High field (W-band, 95 GHz) pulsed electron-nuclear double resonance (ENDOR) measurements were carried out on a number of proteins that contain the mixed-valence, binuclear electron-mediating CuA center. These include nitrous oxide reductase (N2OR), the recombinant water-soluble fragment of subunit II of Thermus thermophilus cytochrome c oxidase (COX) ba3 (M160T9), its M160QT0 mutant, where the weak axial methionine ligand has been replaced by a glutamine, and the engineered "purple" azurin (purpAz). The three-dimensional (3-D) structures of these proteins, apart from the mutant, are known. The EPR spectra of all samples showed the presence of a mononuclear Cu(II) impurity with EPR characteristics of a type II copper. At W-band, the g^ features of this center and of CuA are well resolved, thus allowing us to obtain a clean CuA ENDOR spectrum. The latter consists of two types of ENDOR signals. The first includes the signals of the four strongly coupled cysteine beta-protons, with isotropic hyperfine couplings, Aiso, in the 7-15 MHz range. The second group consists of weakly coupled protons with a primarily anisotropic character with Azz < 3 MHz. Orientation selective ENDOR spectra were collected for N2OR, M160QT0, and purpAz, and simulations of the cysteine beta-protons signals provided their isotropic and anisotropic hyperfine interactions. A linear correlation with a negative slope was found between the maximum Aiso value of the beta-protons and the copper hyperfine interaction.
Comparison of the best-fit anisotropic hyperfine parameters with
those
calculated from dipolar interactions extracted from the available 3-D
structures
sets limit to the sulfur spin densities. Similarly, the small coupling
spectral region was simulated on the basis of the 3-D structures and
compared
with the experimental spectra. It was found that the width of the
powder
patterns of the weakly coupled protons recorded at g^is
mainly determined by the histidine He1
protons. Furthermore, the splitting in the outer wings of these powder
patterns indicates differences in the positions of the imidazole rings
relative to the Cu2S2
core. Comparison
of the spectral features of the weakly coupled protons of M160QT0 with
those of the other investigated proteins shows that they are very
similar
to those of purpAz, where the CuA center is the
most
symmetric, but the copper spin density and the He1-Cu
distances are somewhat smaller. All proteins show the presence of a
proton
with a significantly negative Aiso value which
is
assigned to an amide proton of one of the cysteines. The simulations of
both strongly and weakly coupled protons, along with the known copper
hyperfine
couplings, were used to estimate and compare the spin density
distribution
in the various CuA centers. The largest sulfur
spin
density was found in M160T9, and the lowest was found in purpAz. In
addition,
using the relation between the Aiso values of
the
four cysteine beta-protons and the H-C-S-S dihedral angles, the
relative
contribution of the hyperconjugation mechanism to Aiso
was determined. The largest contribution was found for M160T9, and the
lowest was found for purpAz. Possible correlations between the spin
density
distribution, structural features, and electron-transfer functionality
are finally suggested.