in the following paper in this issue, these structures help to illustrate how the delicate and critical interactions between protein, ligand, and metal modulate the function of heme enzymes.
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Hemeprotein
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Imidazole
Linkage isomerism
Nitrogen atom
Carbon atom
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Key words copper catalysis - arylation - histidines - triarylbismuth reagents - imidazoles
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Side chain
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The pH-dependence of selected 13C chemical shifts reflects the state of ionization of the imidazole ring in both imidazole and L-histidine. Titration of the amino and carboxyl groups of histidine also perturbs the shifts. The coupling constants 1J (13C(2),H) and 1J (13C(5),H) for both compounds also vary with pH, but in L-histidine these constants are relatively insensitive to the titration of groups outside the imidazole ring.
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This chapter contains sections titled: Ring System C3N2-C4N: Pyrrolo[3,4-d]imidazole Ring System C3N2-C4N: Pyrrolo[2, 3-d]imidazole Ring System C3N2-C3N2: 1H-Imidazole[2,2-a]imidazole Ring System C3N2-C3N2: 1H-Imidazole[2,5-a]imidazole Ring System C3N2-C3N2: 1H-Imidazole[2,5-c]imidazole Ring System C3N2-C3N2: Imidazole[1,2-b]pyrazole Ring System C3N2-C3N2: 1H-Imidazo[1,5-b]pyrazole Ring System C3N2-C4O: 1H-imidazo[1,5-b]pyrazoles Ring System C3N2-C4O: 1H and 4H-Furo[3,4-d]imidazoles Ring System C3N2-C3NO: Imidazo[2,1-b]oxazoles Ring System C3N2-C3NO: Imidazo[5,1-b]oxazoles Ring System C3N2-C3NO: Imidazo[1,5-c]oxazoles Ring System C3N2-C3NO: Imidazo[1,5-b]isoxazole Ring System C3N2-C4S: Thieno[2,3-d]imidazole Ring System C3N2-C4S: Thieno[3,4-d]imidazole Ring System C3N2-C3NS: Imidazo[1,2-b]imidazole Ring System C3N2-C3NS: Imidazo[2,1-b]thiazole Ring System C3N2-C3NS: Imidazo[5,1-b]thiazole Ring System C3N2-C3NS: 1H,3H-Imidazo[1,5-c]thiazole Ring System C3N2-C3NS: Imidazo[1,2-c]thiazole Ring System C3N2-C4Se: Selenolo[2,3-d]imidazole Ring System C3N2-C4Se: 1H-Selenolo[3,4-d]imidazole Ring System C3N2-C4NSe: Imidazo[2,1-b]Selenazole
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Isoxazole
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A simple access to histidines that contain a carbon branch at the C2 atom of the imidazole ring is provided by reaction (a). The protected monoiodo histidine 1 is therefore a versatile coupling partner.
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N-(α)-Benzyloxycarbonyl-N(π)-t-butoxymethyl -L-histidine (1)and N(α)-fluoren-9-ylmethoxycarbonyl--N(π)-t-butoxymethyl-L-histidine(2)have been prepared and their use for the synthesis of peptides containing histidine residues has been demonstrated in two simple exercises; no difficulties were encountered, the base-and hydrogenolysis-resistant imidazole imidazole protecting group ultimately being removed by mild acidolysis.
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Hydrogenolysis
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Protecting group
Base (topology)
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The copper(I) complexes of peptides containing sulfhydryl and imidazole groups have been investigated in aqueous solution by proton magnetic resonance (1H NMR). The N-mercaptoacetyl-L-histidine coordinates through the sulfhydryl sulfur and imidazole nitrogen atoms to form the unique 1:1 Cu(I) complex species containing bridged imidazole. The sulfhydryl and imidazole groups are adequate ligands for not only Cu(II) but also Cu(I). The 1H NMR spectra show that the peptide nitrogen group is not involved in complex formation of Cu(I) with N-mercaptoacetyl-L-histidine and 3-mercaptopropionyl-L-histidine. The results have been discussed with respect to the protein ligands for 'blue' copper centers.
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Copper protein
Proton magnetic resonance
Proton NMR
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Abstract The 1 H‐nmr titration curves of chemical shifts versus pH were observed for the protons of D,L ‐histidyl‐ D,L ‐histidine as representative of cases with two or more ionizable groups with similar p K a values. The titration curves of L ‐histidyl‐ L ‐histidine and D ‐histidyl‐ L ‐histidine were individually analyzed according to two mathematical models: one of a macroscopic dissociation series and one of a microscopic dissociation series. Most‐probable values and standard deviations were obtained for p K a values and intrinsic chemical shifts. An analysis including the microscopic dissociation series yielded an electrostatic interaction between twoimidazole rings of about 0.3 pH units for L ‐histidyl‐ L ‐histidine and about 0.7 pH units for D ‐histidyl‐ L ‐histidine. The difference of the magnitude of imidazole‐imidazole interactions between L ‐histidyl‐ L ‐histidyne and D ‐histidyl‐ L ‐histidine was interpreted in terms of the spatial arrangement of two imidazole rings in each molecule based on the solution conformation estimated from Gd(III)‐induced relaxation enhancements.
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Abstract The N‐arylation of the side chain of histidine by using triarylbismuthines is reported. The reaction is promoted by copper(II) acetate in dichloromethane at 40 °C under oxygen in the presence of diisopropylethylamine and 1,10‐phenanthroline and allows the transfer of aryl groups with substituents at any position of the aromatic ring. The reaction shows excellent functional group tolerance and is applicable to dipeptides where the histidine is located at the N terminus. A histidine‐guided backbone N−H arylation was observed in dipeptides where the histidine occupies the C terminus.
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Dichloromethane
Side chain
Phenanthroline
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