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.
Kim
Sun Hee
SH
Department of Chemistry, University of California, Davis, California 95616, USA.
Aznar
Constantino
C
Brynda
Marcin
M
Silks
Louis A
LA
Michalczyk
Ryszard
R
Unkefer
Clifford J
CJ
Woodruff
William H
WH
Britt
R David
RD
D003198:Computer Simulation; D003432:Cross-Linking Reagents / Q000737:chemistry; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D005583:Fourier Analysis; D006418:Heme / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010088:Oxidoreductases / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
D003432:Cross-Linking Reagents; D014443:Tyrosine; D006418:Heme; D006639:Histidine; D010088:Oxidoreductases; C024748:copper oxidase