A systematic approach for studying Cu imidazole interactions using multi-frequency ESEEM: Application to Cu(II)-bleomycin and model systems

Many copper containing proteins exhibit well defined ESEEM signals detected at X-band and C-band. In these systems the Cu(II) ion is coordinated to one or several histidine residues. The main sharp features measured in the ESEEM spectra originate from the interaction of the unpaired electron with the remote nitrogen nucleus of the histidine ring. The relative intensities of these features contain information about the orientation of the NQI-tensor in the molecular axis frame as defined by the principal axes of theg-matrix. This information can be related to the orientation of the imidazole ring in the complex. We present a systematic approach to determine the constraints of the Euler angles, α, β, ψ of the NQI-tensor in theg-matrix principal axis system. The first step is to analyze the intensity ratios of the quadrupole peaks and the line shape of the double quantum feature measured on the canonical positions in the EPR spectrum. This will lead to a constraint in the angles (α, β) as well as the effective hyperfine interaction. This information is further refined using spectra on other orientation selective positions. We have applied this method to Cu(II)-Bleomycin and two model compounds: Cu(II)-pypep and Cu(II)-PMA of which we have determined the principal quadrupole values and the orientation of the quadrupole tensor with respect to theg-matrix axis system.