Wide zero field interaction distributions in the high-spin EPR of metalloproteins

The EPR spin Hamiltonian parameters of transition ion active centres in frozen aqueous solutions of proteins are distributed as a reflection of distributions in spacial conformations. This phenomenon is generally referred to with the generic term ‘g-strain’, however, its manifestation is not limited to a distribution in the g-values. The equivalent name ‘D-strain’ applies to the situation common for biological half-integer high-spin systems whose powder EPR shape is predominantly modified through the distributive effects in the second-order terms of the spin–spin interaction Hamiltonian. A simple, one-parameter model is developed to account for D-strain, and this forms the basis for an efficient and practical numerical analysis procedure for D-strained spectra in the weak field limit. Analysis of S = 5/2, 7/2, and 9/2 protein examples is used to illustrate the drastic modification of relative intensities and widths and the occurrence of extra turning points in these spectra as a consequence of D-strain.