Examples of high-frequency EPR studies in bioinorganic chemistry

Abstract

Low-temperature EPR spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T has been used to study metal sites in proteins or inorganic complexes and free radicals. The high-field EPR method was used to resolve g-value anisotropy by separating it from overlapping hyperfine couplings. The presence of hydrogen bonding interactions to the tyrosyl radical oxygens in ribonucleotide reductases were detected. At 285 GHz the g-value anisotropy from the rhombic type 2 Cu(II) signal in the enzyme laccase has its g-value anisotropy clearly resolved from slightly different overlapping axial species. Simple metal site systems with S>1/2 undergo a zero-field splitting, which can be described by the spin Hamiltonian. From high-frequency EPR, the D values that are small compared to the frequency (high-field limit) can be determined directly by measuring the distance of the outermost signal to the center of the spectrum, which corresponds to (2 S-1)* mid R: Dmid R: For example, D values of 0.8 and 0.3 cm(-1) are observed for S=5/2 Fe(III)-EDTA and transferrin, respectively. When D values are larger compared to the frequency and in the case of half-integer spin systems, they can be obtained from the frequency dependence of the shifts of g(eff), as observed for myoglobin in the presence ( D=5 cm(-1)) or absence ( D=9.5 cm(-1)) of fluoride. The 285 and 345 GHz spectra of the Fe(II)-NO-EDTA complex show that it is best described as a S=3/2 system with D=11.5 cm(-1), E=0.1 cm(-1), and g(x)= g(y)= g(z)=2.0. Finally, the effects of HF-EPR on X-band EPR silent states and weak magnetic interactions are demonstrated.

Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

“…(2)) but may also provide clues as the degree of homogeneity associated with the binding site of the metal cofactor. By going to higher field and frequency, the individual spin transition manifolds become more isolated, greatly simplifying results from the application of advanced pulsed techniques such as ENDOR [115][116][117]. As described in Sect.…”

Crystallographic and Spectroscopic Studies of Peroxide-derived Myoglobin Compound II and Occurrence of Protonated FeIV–O

“…EPR Measurements-As described previously (55), X-band EPR spectra were obtained on a Bruker ESP 300 equipped with an Oxford instrument 900 cryostat, and also at the Grenoble High Magnetic Field Laboratory (71,72). A few hundred protein crystals in quarts EPR tubes or in High Field/Frequency EPR cylindrical Teflon buckets (71, 72) were exposed to 22 mM hydrogen peroxide and rapidly frozen after 30 s of incubation (300 s was also tried) in liquid nitrogen, and subsequently analyzed at 10 K. Samples were also analyzed with a customized High Field/Frequency EPR instrument (71,72) without a cavity in a single-pass transmission spectrometer.…”

Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves

“…The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radio frequency or microwave magnetic fields and nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment directions [1][2][3]. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and metalloproteins.ESR in the terahertz (THz) frequency region can reveal rich information content in chemistry, biology, and materials science [1,2,[4][5][6][7]. In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5].…”

“…ESR in the terahertz (THz) frequency region can reveal rich information content in chemistry, biology, and materials science [1,2,[4][5][6][7]. In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5].…”

“…In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5]. With strong applied magnetic fields (∼10 T), resonances of unpaired electron spins in molecular complexes can be shifted from the usual microwave regime into the THz range, drastically improving the resolution due to enhanced spectral splittings [2,6,7]. In many ferromagnetic (FM) and antiferromagnetic (AFM) materials, intrinsic magnetic fields in the same range put collective spin waves (magnons) in the THz range.…”