A reinvestigation of the synthesis of [15N2]hydroxymethylimidazole: Useful in an improved synthesis of (D,L)-[τ,π-15N2]histidine

Highly Selective Stable Isotope Labeling of Histidine Residues by Using a Novel Precursor in E. coli-Based Overexpression Systems

The importance of NMR spectroscopy in unraveling the structural and dynamic properties of proteins is ever-expanding owing to progress in experimental techniques, hardware development, and novel labeling approaches. Multiple sophisticated methods of aliphatic residue labeling can be found in the literature, whereas the selective incorporation of NMR active isotopes into other amino acids still holds the potential for improvement. In order to close this methodological gap, we present a novel metabolic precursor for cell-based protein overexpression to assemble ¹³ C/² H isotope patterns in the peptide backbone, as well as in side chain positions of a mechanistically distinguished histidine residue.

An EPR, ESEEM, Structural NMR, and DFT Study of a Synthetic Model for the Covalently Ring-Linked Tyrosine-Histidine Structure in the Heme-Copper Oxidases

We report CW-EPR, ESEEM, and structural NMR results, as well as DFT calculations, on model compounds relevant to the unusual cross-linked Tyr-His (YH) moiety at the active site of the heme-copper oxidases. CW-EPR spectra of an (15)N isotopically labeled 4-methyl-2-(4-methyl-imidazole-1-yl)-phenol radical are nearly identical to those of the natural abundance (14)N compound. We obtain good simulations of these EPR spectra without including hyperfine couplings to the nitrogen nuclei. This implies that the electron distribution of the radical is largely localized on the phenol ring with only a small amount of spin delocalized onto the nitrogens of the imidazole. Using three-pulse ESEEM spectroscopy, we have successfully detected the two imidazole ring nitrogens, one near the "exact cancellation" ESEEM condition and the other more weakly coupled. We assign these to the imino and amino nitrogens, respectively, based on DFT calculations performed on this radical species. The experimental results and the supporting density functional calculations clearly show that the imidazole substituent has only a minor effect on the electronic structure of the substituted phenol radical.