Electron Paramagnetic Resonance Spectroscopic Study of Copper Hopping in Doped Bis(l‑histidinato)cadmium Dihydrate

Electron
paramagnetic resonance (EPR) spectroscopy was used to
study Cu­(II) dynamic behavior in a doped biological model crystal,
bis­(l-histidinato)cadmium dihydrate, in order to gain better
insight into copper site stability in metalloproteins. Temperature-dependent
changes in the low temperature X-band EPR spectra became visible around
100 K and continued up to room temperature. The measured 298 K g-tensor
(principal values: 2.17, 2.16, 2.07) and copper hyperfine coupling
tensor (principal values: −260, −190, −37 MHz)
were similar to the average of the 77 K tensor values pertaining to
two neighboring histidine binding sites. The observed temperature
dependence was interpreted using Anderson’s theory of motional
narrowing, where the magnetic parameters for the different states
are averaged as the copper rapidly hops between sites. The EPR pattern
was also found to undergo a sharp sigmoidal-shaped, temperature-dependent
conversion between two species with a critical temperature Tc ≈ 160 K. The species below Tc hops between the two low temperature site
patterns, and the one above Tc represents
an average of the molecular spin Hamiltonian coupling tensors of the
two 77 K sites. In addition, the low and high temperature species
hop between one another, contributing to the dynamic averaging. Spectral
simulations using this 4-state model determined a hop rate between
the two low temperature sites νh4 = 4.5 × 108 s–1 and between the
low and high temperature states νh2 = 1.7 × 108 s–1 at 160 K. An Arrhenius
relationship of hop rate and temperature gave energy barriers of ΔE4 = 389 cm–1 and ΔE2 = 656 cm–1 between the two
low temperature sites and between the low and high temperature states,
respectively.